OTHERBOOKSBYADAMFRANK

Astronomy:AtPlayintheCosmos

TheConstantFire:BeyondtheSciencevs.ReligionDebate

AboutTime:CosmologyandCultureattheTwilightoftheBigBang

Tomysister,ElisabethFrank,andourlong,strangeroad.Iamgratefulthatyourhumor,resolve,andCampDawsonfriendshipwereonthispathwithme.

CONTENTS

INTRODUCTION:THEPROJECTANDTHEPLANET

CHAPTER1:THEALIENEQUATION

CHAPTER2:WHATTHEROBOTAMBASSADORSSAY

CHAPTER3:THEMASKSOFEARTH

CHAPTER4:WORLDSBEYONDMEASURE

CHAPTER5:THEFINALFACTOR

CHAPTER6:THEAWAKENEDWORLDS

ACKNOWLEDGMENTS

NOTES

ILLUSTRATIONCREDITS

INDEX

LIGHTOFTHESTARS

INTRODUCTION

THEPROJECTANDTHEPLANET

THECOSMICTEENAGER

Imaginearoomfullofteenagers.Thechairsarearrangedinaloosecircle,andthe air smells of cheap cleaning products and anxiety. The kids aremostly intheir late teens.Someare slumped in their chairs, trying to lookbored; otherslean forward, listening closely.They are here to tell their stories.The sixteen-year-old girl in the Black Sabbath T-shirt and chipped black nail polish gotbustedfordealingdrugsatherhighschool.Theskinnyboywithabadtattooonhishandwasarrested for joyriding inhisgrandparents’car.They’reall in thisroombecausethey’reonthewrongroad.Oldenoughtohavesomepowerovertheirownlives,they’vebeenmakingbadchoices,destructivechoices.

Eachofthekidstakesaturnunspoolinghowtheygothere.Somecamefromfamilies that could barely hold it together. Others were trapped in their ownfeelingsofisolationandinsecurity.Butintellingtheirstories,someofthekidscatch a glimpse of an insight. It’s something they couldn’t imagine, couldn’ttrulyfeel,before.

Theyarenotalone.Theyarenotthefirst.Thecircleandthestoriesgivesomeofthekidsthechancetoseethatit’snot

just them. Their individual stories are not so individual. Other kids their agehavewalkedthisroadbefore,andsomehaveevenfoundawayout.Somehavefoundawaytogrowup.

•••

WEHUMANS,withourprojectofcivilization,arelikethosekids.Themassive collective projectwe call civilization began about almost ten

thousandyearsago,whenthe last iceageendedandourplanet’sclimategrewwarmerandwetter.Inresponse,someofusstoppedournomadicwanderingandsettledintovillages.Aroundthosesmallgroupingsofhutsandstorehouses,theEarthwasput toplow.Wecultivatedgrainsandrice.Wedomesticated theox,thegoat, and the cow.Wecreated anewwayof beinghumanbeyond theoldhunter-gathererwayoflife.Itwasanagriculturalrevolutionthatbroughtwithita radicallydifferentwayofunderstandingourselvesandourplacebeneath thestars.Thisprojectofcivilizationacceleratedwhensomeofthevillagesgrewintothe first cities. There, we developed sophisticated new technologies forirrigation. We forged metals and stored information in writing. Through thetumultofmarketsandtradeandconflict,ourworkbecamespecialized.Someofusbecamemillers,otherstanners,otherssoldiers,andothersstilladministrators.Someofusevenbecameaspecialkindofpriestwhosejobitwastowatchtheskies.Andallthewhile,ournumberssteadilyincreased.ByonethousandyearsafterthebirthofChrist(1000CE),threehundredmillionhumanbeingswalkedtheEarth.1 Then, just five or six centuries ago, a new approach to the naturalworldwasestablished.Harvestingideasfromacrosstheplanet,wediscoveredamethod for directly probing theworld’s behavior—whatwe now call science.Usingit,ourproject’scapacitiesexploded.Welearnedtocrossoceansquicklyinever-larger ships and in relative safely. Improvements in sanitation andmedicines began to keep us from dying young. Machines for farming beganfreeingus from famine. In response, populationgrowth rates exploded, and inthefirsthalfofthe1800s,ournumberscrossedtheonebillionmark.2

In the years around that milestone, we made perhaps the most importantdiscovery for our project of civilization. Using the fruits of our newlyestablishedscientificsociety,welearnedhowtoharvest fossil fuels.Tappingahundred million years of stored sunlight in the form of coal, and thenpetroleum,3 industrial civilization tidal-wavedacross theglobe.Touchingeventhe most remote corners of every continent and every ocean, our capacitiesseemed to grow without limit. By 2011 CE, just about two centuries afterreachingonebillion,ournumbershadclimbedtosevenbillion.4Today,evenamodest-sizedmodern city housesmore people than lived on the entire planetbefore the dawn of agriculture. Using the tools of science and its daughtertechnologies,weexplored the entireplanet.Wemappedeverywhere.Wewereeverywhere. These days, at any given moment, there are even half a millionpeopleflyingmilesabovetheground.5

Ourprojectwasthriving.For the most part, the planet took little notice of our experiment in

civilization building. The clearing of land for farming certainly altered localbalancesof lifeandresources,but theEarthasawhole—meaning thesurface,air,water,andlife—wasn’tsignificantlyandgloballydisturbedfromthestatewefounditinwhencivilizationbegan.Withtheindustrialrevolutioninthe1800s,therelationshipbetweentheprojectandtheplanetshifted.Earthbeganto“feel”ourpresence.Air,water,ice,rock—alltheinterdependent,stronglylinkedpartsof the planet we inhabit—began to change. And, as it has done many timesbefore in its four-and-a-half-billion-yearhistory, theEarthstartedshiftingfromoneplanetary“state”toanother.

The relatively temperate planet our project of civilization was born intobegan sliding into the past. Something new, something as yet unknown, nowwaits for its own time to begin. The planet is changing, and it’s changingbecause of us. Those changes will, without doubt, stress our project ofcivilization.Ifthechangesareextremeenough,theymayevenmakethekindofcivilization we rely on for survival impossible to continue. Our project maycollapse.

And that is why humans, with our project of civilization, are cosmicteenagers (asCarlSaganoftennoted).Our technology and thevast energies ithasunleashedgiveusenormouspoweroverourselvesandtheworldaroundus.It’slikewe’vebeengiventhekeystotheplanet.Nowwe’rereadytodriveitoffa cliff.Unlike those kids, however,we are still blind to the truth.We are stillunabletoseetherealityourprojectofcivilizationonlyrecentlyrevealed.

Wearenotalone.Wearenotthefirst.Across our history, we have never seen our project of civilization—or

ourselves, for thatmatter—as anything but a one-time story.We have alwaysappearedtoourselvesassomethingessentiallynew,somethingentirelydifferent.Everystepwehumanstookwasastepintotheunknown.Therewasnothingtoguideus,nootherhistorieswecouldlooktoandknowwhatmightbeexpected.

Itistimetoputthatstorytorest,becausewehavegrownpastit.Throughlongeffort,wehavemappedoutthefour-billion-yearhistoryoflife

onEarth, and it showsus thatwearenot the first.Wearenot the first timeaspecieshaschangedaplanet’sclimatethroughitsownsuccess.TheEarthanditsinhabitantshavebeenevolvingtogetherforeons,andwearejustthemostrecentinalonglineofitsexperiments.

Butthereismore.Our science has also shown us something we did not know even twenty

years ago. The universe is awash in planets and they are, in principle, not so

different fromourown.There isevery reason toexpect thatonmanyof theseworlds therewill be oceans and currents.Therewill bemountainswith fiercewindsandvalleys thatbegin thedayshrouded inmorning fogandend itwithfallingrain.

Andtherewillbelife,too.Sure,itispossiblewe’reontheonlyworldtohostlife in all of cosmic history. Science has, of course, been arguing about theexistence of life on other worlds for centuries. But the explosion in ourknowledge about other worlds sheds new light on this question, revealingsomething remarkable. The discovery of all those new planets means we canonlybeunique if the lawsof theuniverse are stronglybiased against life andintelligence.Inotherwords,therearesomanyplanetsintherightplaceforlifeto formthat theburdennowfallson thepessimists. It’sup to thenaysayers todemonstratehow,withsomanyworldsandsomanypossibilitiesoverthewholeofcosmicspaceandtime,wesomehowarethefirstandtheonly.

So,whileitisimportanttorememberthatthequestionofotherlifeonotherworldsremainsopenandundecided,wecannowseethattherehas,mostlikely,beenlifebeforeus.Andonsomeworldsthatlifewill,mostlikely,buildrichandcomplexbiospheres.Goingfurtherstill,overthelonghistoryofthecosmos,lifeon some of those otherworldswill,most likely, havewoken up. It will havelearnedtothink,toreason,andeventobuilditsownprojectsofcivilization.

Onewayoranother,sciencepointstothefactthatwearelikelynotthefirst.Nowit’stimetotakethoseinsightsfromastronomyandearthscienceseriously.In light of our maturing knowledge, it’s time to tell a different story aboutourselvesandourfateamongthestarsandtheirmanyworlds.

SCIENCEANDMYTH

Try to get a teenager to change his or her driving behavior only by quotingstatisticsabout traffic fatalities,andyou’re likely tobemetwithablankstare.That’s because we humans needmore than numbers or the rising curve on agraphtounderstandtheworld.Wearefundamentallystorytellers.Askthekidsin that group of troubled teens about themselves, and they’ll respond with anarrativeaboutfamiliesandfights,theirisolationatschool,orthetimetheyranawayorthedayaparentskippedoutonthem.Weallusestoriestomakesenseofourselvesintheworld.Andwhat’strueofindividualsisalsotrueofculturesandthesweepoftheirhistory.

Formostofhistory,wehaveusedmythtotellourbiggeststories.Whenyouhear thewordmyth, you’re likely to think of a false story.But, taking a long

view of human evolution, myths are often more than just true or false, andthey’vealwaysplayedanessentialroleforus.Everysociety,ineverytimeandplace,hashadasystemofmyths,aconstellationofstoriesthatprovideabasicsenseofmeaningandcontext.Somespeakonlytoourinternallifeaswemakeourtransitionstoadulthood,toparenthood,andtooldage.Butsometellthebigstories. Through these mythic-scale big stories (including their forms inreligion),peoplecametounderstandhowtheirculturethoughttheuniversewasborn,howtheEarthwasformed,andhowpeopleweremade.

Inourage,thatrolefallstoscience.Insteadofgodsandspirits,wenowhavetheBigBangandDarwin’sstoryofthedescentofman.Withscience,wefoundanewwaytoenter intoadialoguewith theworld,onewhereexperimentationand evidence led the way. That’s how the big stories get put together for usmoderns.Butthepowerofthosestoriesasstoriesneverwentaway.

When it comes to the fate of our civilization in a climate-changedworld,however,wedon’thaveabigstory thatcanconvert risingglobal temperaturesandmeltingGreenlandicesheets intoagrandnarrativewithus in it.Theonlythingcloseisastorythatgoessomethingalongthelinesof“wesuck.”Humanbeingsaregreedyandselfish.Wearenothingbutaplagueontheplanet.

That story isnotonlyunhelpful and impoverished, it’s alsoentirelywrongfromtheperspectiveofthenewunderstandingoflifeandplanetswe’verecentlygained.Peopleoftencasttheclimatecrisisintermsof“savingtheplanet.”ButasthebiologistLynnMargulisonceputit,theEarth“isatoughbitch.”6It’snottheEarth that needs saving. Instead, it’s us andourproject of civilization thatneedanewdirection.Ifwefailtomakeitacrossthedifficultterrainweface,theplanetwilljustmoveonwithoutus,generatingnewspeciesinthenovelclimatestates it evolves. The “we suck” narrative makes us villains in a story that,ultimately,hasnone.Whatthatstorydoeshaveareexperiments—theones thatfailedandtheonesthatsucceeded.

This largerperspective,gained in lightof the stars, doesnot absolve thosewhodrive climatedenial for reasonsof greedor political gain.They are fullyculpablefortheirfolly.Fromaplanetaryperspectiveanditslongview,theywillbecomethereasonwhyEarth’sexperimentincivilizationbuildingfailstoreachitshigherpotential.

So there is an entirely new “big story”we can tell. It’s a drama that putshumanitybackintothelifeoftheplanet.It’sanarrativethatputsEarthanditslifebackintothepropercontextofauniverseawashinplanets.

In this new story, we aren’t collectively villains but we may collectivelybecomelosers.

ASTROBIOLOGYANDTHEANTHROPOCENE

Overthelasthalfcentury,ourprojectofcivilizationhaslearnedtolookoutandlookbackasneverbefore.WehavelookedbackbillionsofyearstouncovertheEarth’sdeephistory.We’veseenhowour speciesand itscivilizationcomprisejustanother“expressionoftheplanet,”aswriterKimStanleyRobinsoncallsit.7

Theevolutionoftheplanetanditslifecannotbeseparated.Thatiswhatourscience has shown us. Earth and its life must be thought of as a whole that“coevolves” together. Two and a half billion years ago, for example, it wasmicrobes that reworked theworld by creating the oxygen-rich atmospherewenowbreathe.Intheprocess,thosespecies“polluted”themselvesoffmostoftheplanet’ssurface.8Using this new oxygen-rich air, theEarth thenmoved on tocreatenewversionsof itself, like theonewith thefirst largesea-creatures, theonewith enormous dinosaurs, and the one carpeted by vast grasslands.Thosefish,dinosaurs,andgrasslandswereoncenewactorsontheworld’sstage.Theyappeared and took their places in the long drama of Earth’s undirectedexperimentsincoevolution.TheEarthhasrunmanyexperimentsinlifeanditspossibilitiesoverthelastfourbillionyears.We’rejustthelatestversion,andinthatwaywe’renotsounique.

AndjustasoursciencelookedbacktorevealEarth’shistory,italsolookedoutward, traveling across billions ofmiles to explore the other worlds of oursolar system. These audacious journeys showed us that “climate” is notsomethinglimitedtopatternsinyourlocalweatherreport.OnVenus,220-mile-per-hourwindsblowhighintheatmosphere.9OnMars,icyfogformseachnightnearitsnorthernpole.10There’sevenrain(madeofgasoline)driftingoverforty-mile-wide lakes on Titan, the giant moon of Saturn.11 In terms of having aclimate,ourplanetisalsonotthatunusual.

Finally,wehavealsolookedouttothestarsanddiscoveredthattheuniverseisfecundwithsolarsystemslikeourown.Thenumbersfromthesemonumentalstudies tell us that projects of civilization like oursmay likely have occurredelsewhere at other points in cosmic history. As long as the universe isn’texceptionallybiasedagainstit,wearenotthefirst.Others,onotherworlds,willlikelyhavecomebeforeus.Andwithournewknowledgeofplanetsand theirlaws,wecanalsoseethat theytoowill likelyhavefacedasimilardilemmatothe one staring us down today. Even our climate crisis may likely not be souniqueandnoteventhatunusual.

Sooursciencehasdoneitsjob.Whenitcomestotherelationshipbetweenlifeandplanets,ithasshownusentirelynewrealitiesandnewpossibilities.This

scienceisnew,it’srevolutionary,andit’scalledastrobiology.Throughthetrulyheroic efforts of scientists across theworld, astrobiology has opened up new,universal truths for us about the braided potential for planets and life. It hasshownuswhathashappenedhere,andwhatmighthappenelsewhere.

Thatknowledgecomesatastrangelyauspiciousmoment,bestowingitwithgreatconsequence.

Ten thousand years ago, our project of civilization was born after thebeginningofwhatgeologistscalltheHolocene,aplanetaryepochofwarm,wetconditions following the end of the ice ages. But in driving climate change,we’renowpushingtheEarthoutoftheHoloceneintoanewerainwhichhumanimpacts dominate the planet’s long-term behavior. The new era is called theAnthropocene.12

We all want our project of civilization to continue deep into theAnthropocene. But our efforts so far havemostly failed.We’ve known aboutglobalwarming,themostobvioussymptomoftheemergingAnthropocene,formore than fifty years.13 Despite having that knowledge, we’ve done almostnothing to deal with climate change and its consequences. Our politics, oureconomics,andevenourmoralphilosophyhaveall failed todriveactions thatcouldensurethelong-termsustainabilityofourprojectonachangingplanet.

That failure is rooted in themistaken view thatwe, and our project, are aone-time story. But we can be forgiven for that failure because, until veryrecently,wedidn’thave the toolsor the information to rise above such tunnelvision.Wedidnotyethavetheastrobiologicalperspective.Butnowwedo,anditcanchangethepathtoourfuture.

This book explores what might be called the astrobiology of theAnthropocene,andit’sbuiltoutoftwobraidedquestions.

• What can the revolutions of astrobiology tell us about life on otherworlds,evenotherintelligencesandtheircivilizations?

• What can life on other worlds, even other intelligences and theircivilizations,tellusaboutourownfate?

Theseinterwovenquestionswillleadustoafundamentallynewstoryaboutwhatweareandwhat’shappeningtousatthiscrucialmomentinourcivilizationbuilding. It’s a narrative built from space telescopes, deep-sea submersibles,robotsdivingintocometsandgeologistsscramblingoverdeadlyglacialchasms.Intellingthatstorywe’llencountersciencethatisnothinglessthanthrilling.

TheAstrobiologyof theAnthropocenewill showus imagesof steep cliffsunder coral skies on Mars that help to vastly enlarge our understanding of

climateandclimatechange.Itwilltakeustodarkcrazy-quiltecosystemsdeepintheoceanthatgiveusatime-machineviewofEarthbillionsofyearsagowhenlifewasstillnew.

Andthenthereareallthenewplanets.TheastrobiologyoftheAnthropocenewillalsotakeusacrossthegalaxyto

seeentirelyunexpectedclassesofplanetshavejustbeenaddedtothetextbooks:“hot”worldssnuggledtightlyagainsttheirparentstarsand“super-Earths”manytimesthesizeofourown.14

In telling this new story, we will also encounter the most thrilling of allpossibilities:aliens.

Our exploration of the astrobiology of theAnthropocenewill lead us to aradicalclaim.It’stimetotaketheexistenceofaliens—bywhichwereallymeanexo-civilizations—seriously. Everything that has been learned in theastrobiologicalrevolutionsofthelastfewdecadesnowallowsustoseejusthowimprobable it is for us to be theonlyproject of civilization in cosmichistory.That realization tells us that if we ask the right kinds of questions, the onesbacked by the hard numbers of the new exoplanet discoveries, we can beginmakingout thecontoursofa scienceofexo-civilizations that’s relevant toourowncrisisonEarth.

The new science this book exploreswon’t tell us if the galaxy is teemingwithothercivilizationsnow.Itwon’ttellusifwe’regoingtocatchevidenceoftheirexistencesoon. Itwon’t tellus if theyhavepointyearsorseven-fingeredhandsorareshapedlikelizards.Whatitcando,however,isshowushowthis—meaningeverythingyouseearoundyouinourprojectofcivilization—hasquitelikelyhappenedthousands,millions,oreventrillionsoftimesbefore.

From thevantagepointof theastrobiologicaldata,wecan take thoseexo-civilizations seriously as a subject of scientific inquiry. It’s hard to avoid thegigglefactorwhenwetalkaboutaliens.YearsofbadTVsciencefiction(aswellasthecrazyworldofUFOconspiracytheorists),haveleftabadtasteformostscientistswantingtothinkaboutintelligentlifeonotherworlds.Worsestill,foryears there wasn’t much in the way of scientific constraints on the question.Withoutsuchconstraints, thediscussionfallsdangerouslyclosetopurefiction.Butifweasktherightkindofquestions,thelawsofplanetarybehaviorwehavenowgraspedcanactasguardrailsforhowwethinkaboutexo-civilizations.Thatmeansifweasktherightquestions,wecangetanswers.Attheveryleast,withtherightquestions,thoselawsofplanetswe’veuncoveredwillhelpusputlimitsonwhatanswerscanlooklike.

Remarkably, theonedomainwhere suchquestions exist lies exactly at theintersection of astrobiology and the Anthropocene. Our new understanding of

planetary laws iswell tuned toaddress thequestionwemostcareabout:Howwillacivilization(meaninganycivilization)coevolvewithitsplanet(meaningany planet)? If other civilizations have likely existed across cosmic time andspace,wecan take themseriouslybymaking them the subjectofour science.We can bring to bear all we’ve learned from Earth, Venus, Mars, and thethousands of planets discovered outside our solar system.We can deploy thelawsofphysicsandchemistryinherentinthatknowledgetobegindoingscienceintheformofdetailedmodelsandsimulations.

From this perspective, civilizations become just another thing the universedoes,likesolarflaresorcometsorblackholes.Wecanusewhatthestarshavelaidoutbeforeusinourastrobiologicalstudiestoexplorehowanycivilizationonanyplanetcan—or,intheworstcase,cannot—evolvetogether.Wecantreatthosepossibleexo-civilizationsasotherhistoriesthatcantellusaboutourownfuture.

Theadvantagesofthisastrobiologicalperspectivecanbegainedevenifnoother civilizationever existed.Thinkingabouthypothetical exo-civilizations isvaluableindealingwiththechallengeoftheAnthropocenebecauseitteachesusto “think like a planet.” It teaches us to frame our pathways to a long-termprojectofcivilizationintermsofthecoevolutionbetweenlife(includingus)andtheEarth.Throughtheastrobiologicalperspective,wecanmapoutthecontoursofourownfateandourownfuture.

ANEWSTORY

If,however,wetakethepossibilityofothercivilizationsseriously,thenwefinda new door open to us in facing theAnthropocene.Across a cosmoswith somany planets experimenting with life, we can see that some technologicalspeciesmaylearnhowtomakeit.Theywilllearnhowtonavigatethedifficultbottleneck of the climate feedback they generate. Others, however, will fail.That’showthisnewbigstorybecomesmeaningful. Itbeginswith thescience,but ends with showing us how to face the hard choices our version of theAnthropocene forces on us. A long-term sustainable version of our project ofcivilizationwillmeanwemustbecomepartners, insomeas-yet-unknownway,withtheplanet.

Wemustthereforebecomeapower—inourownright—alongsidetheEarth.But as Spider-Man so famously discovered, with great power comes greatresponsibility.Doesbecomingawinner in thegameofcosmicevolutionmeanweholdtheEarthinaperpetualversionoftheHolocene?Willweneverallow

another ice age to form? If that’s true, thenwhat about the species thatmighthaveemergedintheiceagesweblock?DowehavetherighttokeepthemfromeverenteringtheEarth’sdrama?

Andwhich species from the current version of theHolocene dowe carrywithusintotheAnthropocene?Imagesofpolarbearsadriftonlonelyicefloestugatourhearts.Butenteringintoatrue,long-termpartnershipwiththeplanetwill demand hard choices. Those decisions will not just be the domains ofscience.Theywillalsodependonwhatwevalue,whatweholddear,andwhatwe believe to be sacred. These are all the domains of meaning. That is whygetting the story right—the story with us in it—is just as important now asgettingthescienceright.

The astrobiological perspective on the Anthropocene is science at thegrandestofscales.It’sanarrativeofourowncollectivelifeandfatesetagainstthestars,whoseownstoriessuddenlymatterasbothourguidesandourteachers.Itismorethandata,morethaninformation,morethanknowledge.We,andourcherishedprojectofcivilization,arecrossingoverafrontierasourplanetenterstheAnthropocene.Thenewsciencewe’ll explore inwhat followscanhelpusmapthisnewterritory.Itcanalsohelpustonavigateitsburningedgesandmakeitthroughtotheotherside.

CHAPTER1

THEALIENEQUATION

THEFERMIPARADOX

Onewarmandbrightsummerdayin1950,fourcolleagueswalkedthroughtheatomic weapons complex at the Los Alamos National Laboratory in the highdesert of northernNewMexico. The ColdWarwith the Russianswas in fullswing,andtherewerenewfaceseverywhere.Eachofthemen,however,wasanold hand at the lab, and each played a key role in developing the bombs thathelpedwinWorldWarII.

FirstamongthemwasEnricoFermi, theItalian-bornNobel laureatewhosebrilliancehadpiercedthemysteryoftheatomicnucleus.Fermiwasfamousforhis almost superhuman scientific abilities.C.P.Snowoncewrote that if Fermihadbeenbornjustabitearlier,hemighthaveinventedallofatomicsciencebyhimself.“If[that]soundslikehyperbole,”Snowwrote,“anythingaboutFermiislikelytosoundlikehyperbole.”1

Walking alongside Fermi was Edward Teller, the brooding Hungarianphysicistwhoseworkwouldbecomesynonymouswith the terrifyinghydrogenbomb.WhileFermiwasnotinfavorofTeller’spushforthe“super”bomb,thetwomenremainedfriendsthroughout their lives.2Roundingout thegroupthatdaywereAmerican nuclear scientists Emil JanKonopinski andHerbertYork,bothhighlyregardedresearchersintheirownright.

ThefourscientistsmadetheirwayfromthelabbuildingstotheFullerLodgewherelunchwasserved(itwasoneofthefewstructuresleftoverfromthesite’searlierincarnationasaboys’camp).Astheywalked,theconversationturnedtounidentified flyingobjects.3 Since the end of thewar, sightings ofmysterious

lightsintheskyhadbeenincreasing.Arecentincidenthadjustmadethelocalpapers, reminding York of a whimsical New Yorker cartoon in which flyingsaucerswereblamedforarashofManhattangarbagecandisappearances.Giventhephysicists’inclinationforanalysis,theUFOstoryledtoatussleofquestionsaboutfaster-than-lighttravelanditslimitations.Soon,however,theconversationwanderedofftoothertopicsasthefourscientistscontinuedalongthepathlinedbypine treesand juniper. Itwasonly later, in themiddleof lunch, thatEnricoFermiblurtedout,“Butwherearethey?”4

AlanDunn’s cartoon ofUFOs abductingNewYorkCity garbage cans,which appeared inTheNewYorkerin1950.

Teller, York, and Konopinski all broke into laughter at Fermi’s outburst.They recognized theircolleague’s sharp insight.Fermihadahabitof reducingcomplexproblems to thebarest essentials.Present atTrinity, thedesert test ofthefirstatomicbomb,Fermihadfamouslycalculatedtheexplosion’spowerbysimplydroppingscrapsofpaperandnotinghowfarsidewisetheyweresweptbywindsfromtheblast.5

But on that summer day over lunch, Fermi had identified a core question

destined to haunt all subsequent discussions of intelligent life in the cosmos.Fermi’sobservationwasasstraightforwardasitwaspenetrating.Iftheevolutionof extraterrestrial intelligent species was common, why didn’t we see them?Why hadn’t our telescopes found indications of their existence? Why hadn’taliensalreadylandedontheWhiteHouselawn?

Fermi’squestionwasnotaimedatUFOs.That topicwas,andcontinues tobe, a morass of weak reasoning, poor observations, fakery, and conspiracytheories.Instead,hisquestionwouldcometorepresentoneofthefirstdistinctlymodern and scientifically manageable questions about extraterrestrialtechnologicalcivilizations(we’llusethetermexo-civilizations).6

Overtime,Fermi’squestionwouldcometobeknownasFermi’sParadox.Itsformal statement might go as follows: If technologically advanced exo-civilizations are common, then we should already have evidence of theirexistenceeitherthroughdirectorindirectmeans.

In the decades to come, other scientists would give Fermi’s question theprecision it needed to have a scientific bite. In 1975, astrophysicist MichaelHart’s paper “An Explanation for the Absence of Extraterrestrials on Earth”addressed a number of objections to the reasoning behind Fermi’s paradox,includingonesassociatedwithphysics,biology,andsociology.7Hisconclusionwasthatnoneoftheobjectionswasstrongenoughtoputofftheparadox’slogic.Hart laid bare the essence of Fermi’s insight by demonstrating that just onespecies could “quickly” colonize the galaxy. Assuming an exo-civilizationappearedthatbuiltshipscapableoftravelingat10percentofthespeedoflight,Hartshowedthatwithinjust650,000yearsthesecreatureswouldcrossthewidthof the galaxy. In thisway, a single species could send ships in all directions,radiating outward from their home world, and quickly colonize every starsystem.

Of course, a fewmillion years seems like a long time tomost of us.Ourspecies,Homo sapiens, has been around on the Earth for less than a millionyears.Butwhat’slongforusisshortforthelifeofthegalaxy.TheMilkyWay,ourhomegalaxy,isavastandancientmetropolisofstars.Itwasbornsometenbillionyearsago.Soitwouldtakeaboutoneten-thousandthoftheMilkyWay’sageforHart’sspacefaringcivilizationtocrossthegalaxy.Harthaddemonstratedthat, in a time scale that is small compared to the galaxy’s existence, just onerandomly appearing interstellar civilization could reach all theplanets orbitingallthestarsinthesky—includingourown.

For some researchers, Hart’s work filled the night with a disquietingemptiness.Intheireyes,therewasastraightforwardlogictotheFermiParadox

thatsaidwemustbealone.Theobviousabsenceofanaliencivilizationinoursolar system, alongwith the lackof evidence for those civilizations’ existenceamong the stars, mustmean no other form of life anywhere had reached ourlevelofintelligenceandtechnology.WewerethesolespeciesintheMilkyWaythathadmadeituptheladderofevolutiontobuildanadvancedcivilization.Inresponse to theFermiParadox,physicistandscience fictionwriterDavidBrinspoke of the stars’ “Great Silence.” It was an apt term, capturing the cosmiclonelinessthatFermi’sParadoxseemedtoimply.8

AlongwiththeGreatSilence,anincreasinginterest inFermi’sParadoxledto the idea of a “Great Filter.”9 The absence of evidence for advancedcivilizations in the galaxy does not imply that Earth is the only life-bearingplanet. The Fermi Paradox only speaks to the existence of technologicalcivilizations like ours, or ones even more advanced.Microbes or shellfish orevendinosaursmightexistoneveryworldinthecosmos.Soifwedon’tseeexo-civilizations, some scientists argued, there must be a filter keeping evolutionfromspawningthem.Inotherwords,ifwearealoneinthecosmos,thensomekindofevolutionarywallblocksotherplanetsfromreachingourlevel.

ButaGreatFiltermightlieanywherealongthatevolutionarypath.PerhapssimplelifeissodifficulttoformthatitconstitutestheGreatFilter.Inthatcase,Earth would be one of the few worlds with life. On the other hand, theemergence of even simple formsof intelligencemight be theGreatFilter. So,whilelizardsmightappearonmanyworlds,dolphinsandapeswouldnot.Ifthatwere true, the difficulty in evolving intelligence filters out even those worldswherelifehasformedfrommovingfurthertowardatechnologicalcivilization.

Ironically,attheexacthistoricalmomentthatFermiandhiscolleaguesweresittingdowntolunch,anewkindofevolutionarydeadendfor theGreatFiltermade its appearance. Fermi posed his question at a laboratory dedicated todevelopingweaponsofunprecedenteddestructiveenergies.Itwasinthe1950sthat humanity first gained the power to bring civilization to an abrupt anddecisiveendthroughfull-scalenuclearwar.

AtomicArmageddonmadeitpossibletoimaginethattheGreatFilterlaynotin thedistantevolutionarypast (inwhichcasewehadbeen lucky toavoid it);instead,itmightwaitlikeaviper,hidinginthetallweedsofourfuture.Maybethe night sky was silent—and our planet unvisited—because no advancedcivilizationwassmartenoughtohandlethepressureofitsownexistence.

IfsomeonecouldhaveaskedFermiforhistopchoiceforaGreatFilter,hewould likely have answered nuclear war. These days, however, we have abroader understanding of civilizations and their existential challenges. In the

1950s,when Fermi posed his question, therewas only a small community ofEarth scientists awakening to the possibility of human-driven climate change.The idea that humans could unintentionally change the behavior of the entireplanet through nothingmore than our collective daily activitywas an idea soradical, it had barely been formulated in a scientific way. Now, however, weknowbetter.

Earth’s passage into its human-dominated era, increasingly known as theAnthropocene,showsusapotentiallymorepotentcandidatefortheGreatFilter.Civilizationslikeourownareacomplexwebofinterdependentsystems.Wherewouldyougetyourfoodiftheelectricitywentoutforayear?Howwouldyouheatyourhomeifthepipelinesdeliveringpetrochemicalsshutdown?Therearea million ways we all rely on the smooth operation of these systems. But asignificantshiftinEarth’sclimatestatewouldupendthosesystemsinwaysthatwoulddeeplychallengetheiroperation.

ThinkabouttheGulfStreamforamoment.Itcycleswarmwater(andwarmweather)upfromFloridatoBoston,andthenoutacrosstheAtlantic.Hundredsofmillions of people in some of Earth’smost technologically advanced citiesrelyon themildclimatedeliveredby theGulfStream.But theGulfStream isnothing more than a particular circulation pattern formed during a particularclimate state the Earth settled into after the last ice age ended. It is not apermanentfixtureoftheplanet.Iftheclimatechangesenough,theGulfStream,andthemildweatheritdelivers,couldbecomeathingofthepast.10

SowhatwecalltheAnthropocenemaybeafarmorepotentcandidatefortheGreatFilter thannuclearwar.Anall-outnuclear exchangewould, after all, beintentional. It would be someone’s decision. But it’s easy to imagine othercivilizationslessaggressiveandwarlikethanours.Theymightnoteventhinktobuildnuclearweapons.Climatechange,however,islikelytobeuniversal.Aswewillsee,itislikelytobeaconsequenceofanyprojectofadvancedcivilizationbuildingonanyplanet.Long-termdramatic climatechangeneednot lead to acivilization-building species’ extinction. It only needs to make conditionsdifficult enough that their projectof technological civilization is disrupted andunabletorecoveronthenow-climate-changedplanet.11

All these issues surrounding the Great Filter really illustrate the power ofFermi’sinsight.Makingprogressinscienceoftenhingesonaskingtherightkindofquestion.Withoutawell-posedquestion,discussionsbecomelittlemorethanpeople talking(oryelling)pasteachother.Andwithoutawell-posedquestion,there’snoclearpathtowardgatheringdatathatwillyieldanswers.

Findingagoodquestionislikethrowingopentheshadesinadarkroom.It’s

thefirststepinfindinganewwaytotellastoryabouttheworldbecauseitletsus see the world in a new way. A good question reframes what we think isimportant. It tells uswherewe should be looking,wherewe shouldbe going,andhowtobeginorganizingoureffortstogetthere.

Fermi’s1950questionhelpedplaythatrolefortheissueofexo-civilizations.As developed byHart and others, Fermi’s Paradox asks us to consider if andwhyhumanitymightbealoneintheuniverse.

ButtotrulyunderstandtheimportanceofFermi’squestionforourfuture,weneedtotravelbackafewthousandyearsintoourpast.

THEPLURALITYOFWORLDS

TheGreekphilosopherEpicurusmadethefirstexpressionofwhatwemightcall“exo-civilizationoptimism”almost2,200yearsago:“Thereare infiniteworldsbothlikeandunlikeourown....Furthermorewemustbelievethatinallworldstherearelivingcreaturesandplantsandotherthingsweseeinthisworld.”12

Epicurus’s interests ranged from ethics to the nature of suffering, but firstandforemosthewasanatomist.Theworldforhimwascomposedofaninfinityof tiny components, arrayed in infinite combinations. That belief served as afoundation for the atomists’ belief that the universemust also be infinite, andthusmustcontainaninfinitenumberofotherinhabitedplanets.

NotallGreekphilosophers,however,shared theatomists’ faith ina fecundcosmos.“Therecannotbeseveralworlds,”wroteAristotle innearly thesameera.13 Aristotle was an exo-civilization pessimist. For him, the Earth was thecenteroftheentireuniverse.Sincetherecanonlybeonecenter,theEarthmustbe unique. Aristotle was certain that no other worlds, and certainly no otherworldslikeEarth,existed.

Theconflictbetweentheseconvictions—offecundityoftheuniverseononehandandtheuniquenessof theEarthontheother—wouldechodownthenexttwenty centuries. From the Greeks, through the Middle Ages, to theRenaissance,andonintotheearlytwentiethcentury,optimismconcerningotherinhabitedplanetswaxedandwaned.

From one century to the next, philosophers, physicists, theologians, andastronomers asked the same questions:Arewe alone?Arewe the first? Eachgenerationposedthequestionusingtheprejudices,ideas,andtoolsoftheirtime.The argumentswere always fierce; sometimes they even turneddeadly. In themedievalperiod, theCatholicChurchconsidereddiscussionofotherworlds to

be heresy. That did not stop philosophers and theologians from struggling tounderstandwhyaninfinitelypowerfulGodwouldcreateonlyasingleinhabitedworld. In the thirteenth century, Thomas Aquinas answered this dilemma byclaimingGodcouldhavecreatedotherinhabitedplanets,buthadchosennotto(adistinctlyunsatisfyingsolution).14

Bythesixteenthcentury,anewgenerationofthinkerswaspushingbackonthequestionofotherworlds.CopernicusfamouslydethronedtheEarthfromthecenteroftheuniverseinOntheRevolutionofHeavenlySpheres,firstpublishedin1543.Inhisversionofastronomy,radicalforitstime,ourworldwasjustonemoreplanetorbitingtheSun.15Copernicusneverexpressedopinionsaboutotherplanets orbiting other stars. But his work removed Earth from its privilegedcosmicpositionandopenedthedoorforotherstopubliclyexplorewhatbecameknownas“thepluralityofworlds”question.

Foratime,theChurchtoleratedsomediscussionofCopernicanastronomy.But in the late1500s theradicalDominicanmonkGiordanoBrunopushed thelimits of that tolerance until it broke. Bruno not only publicly advocated forCopernicanastronomy,hewent further, arguing that theuniversemustcontaininfinite worlds with infinite varieties of inhabitants. These views helped earnhimtheattentionoftheInquisition,andin1600theChurchburntBrunoatthestakeforheresy.16

By the time the scientific revolutionwas in full swing, IsaacNewton hadrevealed powerful, unifying laws governing the motion of celestial andterrestrialobjects.Astronomywasmakingswiftprogress,asnewplanetssuchasUranusandNeptunewerediscoveredandtheorbitsofcometswereunderstood.The intellectual tumult shifted debate about life on other worlds for bothscientists and an increasingly literate public. The influential French writerBernarddeFontenelle,forexample,scoredtheequivalentofanEnlightenment-erabestsellerwithhis1686bookConversationsonthePluralityofWorlds.

The book was framed as a series of late-night discussions between aphilosopherandaquick-mindedyoungbaroness.Expressingtheoptimismofhisage, de Fontenelle imagined thatmany of the planets orbiting the Sun hostedpeoples. He even thought the Moon had intelligent inhabitants. Turning hissights beyond our solar system, de Fontenelle wrote, “The fixed stars are somanySuns,everyoneofwhichgivesLighttoaWorld.”Andonmanyoftheseworlds,deFontenellewascertainthatlifethrived.17Oneinfluentialimagefromthe book gives a graphic representation of de Fontenelle’s optimism. Thefrontispieceofanearlyeditionshowsoursolarsystemnestledsnugglyamidstacosmosdensewithotherstarsandotherworlds.

Itwasanoptimismthatprevailedwellintothenineteenthcentury.Darwin’stheory of evolution brought a new twist to the discussion of life and planets.Writers like Camille Flammarion, the French Carl Sagan of his day, thrilledaudienceswithvisionsoflifeevolvinginentirelynovelformsonafertileMarsandVenus.18Adding evolution theory to debates about theplurality ofworldsgave writers like Flammarion the chance to imagine how nature shaped theinhabitantsofotherplanets.Sinceevolutionrespondedtothespecificconditionson a given planet, the transformations a species undergoes must fit thoseconditions. In this way, Flammarion argued that life on Mars must be verysimilar to life on Earth, since both planets (he thought) presented similarenvironments.19

Illustration of our solar system surrounded by other stars and their planets from Bernard deFontenelle’s1686book,ConversationsonthePluralityofWorlds.

Marswouldlaterbecomethefocusofaverypublicversionofoptimism.Attheturnofthetwentiethcentury,AmericanmillionairePercivalLowellfoundedan observatory in Flagstaff, Arizona (which had yet to achieve statehood andwasstillaterritory),tostudyso-called“canals”ontheRedPlanet.20Lowellwasconvinced thatMarswas inhabited. Through books and lectures, he dedicated

his final years to convincing others. His efforts were successful enough thatmanyinthegeneralpublictookitasagiventhatMarswasalivingworld.

During the latter half of the nineteenth century, however, a pessimisticpushbackonexo-civilizationsemerged,bothfromoutsideandwithinscience.In1853,WilliamWhewell,anEnglishscientist,philosopher,andAnglicanpriest,wroteascathingcritiqueoftheoptimists’positioninhisbookOfthePluralityofWorlds.Turningfrommerehopesexpressedbyotherwriterstotheastronomicalfactsofhisday,Whewellwrote, “Noplanet,noranythingwhichcan fairlyberegarded as indicating the existence of a planet revolving about a star, hasanywhere been discovered.”21 Whewell also argued strongly against usingEarth’shistoryasaguidefor life’sprogressonotherworlds. “Theassumptionthat there is anythingof thenatureof a regular lawor order of progress from[interstellar]materialtoconsciouslife...isinthehighestdegreeprecariousandunsupported.”22

AnotherdissentingvoicecamefromAlfredRusselWallace,who,alongwithDarwin,isconsideredoneofthefoundersofevolutiontheory.Inhis1904bookMan’sPlaceintheUniverse,Wallaceappliedhisowndetailedunderstandingofbiology to thequestionof lifeonotherworlds.Using theavailabilityof liquidwaterasaguide,WallaceconcludedEarthwastheonlyhabitablesolarsystemworld.Going further, he claimed fewplanets in thegalaxywouldbe earthlikeenoughtoallowforintelligence.23

By the early twentieth century, a more determined pessimism about theexistenceof planets aroundother stars (nowcalled “exoplanets”) tookhold. Itwasaviewthatproveddamningforscientificviewsofexo-civilizationsaswell.Thisnewpessimismfocusedontheprevalenceofplanetsandrestedonamodelforplanetformationcalledcollision theory.Theoreticalstudiesbyastronomersintheearly1900sarguedthatplanetscouldonlyformwhentwostarspassedinaclose encounter. As the suns shot past each other in a near collision, gravitywouldpullsomeoftheirgasintospace,leavingittofallintoorbitaroundoneofthe stars. Eventually, the extruded gaswould cool and coalesce into a planet.James Jeans, the leading astronomer of his day, soon demonstrated that thesekindsofstellarnearmisseswouldbeexceedinglyrare.BecauseofJeans’swork,bythemiddleofthetwentiethcentury,manyastronomersbelievedplanetswerefewandfarbetweenintheuniverse.24Thatmeantlifewouldalsoberare.

So, by the time Fermi and his companions sat down to lunch that day in1950,thebuoyantoptimismofdeFontenelleandFlammarionhadbeenstalled.Many scientists thought planets were rare. Even if they weren’t, biologicalarguments like thoseofAlfredWallace couldbemarshaled tomake life seem

likeanimprobableevent.Evenworseforthosewhowantedtotakelifeonotherworldsseriously,Lowell’sobservationsofMartiancanalshadbecomeajokeinthe scientific community.25 In the early 1950s, the possibility of life andintelligence in the universe remained a question that few scientists wereseriouslyconsidering.

Butsciencedoesnotexistinavacuum.Itisahumanendeavor,anditsstoryevolveswith thestories laidoutby the restofaculture,evenas it shapes thatculture. The narrativeswe could tell ourselves about life in spacewere set toshiftfortheworstofreasons.

ROCKETS,BOMBS,ANDSATELLITES

WhenFermiblurtedouthis famousquestion in1950, theUSwas still reelingfromnewsthatRussiahaddetonated itsownatomicweapon.At that time, thetotal US inventory of atomic bombs numbered in the hundreds. By 1960,however, the global weapons stockpile had grown to more than twenty-twothousand.26Moreimportantly,theearlybombshadbeenbasedonnuclearfission—the splitting of the nucleus of a heavy atom like uranium. The carnage atHiroshima andNagasaki haddemonstrated that these “atomic”weapons couldwipeoutalargeportionofacityinaninstant.By1960,boththeUSandRussiahad deployed weapons based on thermonuclear fusion. These bombs werepowered by slamming atoms of hydrogen, the simplest element, together tocreatesomethingheavier,followingthesamebasicprocessthatpowersstarsliketheSun.Thenewhydrogenweaponswere terrifyinglypowerful.27Amedium-sized H-bomb could destroy an entire metropolitan area. The largest H-bombcouldblowasmallportionoftheEarth’satmosphereintospace.

The race towardevermorepowerfulnuclearweaponsdefinedmuchof the1950s.Butthebombstriggeredanotherraceduringthatdecade,andthissecondtechnologicalsprintwouldhaveanevengreaterimpactonreimaginingthefateoffar-flungexo-civilizations.

Buildingmore powerful bombsmeant little to nuclear weaponeers if theycouldn’tbedelivered to their targetsmorequickly than thoseof theenemy. Inthisway,thelogicoftheColdWarmovedinexorablyfromthetechnologiesofjetbomberstothoseofrocket-poweredmissiles.

In thefinalyearsofWorldWarII,NaziV-2guidedmissileshad terrorizedBritain and proven the power of long-range rockets. After the war, both theRussians and the US snapped up captured German V-2 scientists, and each

nationvigorouslypursuedthedevelopmentofcontinent-crossingrocketscalledintercontinentalballisticmissiles(ICBMs).TheRussiansprovedfasterandmorenimbleintheirdevelopment.OnAugust21,1957,aSovietR-7missileblastedacross3,700miles,reachinganaltitudeoftenmiles.28

Thetruepoweroftheserocketsbecameapparenttwomonthslater,whentheworld woke up to find we’d acquired a second moon. On October 4, 1957,anotherRussianR-7 rocket punched the 184-poundSputnik above the Earth’satmosphere and into orbit,where it became theEarth’s first artificial satellite.Wheelingafewhundredmilesoverhead,Sputnikbroadcastperfectlytimedradio“beeps” for anyone with the right equipment to hear.29 And the world waslistening. While Russian politicians gloated and Americans panicked, it wasclear that an ancient threshold had been breached. Humanity’s space age hadbegun.

There was, however, only one way to talk to a hypersonic rocket in theatmosphere or a satellite orbiting high above the planet. Communications atthese ranges required the use of sophisticated radio technology. And it wasexactly in those technologies that the political and military urgencies of the1950sdovetailedwiththefirstscientificefforttodetectalienintelligence.

Until the1950s, astronomywas carriedoutwith telescopes fashionedwithglasslensesandmirrors.Thatmeantastronomywasdoneonlywithvisiblelight—thekindoureyeshadevolved toperceive.Butvisible light isnothingmorethanwavesofelectromagneticenergywithwavelengthsthatfallwithinacertainrange.(Wavelengthisthedistancebetweenthepeaksinawave.)

In the mid-1800s, physicists discovered there was an entire spectrum ofelectromagnetic waves. These waves stretch from very short, atomic-scale X-rays and gamma rays all the way to radio waves the size of buildings.Astronomical objects tend to emit energy across a large fraction of thiselectromagneticspectrum.

Evolution tuned our eyes to see electromagneticwaves only in the visible“band”ofthespectrum.It’snocoincidencethatthisvisiblebandhappenstobewheretheatmosphereismosttransparenttosunlight.ButtheSunalsoproducesX-ray“light,”ultraviolet“light,”andradio“light.”

Buoyed by the advances in radio engineering during World War II,astronomers in the1950sbeganopeningtheirfirstnew“window”onthenightsky by using light outside the spectrum’s visible band. With radio waves,researchers found theycouldmapout the entire galaxyor capture the echooflong-deadstarsinwaysthatwereimpossibleusingvisiblelight.

Radio astronomy, as it was called, constituted one of the most exciting

frontiers in science as the 1950s progressed. If you were young, gifted, andscientificallyambitious,radioastronomywastheplacetobe.Thatwashow,atthe end of the decade, a newlyminted astronomer named FrankDrake foundhimselfinthewildsofWestVirginia,searchingforsignalsofaliencivilizations.

LISTENINGTOTHESKY

FrankDrakehadalwaysbeenagearheadwithvision.Themanwhowouldhelpdefinemuchofthemodernsciencearoundexo-civilizationswasbornin1930onthe south side of Chicago, just as the Great Depression began. His father, achemicalengineerforthecity,oftenbroughthomegadgetsforhissonthatendedupintheboy’sbasement“lab.”TheyoungDrakespenthoursinthatbasement,playingwithmotors,radios,andchemistrysets.Butitwasthefrequentbiketripsto the city’s Museum of Science and Industry that took Drake’s imaginationbeyondthedetailsofhisradios.There,heandafriendfoundfull-scalemodelsofatomsthatmadetheinvisiblereal.“Someoftheexhibitsweresodramatic,itwouldalmostknockyoutothefloor,”Drakelaterwrote.30

WhenDrakewas just eight years old, his father told him therewere otherworlds “just like Earth.” The idea gave him a vision of other life and otherplanetsthatneverfaded.TheOzstorieswerealsoafavoriteofyoungDrake.Asa child, heownedmanyof thesebooks about anotherworld.AuthorL.FrankBaumhadwrittenthirteenvolumesbeyondthefirstone,TheWonderfulWizardofOz,manyofthemfeaturingPrincessOzma,therulerofOz.31

Theboygrewintoatall,handsomeyoungmanwithanaffinityforsciencethat landedhimatCornellUniversitywith aReserveOfficers’TrainingCorpsscholarship.WhileDrake didn’t begin his undergraduateworkwith a specificinterest in astronomy, he soon found himself drawn to the subject. Andthroughout his introductory astrophysics courses, he never lost his fascinationwiththequestionhisfatherintroducedtohimasaboy:Arethereotherinhabitedworldsintheuniverse?Butitwasnotaquestionhewaswillingtoposetohisprofessors,forfearofsoundinglikeafool.Thatreticencewouldfadethroughachance encounter with Otto Struve, one of the world’s most famousastrophysicists.

Struvewasalarge,intimidatingmanwhowasaleaderinthestudyofstars.In1951,hewasinvitedtopresentalecturetotheCornellcommunity,andDrakewas in attendance. The lecture focused on what was known about how starsformed from clouds of interstellar gas. As he neared the end of his talk, the

imposingRussian-Americanpivotedtothetopicoflifeinitscosmiccontext.Heclaimedtherewasmountingevidencethatatleasthalfofthestarsinthegalaxyhad their own planetary systems. The old collision theory of planet formationwas falling from favor, andStruve said therewasno reasonwhy life couldn’texistonsomeof thoseplanets.32A lightwenton inFrankDrake’shead.Herewas someone older and established, asking the same question he’d beenfascinatedbysincehewasaboy.

Struve’s inspiration was still alive in Drake in the spring of 1958 as hepiloted an old white Ford, stuffed with all his belongings, through thebackwoods of rural West Virginia. He was on his way to the newly mintedNationalRadioAstronomicalObservatory’sGreenBankfacility.There,hewastobecomeamemberoftheobservatory’sfledglingscientificstaff.

TheresearchenginesoftheColdWarwerechurning,andfundinghadbeenopenedforanyprojectthatcouldpushAmericancapabilitiesforward.InDrake’swords,GreenBank“hadbeengivenwhatamountedtounlimitedfundstobuildthebest radioobservatory in theworld.”33Nestled in a remote,verdantvalleyvalued for its radio (and actual) isolation, Green Band was the new home ofAmericanradioastronomy.

Soon after Drake’s arrival, the towering metalwork of an eighty-five-footradio dishwas completed.The astronomers atGreenBank planned to use thenewly commissioned telescope to study everything from the structure of ourgalaxy’spinwheelshapetoitshiddencenter.34Drakewouldbepartofmanyofthese efforts. But the inhabitedworlds in Drake’s imaginationwouldn’t leavehimalone.Itwasn’tlongbeforehewasthinkingofwaystousethisgiantradioeartofindthem.

“I calculated just how far our new 85-foot telescope could detect radiosignalsfromanotherworldiftheywereequaltothestrongestsignals[onearth],”Drakelaterwrote.35Theanswerturnedouttobeabouttenlight-years,orsixtytrillionmiles.SincehebelievedstarsliketheSunhadthebestchanceofhostingaworldlikeEarth,hisnextstepwastocheckthestarcharts.Luckily,therewereat least few sunlike stars within ten light-years.36 Drake saw he had thebeginningsofarealresearchproject.

After his initial calculation, Drake needed to get his colleagues at theobservatory to buy into something as seemingly crazy as a search for aliencivilizations. The scientists who lived at Green Bank often ate together at aroadsidedinerafewmilesaway.Overlunchthereonewinterday,Drakemadehis pitch to use the telescope to search for signs of intelligent life on otherworlds.

Frank Drake and the early telescope at the National Radio Astronomy Observatory in GreenBank,WestVirginia,in1964.

“Atthetime,thedirectoroftheNationalRadioAstronomyObservatorywasLloydBerkner,[whowas]somethingofascientificgambler,andhewasallforit.Soas thelastgreasyfrenchfrywaswasheddownbythelastdropofCoke,ProjectOzmawasborn.”

“Project Ozma.” True to the exuberance of his childhood dreams, DrakenamedhissearchaftertheprincessoftheEmeraldCity.Withtheblessingsoftheobservatory administration, the team began building the equipment needed tocarryoutProjectOzma.Bythespringof1960,theamplifiers,filters,andother

radioengineeringgearwereready.37For six hours each day that year, from April to July, Drake aimed the

telescopeatoneof two targetstars.The firstwasTauCeti in theconstellationCetus(theWhale).ThesecondwasEpsilonEridaniintheconstellationEridanus(theRiver).38

Helaterwroteofremembering“thebattleagainstthecoldeachmorningasIwouldclimbtothefocusofthedish.. . .Andthenofthatmomentonthefirstdayofthesearchwhenastrong,pulsedsignalcameboomingintothetelescopejustassoonaswehadturnedittowardsEpsilonEridani.”39

Theheart-poundingexcitementof the “booming signal” turnedout tobe afalsealarm.Thatsourceturnedouttobeman-made.Itwas,infact,justabouttheonly time Drake thought they’d detected another civilization. Project Ozmanever captured any alien signals, but it did capture something else of greatimportance:theworld’simagination.40Just tenyearsafterFermihadaskedhisquestion among a small group of friends, at least some in the scientificcommunitywerereadytotakethequestionofexo-civilizationsseriously.

As Drake was working out the details of his search at Green Bank, twophysicists named Giuseppe Cocconi and Philip Morrison published agroundbreaking study titled “Searching for Interstellar Signals.” The paperappeared in a 1959 issue of Nature, one of the most prestigious journals inscience. The two physicists argued that the bestway to look for signals fromadvanced exo-civilizations was by using radio astronomy. Dust blocks visiblelight,making theMilkyWay seem blotchy to our eyes. But radio “light” haslongwavelengthsthatpassunobstructedthroughdustyregionsofthegalaxy.So,withradiowaves,thegalaxybecomestransparent,allowingastronomersto“see”fromoneendtotheother.Thismeantacivilizationemittingradiowavescouldbeseenatfargreaterdistancesthanoneemittingvisiblelightsignals.41

Drake had already reached the same conclusion. But the publication ofCocconi and Morrison’s paper meant others were thinking exactly along hislines. Itwasadevelopment thatworried thenewdirectorofGreenBank,whowasnoneotherthanDrake’sinspiration,OttoStruve.Untilthen,Drakehadbeenkeeping a tight lid on his search. Struve, however, feared getting scooped.Withinafewweeks,StruveusedaninvitedlectureatMITasanopportunitytorevealProjectOzma’sexistencetotheworld.42

Soon, Drake was hosting a steady stream of visitors. Award-winningjournalists,theologians,andleadingbusinessmenmadethetrektoGreenBank.Project Ozma, along with the publication of Cocconi and Morrison’s paper,marked a turning point in the way science engaged with the issue of alien

civilizations.By1960,humanitywasdoggedbyquestionsofitsownimminentdestruction on one hand,while it watched the space age dawn, offering freshpossibility,ontheother.Thesetwotechnologicaldevelopmentswerereshapingpoliticsandculture,andtheyservedasakindofimaginativeether,launchingthefirsttruescientificsearchforothercivilizations.

WithProjectOzma,aspecificscientificquestionaboutexo-civilizationshadfinally been posed in a way that could be explored using a specific set ofappropriate scientific tools. As this crucial threshold was crossed, exo-civilizationsroseforthefirsttimefromthepurelyspeculativerealmofsciencefiction.Oneyear later, theyoungFrankDrakewould see theconsequencesofthisworkbecomemanifestinafatefulcallfromWashington,D.C.

THEGREENBANKCONFERENCE

J.PeterPearmanwasastaffofficerofBritishoriginattheNationalAcademyofSciences. In the summer of 1961, he calledDrakewith a remarkable request.PearmanwaspartoftheAcademy’sSpaceScienceBoard,andhewantedDraketo host a meeting exploring the research possibilities for “extraterrestrialcommunications.” Drake had spent the year after Project Ozma nervouslywondering which of his colleagues might be snickering behind his back. Heagreedimmediatelytorunthemeeting.43

Thediscussionthenturnedtoinvitations.DrakewashappytodiscoverfromPearman that not only were other scientists taking up the question ofextraterrestrial life, but there were two government-sponsored committeesalreadyexploringtheproblem.Together,theydrewupalistoftenscientistsforthemeeting.

First,therewouldbeCocconiandMorrison,theauthorsoftheNaturepaper.DrakethensuggestedDanaAtchley,aradioengineerwho’ddonatedakeypieceof equipment for Project Ozma. Barney Oliver, a Hewlett-Packard “researchmagnate”who’d visitedDrake duringOzma,was also included.As a leadingastronomer and head of Green Bank, Otto Struve was asked to serve as themeeting’schairperson.StruvethenaskedthathisformerstudentSu-ShuHuangjoin the group. For expertise in the chemistry of life, the pair chose MelvinCalvin, a Berkeley scientist who discovered the chemical pathways ofphotosynthesisthatallowplantstoturnsunlightintofood.RumorswereflyingthatthenextNobelPrizeinchemistrywouldhaveCalvin’snameonit.

Running over their list, Drake joked, “We’ve got astrophysicists,

astronomers,electronicsinventors,andexobiologyexperts.Allweneednowissomeonewho’sactuallyspokentoanextraterrestrial.”44Withoutmissingabeat,Pearman, inhisperfectOxfordaccent, toldDrakehehadexactly that.JohnC.Lillywasabiologistwhohadbecomefamousforhisworkwithdolphins.Lillyclaimed his research demonstrated that dolphinswere as intelligent as people.Lillyalsobelievedtheypossessedasophisticatedformoflanguagethathecoulddecipher.DrakeagreedthatLillyshouldbeonthelist.

TherewasonemorescientistPearlmanandDrakewantedtoinvite.Hewasyoungerthanalltheotherinvitees,buthisname,likeDrake’s,wouldshapethefuture of astrobiology. In the summer of 1961, Carl Saganwas newlymintedPhD with a fellowship at Berkeley. There, he’d been working with Calvin,developinglaboratoryexperimentsontheformationoflife.Thoughonlytwenty-seven,Saganhadalreadymadeanameforhimselfasbothbrilliantandbrash.45

ThemeetingwasscheduledforOctober31,1961.Invitationsweresentout,and Drake and Pearman were soon delighted to find that almost all wereaccepted. Only Cocconi declined (he would never engage in astrobiologicalresearchagain).Butasthemeetingapproached,aconflictappeared.ThegrouphadgottenwordthatCalvinwasgoingtogethisNobelPrizeinchemistry,andtheannouncementwouldcomeduringthethreedaysoftheGreenBankmeeting.CalvinwasmorethanwillingtotakethecallfromSwedenatGreenBank,butPearmanandDrakeknewsomechampagnewouldbeneededforacelebration.Procuringbubbly,however,poseditsownkindofchallenge.

“[Getting champagne was] no mean feat in the semidry state of WestVirginia,” Drake later recalled. “West Virginia apportioned one state-operatedliquorstore toeachcounty.Theoneclosest to theobservatorystood ina littlelumber town called Cass, about ten miles away. The observatory’s staff nowincluded a driver—aWest Virginianwith the fairly common (for those parts)firstnameofFrench,andtheimprobablesurnameofBeverage.ForamomentIconsideredsendinghimtobuythechampagne,butitwouldhavebeentoosilly.Instead,IdroveovertoCassmyselfthatweekend.”46

Drake purchased a case of champagne and made his way back to GreenBank.

With the invitations complete and the champagne hidden away, the onlythingleftforFrankDraketodowastosetanagenda.“Isatdownandthought,‘Whatdoweneedtoknowabouttodiscoverlifeinspace?’”47

Drakesimplywantedawaytoorganizethediscussion,butthepathhechosehadconsequencesfarbeyondtheGreenBankconference.ThoughDrakecouldnothaveknownitat the time,his ideawouldestablishanorganizingprinciple

fortheentirefutureofastrobiologicalscience.Since the purpose of the meeting was to explore possibilities for

communicationwithexo-civilizations,Drakeunderstoodthatthefirstandmostimportant question would be how many exo-civilizations there were tocommunicatewith.That translated into a single, specific question themeetingneededtoanswer:WhatisthenumberoftechnologicallyadvancedcivilizationsinthegalaxythatcanemitradiosignalsdetectableonEarth?

The galaxy contains about four hundred billion stars.48 If the number oftechnologicalcivilizations(call thenumberN) turnedout tobesmall, then thesearchforexo-civilizationwouldbeunlikelytosucceed.Therewouldbejusttoomanystarstosearchandtoofewinhabitedsystemstofind.ButifNwerelarge(inthebillions,perhaps), thenastronomerswouldn’thavetosearchmanystarsbeforeanexo-civilizationpoppedup.

So,whatDrakeneededwasawaytoestimatethevalueofN.Toaccomplishthis,hebroketheproblemupintosevenpieces.Eachpiecerepresentedadistinctsubproblem the scientists at the meeting could discuss in detail. Mostimportantly, each piece could be expressed as a factor in an equation for thenumberofgalacticexo-civilizations—theall-importantquantityN.

Let’srundownthesevenpiecesofDrake’sequationandhisexo-civilizationquestion.

1.TheBirthRateofStars

BasedonourownexperiencehereonEarth,lifewillformonplanets.Ofcourse,it is perfectly reasonable to askwhether life canbypassplanets by forming insomethinglikeaninterstellarcloud(astronomerFredHoyleassumedthisinhisfamoussci-fistoryTheBlackCloud).49Givenwhatwedounderstandaboutthemechanismsof life,however, it’s farmore likely thata solidplanetarysurfacewith lots of liquid water and other chemicals is a requirement to get biologygoing.Assumingafocusonplanetsbringsusstraighttoafocusonstars.Ifwewant to know howmany planets host exo-civilizations in the galaxy,we firsthavetoknowhowmanyplanetsexist,andthatmeanswefirsthavetoknowhowmanystarsexist.50SoDrake’sequationbeginswiththenumberofstarscreatedinthegalaxyeachyear.AstronomersrepresentthisbythesymbolN*(readas“Nsubstar”).

2.TheFractionofStarswithPlanets

Onceweknowthenumberofstarsformingperyear,wecanthenaskhowoftenplanetsgetcreatedaroundthesestars.Isplanetformationaveryrareoccurrence,or a common one? As we saw in our brief tour of history, this is an ancientquestion.Andbythemiddleofthetwentiethcentury,planetformationhadonceagainbecomethesubjectofintenseastronomicaldebate.

Drakeexpressed thisquestion in termsof fractions.What,heasked, is thefractionofstarsthathostaplanet?Hewrotethistermasthesymbolfp(readas“fsubp”).

3.TheNumberofPlanetsin“TheGoldilocksZone”

Itisnotenoughtojustaskifastarhostsaplanet.Theplanet’sorbitaroundthestarisalsoakeyfactorinthinkingaboutlife,intelligence,andcivilizations.Ifaplanetisveryclosetoitsstar,thenthetemperatureonitssurfacewillbesohighthatlifegetsfrieddowntoitsatoms.If,ontheotherhand,aplanet’sorbitisverylarge,itssurfacewillbeperpetuallyfrozenandinneardarkness.

AtthetimeoftheGreenBankmeeting,OttoStruve’sformerstudentSu-ShuHuang had just finished work that showed how each star is surrounded by a“habitablezoneoforbits.”Huangdefinedthiszoneasthebandoforbitswhereliquidwatercanexistonaplanet’ssurface.51Liquidwateristhoughttobeakeyfactor inallowinglife toformandthrive.TheinneredgeofHuang’shabitablezonewas theorbitwhere a planet’s temperaturewas just cool enough tokeepsurfacewaterfromboiling.Theouteredgewastheorbitwherethetemperaturewasjusthighenoughtokeepwateronaplanet’ssurfacefromfreezing.

Drake andhis colleagues at theGreenBankmeetingneeded toknowhowmany planets (for those stars that had planets) were in the habitable zone. Inotherwords,howmanyplanetswereonorbitsthatlefttheirsurfacesneithertoohot nor too cold. Thus, the third variable in Drake’s equation would be theaveragenumberof planets in a star’s habitable zone,which is also sometimescalledthe“Goldilockszone.”Thistermisexpressedasnp(readas“nsubp”).

4.TheFractionofPlanetsWhereLifeForms

While the first three terms in Drake’s equation dealt purely with issues of

physics and astronomy, the fourth brings chemistry and biology into thediscussion.Givenastarwithaplanetinanorbitthatleavesitwithliquidwateronitssurface,whataretheoddsthatthesimplestformsoflifewillappear?Onceagain,Drake expressed this question in termsof a fraction,whichhe called fl(readas“fsubl”).

It’sworthnoting that discussions about fl hinge on the chemical pathwaystakingnonlivingmatter intoaself-replicatingstate.Theformationof life fromnonlife is called abiogenesis. Experiments done by Harold Miller at theUniversity of Chicago in the early 1950s had already provided compellingevidence that abiogenesismight not be difficult to obtain on a habitable-zoneplanet.52

5.TheFractionofPlanetsWhereIntelligenceEvolves

Thefifthtermmovesusfromthebiochemistryoflife’soriginintothedynamicsof its changing forms. Assuming life begins on a planet, how often wouldevolutioncarrythatlifeforwardtointelligence?Drakeexpressedthefractionofplanetswhereintelligenceevolveswithatermcalledfi(readas“fsubi”).

6.TheFractionofPlanetswithaTechnologicalCivilization

Thesixth termmovesus fromevolutionarybiology to sociology.Given that aplanet hosts an intelligent species, howoften does a technologically advancedcivilizationthenarise?Thisquestionwasrepresentedbythetermfc(readas“fsubc”),thefractionofplanetswhereatechnologicalcivilizationbegins.

Forpracticalpurposes,Drakesaw“technologicallyadvanced”asmeaningacivilizationwiththecapacitytobroadcastradiosignals.53So,whiletheRomanswerecertainlyacivilization,fromDrake’spointofview, theydon’tcountasatechnologicallyadvancedone.54

7.TheAverageLifetimeofaTechnologicalCivilization

The final factor in Drake’s equation is the most haunting: How long does acivilization likeourownlast?Canweexpectanother fewcenturiesbeforeour

global society flares out, or are there manymillennia of development ahead?Assuming that technological civilizations have occurred often enough for anaveragetobewelldefined,whatistheiraveragelifetime?

Withthislastterm(writtenasL),Drakewasaskingtheothersatthemeetingtoconsideraliensociologyonadeeper level.Somediscussionwasdevoted totheoverconsumptionofresources,butgiventheheightenedfearsofnuclearwarin 1961, aggression was the focus of Drake’s final variable.55 Are mostcivilizations as aggressive and warlike as our own? Do they become morepeacefulastheyevolve?Howlong,onaverage,cantheylastwithoutdestroyingthemselves?

ONEEQUATIONTOBINDTHEM

Each of the seven terms Drake chose to set the agenda for his Green Bankmeeting was a problem that, in principle, had a quantifiable answer. Eachcontaineditsowncompellingmysteries,andeachwasasteponaladdertothatoverarchingquestion:Arewealone?

To be specific, though—and the whole point of the meeting was to bespecific—Drake’soverarchingquestionwas:HowmanyradiosignalproducingtechnologicalcivilizationsotherthanourownresideintheMilkyWaygalaxy?InthelanguageofDrake’sagenda,whatisthevalueofN?

Withallhissubproblemsmappedout,Drakewasfinallyinapositiontoputthem together into a single equation. Here it is, written out in mathematicalform:

N=N*fpnpflfifcL

Inwords,Drake’sequationsaysthenumberofexo-civilizationsfromwhichwecangetsignalsequalsthenumberofstarsformingeachyear(N*),timesthefractionof thosestarswithplanets(fp), times thenumberofplanetswhere lifecan form (np), times the fraction of planetswhere life actually does form (fl),timesthefractionofthoseplanetsthatevolveintelligence(fi),timesthefractionof those intelligences thatgoontocreate technologicalcivilizations(fc), timestheaveragelifespanofthosecivilizations(L).

Here,youcanseewhyscientistslikeequationssomuch.Anideathattakesamouthfulofwordstoexpressgetscapturedprettycleanlyinjustoneshortlineofsymbols.

OnthemorningofNovember1,1961,with theparticipantsatGreenBankmeetinggathered around the conference table,Drake stood andwrotehis newequationontheblackboard.Scrawledinchalklikeahaiku,itwasneverintendedtobeanythingmorethanaguide,anoverview,anorganizingprinciple.

Itturnedouttobemuchmore.Asearchof“Drakeequation”on theGoogleScholar searchengine returns

thousandsofpapers.Asimilar searchofAmazonbringsbackscholarlybooks,sciencefictionnovels,T-shirts,andevenatungstencarbideringimprintedwiththe formula. Since the Drake equation was introduced, it has appeared in astunning number of scientific conferences, magazine articles, anddocumentaries.

“It amazes me to this day,” Drake wrote later, “to see [the equation]displayed prominently in most textbooks on astronomy, often in a big,important-lookingbox.”Withhumility,Drakeadded, “I’malways surprised tofinditviewedasoneofthegreaticonsofsciencebecauseitdidn’ttakeanydeepintellectualeffortorinsightonmypart.Butthenasnowitexpressedabigideainaformthat...evenabeginnercouldassimilate.”56

InconsideringtheimportanceoftheDrakeequation,youhavetobeginwithwhat it is not. It is not a lawof physics.Einstein’s famous equationE=mc2expressesafundamentaltruthaboutthebehavioroftheworld.Itisastatementofourunderstandingabouthownatureworkson itsown.TheDrakeequation,ontheotherhand,isreallyastatementofour lackofunderstanding.It tellsuswhatwewould need to know to get a specific answer to a specific question:Howmanyexo-civilizationsareoutthere?

Before Drake, the scientific consideration of exo-civilizations wasunfocused. What existed was a mix of unconnected musings in scientificjournals, books, and popular articles. There was no structure for building acoherentprogramof study, either through theoryorobservations.Bybreakingthe big question into seven smaller questions, Drake crafted a useful way tothinkabouttheproblemthatalsoleftscientistssomethingtheycouldworkon.Itgavethemsomethingtodo.

Each of the terms in the equation could be explored on its own, usingwhatever means were available. Astronomers could work on the first threeterms; biologists could think about the two that followed; sociologists andanthropologistscouldexplorethelasttwo.Ofcourse,mostoftheworkwouldbespeculative. But at least it would be speculationwith a focus and a scientificfoundation.

With time and patience, advances were made from all sorts of directions.

Computer studies of chemical reactions provided insights into abiogenesis.Evolutionarystudiesof lifeonEarthshowedhowcognitivepatternsleadingtointelligence first appeared. And while some terms, like the average age of acivilization,mightneverbeknown,others,likethefractionofstarswithplanets,were thought to be within grasp at the time of the Green Bank meeting. Inaddition,whileeventhecloseststarswerefiftytrillionmilesaway,theplanetsinoursolarsystemwererelativelyclose.Ifwecouldfindevenoneexampleoflife—in its simplest form—on Mars or anywhere else in our solar system, thatwouldtellussomethingpowerfulaboutthefirstbiologyterm.

WhatDrake’sequationgaveastrobiologywasawaytothinkaboutitself.Intheprocess,itchangedhowweunderstoodlife,civilization,andourselves.

Drake’s equation also ensured the success of the Green Bank meeting.Beginningwith the rateofstar formationandmarchingall theway through totheaveragelifetimeoftechnologicalcivilizations,thenineparticipantsdidtheirbesttomakeinformedestimatesofthedifferentterms.Historyshowstheywerea hopeful group. They assigned values relatively close to one for all thefractions.Most telling, though, they reserved theirpessimismforDrake’s finalfactor:theaveragelifespanoftechnologicalcivilizations.

Thecapacityforacivilizationtoshort-circuititsownevolutionthroughself-destruction vexed the meeting’s participants. It would go on to become abottleneckinallthinkingaboutsearchesforextraterrestrialintelligence(SETI).TheGreenBankparticipantsbelieved,asDrakelaterwrote,that“thelifetimesofcivilizations would either be very short—less than a thousand years—orextremelylong—inexcessofperhapshundredsofmillionsofyears.”57

In the end, the group agreed that the final factorwaswhatmatteredmost.ThenumberofstarswassovastthatthegalaxycouldabsorbalotofwhatDrakeand his colleagues considered pessimism regarding the other terms of theequation.Butthegalaxyalsoneededtobepopulatednow.Thereneededtobeanoverlapintimebetweenourcivilizationandtheirssothattherewouldbesignalsfor us to receive. That meant the other civilizations needed to last at leastmillionsofyears,whichseemedlikeastretchfortheGreenBankgroup.

Justbeforethemeetingended,Drakeandhiscolleaguesbrokeouttheironeremaining bottle of champagne (Calvin’s call from the Nobel Committee hadcome lateon the first nightof themeeting).As they raised their glasses,OttoStruveoffereda toast.“To thevalueofL,” saidStruve.“May itprove tobeaverylargenumber.”58

THEDAYCLIMATECHANGED

In 1965, little more than three years after Struve’s toast, President LyndonJohnsonwould raise the same issue of civilizations’ longevity in the farmorespecificcontextofourownfate.SpeakingbeforeajointsessionofCongress,hesaid, “[T]his generation has altered the composition of the atmosphere on aglobalscalethrough...asteadyincreaseincarbondioxidefromtheburningoffossilfuels.”59

It’sremarkabletonotethat,morethanfiftyyearsago,anAmericanpresidentwas already aware of, and acknowledging, human-created climate change.JohnsonhadbeenbriefedonthedangersofCO2increasesbythefamousclimatescientistsCharlesKeeling andRogerRevelle, amongothers.So, not onlywasPresidentJohnsonawareof the issue,buthewasalreadyconcernedenough toraiseitbeforeCongress.Thatsinglesentenceinhisaddressgivesthelieto theclaimsofsomanyclimate-changedeniersthatglobalwarmingissomekindofrecenthoax.Indeed,thescientificunderstandingofoureffectontheEarthdatesback more than a century. As President Johnson’s speech demonstrates, evenfiftyyearsago,thatunderstandingwasfirmenoughtogainnoticeatthehighestlevelsofpolicyandpolitics.

Butthereisadifferencebetweenacommunityofscientists,atthevanguardof their fields, glimpsing human-driven climate change and the culture as awholemetabolizing the story. A single speech by a president can’t create thekind of intimacy that is the hallmark of humanity’s most powerful narrativesabout itself and itsplace in theworld.That takes timeand theplayof events.Theindustrialrevolution,forexample,didn’tarriveassoonasthefirstfactorywasbuilt.Ittookpeoplemovingenmassefromfarmstocitieswhereday-to-daylifetookonnewrhythmsandtextures.Onlythendidwebegintoseeourselvesas “industrial.” Only then could we tell new stories about ourselves as acivilizationthatconqueredtheplanetwithsteel,rubber,andoil.

Likewise,wearejustbeginningourentryintotheAnthropocene.FiftyyearsonfromPresidentJohnson’sspeech,wehavejuststartedbecomingfamiliarwithimagesofmeltingglaciers,massiveheatwaves,andfloodedcities.Wearejustbeginning to experiencewhat life on a climate-changedworld looks like. ButwhenPresidentJohnsonstoodbeforeCongressin1965,thatstorywasstillnew.

Conservationwastheintendedthemeofthepresident’sspeechthatday.OnlyafewyearshadpassedsincebiologistRachelCarsonhadraisedalarmsovertheenvironmentaleffectsofpesticidesinSilentSpring.Evenlesstimehadelapsedsince the treatybanningatmospheric testingofnuclearweaponshadgone intoeffect.While the ColdWar made instant annihilation a credible threat in the1950s,bythemid-1960ssomewerebeginningtorealizethateventheeveryday

activitiesofourprojectofcivilizationwerenot,intotal,goingunnoticedbytheplanet.

Sustainabilityonaglobalscale,however,isaverydifferentkindofstoryforhumanity to tell itself. Itdemandsavastlyenlargedimaginativepalette.At thetime of President Johnson’s speech, the picture of a climate-threatened futurewas just starting to be painted by scientists as they gained a first foothold onunderstandingtheEarthasaplanet.Theseresearcherswererecognizing,forthefirst time, that Earth needed to be understood in its entirety as single, tightlycoupledsystem—akindofvast,planetary-scalemachine.

Ironically,andasissooftenthecase,theneedforthisnewvisionfounditsfirsturgency in theneedsofwarfare.With the riseof long-rangebombersandintercontinental missiles, cold warriors were busy imagining Earth from wellabove theatmosphere.But theywerealsodeeplyconcernedwithhowweathercouldtipthescalesofbattle.Itwaspartlyattheirurgingthatresourcespouredinto the scientific study of climate. A nuclear-powered laboratory was builtundertheiceofGreenlandtounderstandhowweatherpatternschangedoverthecourseofmillennia.Instrument-ladenshipscrisscrossedtheoceans,studyingtheforces driving deep ocean currents. Most importantly, the same ICBMsthreateningnuclearwarwerestartingtoliftscientificsatellitesintoorbit,wheretheireyeswouldpointdownwardtostudytheEarth.

These were expensive and globally extensive efforts. They laid thegroundwork for a newvision of our project of civilization’s planetary contextandimpact.

ThefirstphotographofEarthcapturedbyaweathersatellite,takenin1960.

In 1960, a still-wet-behind-the-ears NASA launched its first successfulweather satellite, TIROS (Television InfraredObservation Satellite). By 1962,TIROSwasofferingcontinuouscoverageoftheEarth’sweather.60 In thewakeofTIROS,nolongerwouldahurricaneunleashitsviolenceonanunsuspectingpopulation.And for the first time,peoplewere treated to imagesofEarthasaglobesuspendedinspace.Eventheearliestgrainyvideosshowedtheelegantarcof theworld’s horizon as seen from high above the atmosphere, a vision thatwouldrewireourcollectiveimaginations.

Bythemid-1960s,aconvergencehadbegun.ImagesfromTIROS,PresidentJohnson’s address on carbon dioxide, Fermi’s lunchtime insight, and Drake’sGreen Bank conference were pieces in a cultural jigsaw puzzle that wasbeginningtoassembleitself.Eachrepresentedatentativefirststeptowardseeing

ourprojectofcivilizationinanewlight—thelightofthestars.FermiandDrakerepresentedanewawarenessamongscientiststhatthestoryofourownprojectof civilizationmust be set onto a cosmic stage,with all its stars, planets, andpossibilities. Meanwhile, studies of climate funded via Cold War urgenciesshapedan awakening amongother scientists thatEarth’s storymustbe told interms of a mighty planetary system driven by sunlight and shaped by life—including our own. Finally, President Johnson’s address signaled that ourcivilization’s impact on the planet was making its way into the domains ofcultureandpolitics.

Anewhumanstory,anewhumanmythology,wasemerging.Theoutlinesofthis narrative, in which human beings and our project would be inescapablybound to themachinery of planetary evolution,were beginning to take shape.Fewatthetimecouldrecognizethepower,theperil,andthepromisegrowinginthisnewstory.Itwasstilltoonewandtoounformed.Totakethenextstepsinforging this new vision, we would have to leave home. We would have tobecomewayfarersand journey, for thefirst timeinour longhistory,out to thehigh frontier of space.Thatwaswhere the siblingworlds of our solar systemwerewaitingtotellustheirsecrets.

CHAPTER2

WHATTHEROBOTAMBASSADORSSAY

TOBEABUM

TheFloridasunglistenedovertheblueAtlanticwaters,butJackJameswasinablackmood.ItwasJuly22,1962,andithadbeenaverybadday.TheTexas-bornengineerwasprojectmanagerforNASA’sMarinerprogram,whichaimedto sendAmerica’s first emissary to anotherplanet. Jameshadgotten the job alittlemore than twoyearsearlier.Likeeverythingelse in thespaceraceof theearly 1960s, James’s program had been rushed forward at breakneck speed,workingnonstop.Nowthefruitofallthateffortlayinruinsatthebottomoftheocean.

James and his team had been given less than fourteen months to design,build,andlaunchaprobetoVenus.1Untilthen,theMoonhadbeentheprimarytargetofthespacerace,andAmerica’srecordforhurlingoddlyshapedboxesofelectronicsatEarth’s rockysatellitehadbeenamixedbag.TheRussianswerehavingbetterluck.They’dgottenthreeoftheirnineprobestotheMoon.TheUShadonlygottenone there.2NowNASAwasdesperate formore thanawin. Itneeded to upstage the Soviets in a bigway. That’swhy Jameswas given theaudaciousjobofthinkingbeyondtheMoonandgoinginterplanetary.

RocketengineerJackJames(right),withMarinerProjectmanagerDanSchneiderman(left).

Mariner 1 was designed to perform a “fly-by” past Venus, a world thatorbited30percentclosertotheSun,butwithalmostthesamemassandsizeasEarth.3 The probe’s original design called for 1,250 pounds of scientificinstruments,communicationsgear,solarpanels,rocketmotors,andfuel.Butthenew,morepowerfulgenerationofrocketboostersMarinerwassupposedtorideintospacekeptblowingup,andNASAbrasssoondemandedaredesign.James’sengineerswereforcedtoquicklyshedmorethantwo-thirdsofMariner’sweight.

Jamesnavigatedhisteamthrougheverydesignchangeandeverychallenge.Thatwashowtheycameto thisday.With thewholeworldwatching,Mariner

1’sboosterrocketlituptheFloridaskyasitblastedoffthatJulymorning.Forthefirstfewmoments,thelaunchlookedclean.ButthentheAtlasboosterbeganto fishtail.Every launchhas a “range safetyofficer”whose job is toblow therockettobits if it looksasthoughthemissionisfailingandit’sgoingtocrashbacktoEarth.Fourminutesandfifty-threesecondsintotheflight—andjustsixseconds beforeMariner 1 would have safely separated from the main launchvehicle—thesafetyofficerhitthebigredbutton.4

Boom!Forafullminuteaftertherocketexploded,telemetrycontinuedtogetsignals

fromtheprobeasittumbledfromtheskytoitsoceangrave.5AtleastMariner1hadbeenatoughbird.

They’dbeensodamnclose.Justsixsecondsmoreandthey’dhavebeenontheirwaytoVenus.

“BorntoLose,”byRayCharles,playedontheradioasJamesdrovebacktohisrentedapartmentinCocoaBeach.Hewasindespair.Yearslater,he’drecallthemantraofallspaceengineers:“Tobeaherotherearetenthousandpartsthatneedtoworkproperlyonaspacecraft.Tobecomeabumyoujustneedoneofthemtofail.”6

But while James and his team were down, they weren’t out. The now-destroyed probe had a twin.Mariner2waswaiting back atCapeCanaveral.7Therewasstilltimetobeahero.

THEVENUSPROBLEM

ThelogicofthespaceracedictatedthateitherMarsorVenuswouldbethenextdestination after getting probes to theMoon. Bothwere neighbor planets thatcouldbereachedinamatterofmonths,astepupfromthethree-daytriptotheMoon.And each had its own long history of dreamers imagining a temperateworldfitforextraterrestriallife.

Given its proximity to the Sun,Venus gets twice asmuch solar energy asEarth.8That’swhymanyearlyastronomers imaginedVenusasa jungleplanet.In1870,ClaudeFlammarion(theauthorofThePluralityofWorlds)thrilledhisreaders with images of a Venusian landscape made of broad, swampy plainsringedbymountainshigherthantheHimalayas.9

Flammarionassuredhis readers thatVenuswasaworld richwith life: “Ofwhat nature are the inhabitants of Venus . . . ? All we can say is that theorganized life [there]must be little different from terrestrial life, and that this

worldisoneofthosethatresemblesourownmost.”10

AnimaginedviewofVenusfromFlammarion’s1884book,LesTerresCiel.

But with the increasing power of astrophysical observations, this pleasantdreamofajungleVenuswouldcomeunderfire.First,astronomicalobservations

in the late eighteenth century revealed Venus to be perpetually shrouded byclouds.11 Then, in the mid-twentieth century, the Venusian atmosphere wasrevealedtobeheavywithcarbondioxide(CO2).TheEarth’satmosphere is78percent nitrogen, 21 percent oxygen, and one percent everything else. CO2comesinatamere0.039percentoftheairyouarebreathingrightnow.That’saprettysmallfractionforamoleculethat,aswewillsee,hasabigroletoplayinour story. But for Venus, CO2 is pretty much all there is to the atmosphere,accountingformorethan95percentofallitsgases.12

The presence of somuchCO2 was bound tomakeVenus a very differentplace fromEarth, andby1956, astronomers hadgained their first evidenceofjust how different it might be. Using the same kind of radio astronomytechnologiesFrankDrakewouldsoonemployinGreenBank,scientistsfromtheNavalResearchLaboratoryfoundevidencethatVenus’ssurfacetemperaturewaswell above 600 degrees Fahrenheit.13 That’s hundreds of degrees above theboiling point of water. If the NRL result was true, then Flammarion couldn’thave beenmore deluded.HisVenusian swampswould have boiled away longago. More importantly, 600 degrees was far too hot for any form of life tosurvive.ItseemedtheplaceVenusresembledmostwasn’tEarth,butHell.

While geology and its study of the Earth had been around a long time,planetaryscience—whichtakesallplanetsasitssubject—wasayoungfield.TheNRL results set off a firestorm among the small group of researchers whoconsidered themselves planetary scientists. Part of the conflict came because,just a fewyears earlier, another teamhad predictedVenus to be covered by avast,planet-girdlingocean.14Butneitheroceansnorlakesnorevencupsofteacould be squared with the new NRL data, which suggested temperatures onVenuscomparabletotheinsideofapizzaoven.

Inresponse,somescientistsclaimedtheNRL’sdatahadbeenmisinterpreted.Its source, they claimed, wasn’t Venus’s surface but violent, atomic-scaleprocessesoccurringat theboundaryofitsatmosphereandtheharshconditionsofinterplanetaryspace.15

Resolving the dilemma required more power than earthbound instrumentscouldprovide.ThebesttelescopesofthedaycouldnotseethediskofVenusinenoughdetailtodistinguishbetweenahotsurfaceorprocessesoccurringhighintheatmosphere.Gettingupclosewithaspaceprobewasonemeansofgettingthatlevelofdetail.

Butaspacemissionwasn’ttheonlykeythatastronomersneededtounlockthe mystery of Venusian climate. The NRL result was shocking to scientists

because no one could understand how itmight be true.Venus is closer to theSun,but thatproximityshouldonly raise its surface temperaturea few tensofdegrees, not hundreds.16 If the surface temperatures really were 600 degrees,howcouldaplanetsoliketheEarthinsomanywayshaveendedupsodifferentfromourworld?Whatwas neededwas a theory explaining howVenusmightendupwithsuchinsanelyhightemperatures.

Thattaskwouldbetakenonbytheyoung,untested,andnot-yet-mintedPhDstudentCarlSagan.Thoughnooneat thetimecouldhaveguessedit,notonlywouldSagan’sworksolvetheVenusproblem, itwouldalsoset thestageforadeeperunderstandingofourownworld’sentryintotheAnthropocene.

THEGREENHOUSEEFFECT

Though he died in 1996, Carl Sagan remains one of the most recognizablescientific faces in the popular imagination. Born sixty-two years earlier toworking-class Jewish parents in Brooklyn, Sagan’s love affair with sciencebegan as a young boy during a trip to the 1939World’s Fair. The passionateinterestinlifeonotherplanetsthatdefinedhislifecameabitlater,asateenager,withasteadydietofAstoundingScienceFictionmagazineandwriterslikeH.G.Wells.17

SaganattendedtheUniversityofChicago,wherehewastrainedtothinkasbothascientistandahumanist.Itwasacombinationthatwouldlaterprovesocompelling tomillionsvia his popularwritings and televisionprograms.AfterChicago, he moved ninety miles northwest to the Yerkes Observatory inWisconsin,wherehestartedworkonhisPhD.18

Graduateworkinastrophysicalsciencestakesyearsofdedicatedeffort.First,there are advanced classeson thebasicsof theory andobservation.Only afterthis initial phase can students start independent study.Sagan arrived atYerkeswith an interest in lifebeyondEarth, so forhis graduate thesis he chose threeseparate issues at the intersection of planetary science and what we now callastrobiology.ThefirstofthesewouldbetheVenusproblem.

Sagan’squestionwasstraightforward:WhatprocesscouldturnthesurfaceofVenus intoascaldinghell?Combing throughdecadesofscientific literature insearchofananswer,hefoundoneinwhatisnowwellknownasthegreenhouseeffect.

AplanetliketheEarthwouldbeadeep-freezeworldwithoutitsatmosphere.That conclusion requires only a few lines of basic physics to demonstrate.

Sunlighthitting aplanetwarms its surface.Thewarmedgroundemitswhat iscalled heat radiation, which is just electromagnetic waves generated by thejigglingmotionsofheatedatoms.Anyobjectatanytemperatureaboveabsolutezerospewsheatradiationintoitssurroundings.Thatincludesyourownbodyasyoureadthesewords.

For our planet’s temperature to remain steady and unchanging, the energyflowingontoitmustbalancetheenergyflowingout.Heatisjustanotherformofenergy.Thatmeans incoming solar energy and outgoing heat radiation energymust balance if theEarth’s temperature is to stay constant. Scientists call thisbalancetheplanet’sequilibriumtemperature.

Calculating a planet’s equilibrium temperature requires the kind of basicphysicsmoststudentslearnintheirfirst-yearastronomyclasses.Oncetheyworkthroughthemath,thosestudentsallcomeupwiththesamestartlingresult:Earthwithout an atmosphere would have an equilibrium temperature around zerodegreesFahrenheit.That’swellbelowthefreezingpointofwater.19

Asweallknowfromdailyexperience,mostofEarth’ssurfaceisnotfrozen.In fact, the planet’s current average temperature is a balmy 61 degreesFahrenheit.20Somehow,ourplanetmanagestostaywarmenoughformostofitswatertobeinliquidform,ratherthanasasolid(ice)oragas(watervapor).It’stheatmospherethatraisesthetemperature.Theblanketofgasessurroundingtheplanet keeps Earth’s equilibrium temperature well above freezing. But how,exactly,doesthathappen?

ThefactthatyoucanseetheSunonacloudlessdaygivestestimonytothefactthatEarth’satmosphericgasesaremostlytransparenttoourstar’sincomingvisible radiation. The Sun’s visible-range electromagnetic waves pass rightthroughouratmosphereasunmolestedasthroughacleanglasswindow.Buttheheatradiationemittedbythewarmedplanet’ssurfaceisn’tinthevisiblepartofthespectrum.Instead,theplanetradiatesatlongerinfraredwavelengthstheeyecan’tsee.So,whileincomingsunlightpassesfreelythroughtheatmosphere,forthe longer infrared wavelengths emitted by Earth’s warmed surface, it’s adifferentstoryentirely.21

Likeablanketyouthrowoveryourselfonacoldwinternight,theblanketofgasessurroundingourplanetholdsinenergythatwouldotherwisegetradiatedaway.It’sthistrappedenergythatraisesEarth’stemperatureabovefreezing.Anactualgreenhouseworksalongasimilarprinciple,asthewindowsallowsunlightin but keep warmed air from rising away—hence the name, the greenhouseeffect.

The greenhouse effect was old news for scientists studying the Earth. In

1896,SwedishNobelPrize–winningchemistSvanteArrheniushaddiscoveredthehumanimpactonEarth’sgreenhouseeffect.22Usingasimplemathematicalmodel, Arrhenius laid out the physics of Earth’s greenhouse warming,demonstratinghowourplanetwaswarmedbyitsatmosphere.Justasimportant,hiscalculationalsorevealedhowourownactivitywasaddingtothatwarming.Using records of coal consumption, Arrhenius saw we were already puttingenoughCO2 into theatmosphere tochange theenergybalance.Using thecoaldata, he predicted that human beings would eventually raise the planet’stemperature as we continued dumping CO2 into the air. His pencil-and-papercalculation predicted a global increase of about five degrees.23 This isremarkably close to modern estimates. In our current era of climate-changedenial, it’s startling to recognize how far back the understanding of human-drivenclimatechangebegins.

SaganwantedtogofartherthanArrhenius—literally.HesawthatwhatwastruefortheEarthmustalsobetruefordistantplanets.Thegreenhouseeffecthadto be universal. So Sagan set himself the task of calculating the extent of thegreenhouse effect on Venus to see if it could explain that planet’s extremetemperatures.Acrossmany coldWisconsinwinter days,Saganporedover oldpapers in the Yerkes library, teaching himself the basic physics of infraredatmosphericabsorptionand itssubsequentplanetarywarming.Aftermonthsofexhaustingwork,hehadhisanswer.With itsCO2-richatmosphere,Venuswastrappingenoughenergy to raise the surface temperaturenear to the staggering600-degreelevelimpliedbyNRLdata.24Theplanetwasacauldronbecauseofthegreenhouseeffect.

Today, scientists recognize that planets anywhere in the universe must besubject to the samesetof forcesandprocesses.Whileeachworldhas itsownuniquestory,thosestoriesareallenactedbythesamelistofplayers:theflowofwinds,thepullofgravity,thedanceofchemistry.Earthisnodifferent,andthis,aswewillsee,istheprincipallessonoftheAnthropocene.ButwhenCarlSaganwasworkingaloneintheYerkeslibrary,theapplicationofthisuniversalvisionof the universe’s planets was still young. Other than a few nearly forgottenstudies, Sagan was alone in bringing the earthbound process of greenhousewarmingtoanotherworld.“AlmostnobodyontheplanetasfarasIcouldfind,wasinterestedintheVenusgreenhouseeffect...,”hewouldlaterrecall.“Isortofstumbledonitmyself.”25

ALIVINGHELL

RocketengineerandprojectmanagerJackJamesonlyhadaday tomourn theloss of Mariner 1. The launch window in which Earth and Venus werepositioned just right for the calculated flight path would close in a month.James’s teamneeded to getMariner2 ready for launch immediately. Twenty-eight days later, at 2:53 a.m. onAugust 27, 1962, anotherAtlas-Agena rocketliftedfromthegroundatopanotherpillaroffire.

This time, the launchwassuccessful,but justbarely.Afewsecondsbeforethe Atlas booster was to separate fromMariner, one of the rocket’s controlenginesshutdown,drivingitintoanuncontrolledspin.Asfearsroseforanotherfailure, the first of the mission’s “seven miracles” occurred. Control wasregainedatjusttherightmomenttoundoanydamagethespinhadimpartedtotheprobe’scalculatedflightpath.Therocket’ssecondstagefiredandMariner2wasonitswaytoVenus.

It would take three months for the probe to cross more than twenty-fivemillion miles of interplanetary space. Six more times, critical elements inMariner’s systemswould fail: a solar panel stoppedworking; temperatures onthe space probe climbed to dangerous levels; the onboard computer failed toswitch instruments to “encountermode” asVenus approached.But each time,disaster was averted as the problem either fixed itself or James’s team fromNASA’sJetPropulsionLaboratory(JPL)foundaworkaround.26

“I’dgetcalledatalltimesofthenight,”Jamesrecalledlater.“MynerveshadbecomesotautbythistimethatIinstructedeveryonethatcalledmetostartoutwith one of two sentences: ‘There is no problem,’ or, ‘There is a problem.’ ”Morethanafewcallsbeganwith“There’saseriousproblem.”27

Inspiteofallthedifficulties,onDecember14,1962,Mariner2flewwithintwenty-twothousandmilesofVenus,adistanceaboutsixtimesthediametertheplanet.AsdatafromMariner2trickledintoJPL,itbecameclearthattheNRLstudy andCarl Sagan’s greenhouse effect theory had been right. The scaldingtemperatureswerenothighintheatmosphere,butdownontheplanet’ssurface.Venuswasindeedalivinghell.28

The evidence for Sagan’s greenhousemodel forVenus got stronger as thespace agematured. Over the next forty years,more than twenty other probeswould visit our sister planet. Some mapped its surface at high resolution viacloud-penetrating radar. Others made detailed explorations of atmosphericconditions,includingwindswhippingaroundtheplanetathundredsofmilesperhour.TheRussiansevenmanagedtogetprobesdowntothesurface.Theprobesworkedforjustafewhoursbeforesuccumbingtotheplanet’sintenseheatandnuclearsubmarine–crushingpressures.29

What emerged from these studieswas apictureof aworldwhere theCO2greenhouse effect had run amok. The catastrophe was called a runawaygreenhouseeffect,anditsdiscoveryprovedtobeessentialforunderstandingtheclimatecyclesthatrunourownworld.

TheprincipalwaythatCO2getsaddednaturallytoaplanet’satmosphereisthroughvolcanic eruptions.Molten rock explodes through the surface, ventinghuge amounts of CO2. Radar imaging of Venus shows ample evidence forvolcanism in the recent past (meaning the last hundreds ofmillions of years).Butwhat volcanoes give,water can take away. “Weathering” bywater, in theformofrainandrivers,breaksrocksdowntotheirchemicalcomponents.Later,thesemolecularcomponentscanbindwithCO2andgetpackedbackintosolidforms—that is, as new rocks. This is the basic process that creates what arecalled“carbonate”mineralslikethelimestoneunderMiami.

So,CO2belchedintoaplanet’satmosphereviavolcanoescangobackintothe ground in rocks. Eventually, the rocks are subducted (dragged down) intolower regionsof theEarth,where theymelt, allowing theCO2 to find itswaybackintotheatmospherethroughfuturevolcanoes.It’sacyclethatregulatesthecarbondioxide in theatmosphere,andtherefore theplanet’sgreenhouseeffect.It’salsoacyclethatappearstohavebeenbrokenonVenus.30

Atsomepoint,Venus likelyhadmorewater. Itmayevenhavehadoceansandbeenhospitabletolife.Butwhensomeofthatwaterevaporated,itmadeitswayhighintotheatmosphere,whereadeadlyprocessbegan.Closetotheedgeof space, ultraviolet radiation from the Sun (the same kind that causes skincancer) zapped the water molecules and broke them apart into hydrogen andoxygen. Hydrogen, being the lightest of all elements, easily escaped intointerplanetaryspaceassoonasthewatermoleculeswerebrokenapart.Withthehydrogengone,therewasnochanceforthebrokenwatermoleculestoreform.Over time, and high in its atmosphere,Venuswas bleeding its preciouswaterintospace.31

The planet’swater loss resulted inwhat scientists call a positive feedbacklooponclimate.MorewaterlossmeantlessrockerosionandlessCO2boundupin rocks.MoreCO2 in the atmosphere meant a more pronounced greenhouseeffectandhighertemperatures.Buthighertemperaturesmeantmorewaterloss,which...well,yougetthepicture.

OnEarth,thereisnodangeroflosingourwaterinthewaythatVenusdid.Ourplanet’satmospherehasaparticularlycoldlayer,abouttwelvemilesabovetheground,thatcauseswatertocondenseoutandfallasrainorsnow,keepingit

fromevermakingittoveryhighaltitudes.This“coldtrap,”asscientistscallit,mayhaveexistedatonetimeonVenus.Butatsomepoint,itsatmosphericlayerschanged, allowingwatermolecules to begin diffusing up to the heightswheretheycouldbesplitapartandlostforever.32

Withitswatersafelytrappedclosertothesurface,Earth’scarboncycleactsasanegativefeedbackonclimate.Negativefeedbackcycleskeepsmallchangesintemperaturefromgrowingoutofcontrol.ImagineifEarth’stemperaturewereto jump by a few degrees. The negative feedback begins when this highertemperature leads tomore evaporation. Thenmore evaporation leads tomorerain;more rain leads tomoreweathering; andmoreweathering leads tomoreatmospheric CO2 being drawn into rocks. Now there’s less CO2 in the air,meaning the greenhouse effect is reduced and the planet’s temperature comesbackdown.

Bygivingusanexplicitexampleofthegreenhouseeffectgonewrong,Venushelped teach us about the effects of negative and positive feedback loops onplanetaryclimate.Itmadeusthinkmoredeeplyaboutthecyclesofmatterandenergythatgiveaplanetitscharacter—orcausethatcharactertochange.FromMariner2onward,theprobeswesenttoVenusletusseeexactlyhowaplanetthatmighthavebeenakindredtwinhad,instead,becomeamonster.Usingtheearlyunderstandingdevelopedpurely fromstudyingEarth, theVenusmissionsallowedustoflexthemusclesofayoungclimatescienceandbroadenthereachof its knowledge. Like a doctor studying pathological cases of a disease tounderstand the basic workings of a healthy physiology, Venus’s runawaygreenhouse became a laboratory for understanding the complex interplay ofatmospheresandgeologythatshapeaworldlikeours.

By taking our first steps toward the planets, wewere also taking the firststepstowardunderstandingthelawsofplanets.Wewerebeginningtheprocessofusingtheworldsofoursolarsystemtounpackthegeneralandgenericlawsallplanetsmustobey.Ourearlymissionstotheplanets,ledbypioneerslikeJackJames and early theoretical studies byCarlSagan,were also our first steps ingrowingupasaplanetaryspecies.Wewereseeing,forthefirsttime,thedepthofourcommonalitywiththerestofcreation.

It’s worth noting that, while Carl Sagan got the credit he deserved forpredictingVenus’shyperactivegreenhouseeffect,hisnamewasnotonthepaperreportingMariner 2’s results.33 Early in the project, Sagan had been put onMariner’sdesignteam,wherehehad,amongotherthings,arguedforacameratobeincludedonboard(hisproposalwasrejected).ButasJackJames’sgrouppushedhardtomakeitsdeadlines,somefeltSaganwasnotpullinghisweight.

Theirmisgivingsprovedtobecorrect.AcrisishaderuptedinSagan’spersonallifethatkepthimfrommakingtheexpectedcontributionstothemission.

In 1957,whenhewas stillworkingonhisPhD,CarlSaganmarriedLynnMargulis,abrilliantbutas-yet-undirectedstudent(atthetime,herlastnamewasAlexander).Whentheymet,Margulishadnotyetsettledonscienceasacareer.Saganhelpedintroducehertoquestionsconcerninglifeandplanets.Afirewaslitintheyoungwoman’simagination,andevenastheirchildrenwereborn,shetook on the task of graduate work in biology. But Sagan’s relentless workschedule left the full burden of raising their children and managing thehouseholdtoMargulis.Afterfiveyearsoftryingtoholdthedemandsoffamilyand graduate work together, Margulis had had enough. She packed up thechildrenandleftSagantohisovercommittedworkschedule.34Butinoneofthegreat turnsofscientifichistory,LynnMarguliswouldreturntoplayanequallyimportantroleinunderstandingthecoupledhistoriesoflifeandplanets.Beforethatstorycouldplayout,however,SaganandtherestoftheworldwouldhavetocometotermswithMars.

BEDROCKMARS

StevenSquyres, thechief scientist for themultibillion-dollarMarsExplorationRoverprogram,wasnotnervous.Sure,theplanwasinsane,butthatdidn’tmeanhehadtobenervous.ItwasJanuary25,2004,landingnightfortherobotroverOpportunity. Squyres was waiting in the flight control room at NASA’s JetPropulsionLaboratorywhile,more than threehundredmillionmilesaway, theOpportunity rover was bundled in its descent capsule, hurtling at twelvethousandmilesperhourtowardMars.SinceblastingofffromEarthsixmonthsearlier,Opportunity had been on a direct path toward the Red Planet. But itwasn’t going to slowdown and ease into orbit, as in somepreviousmissions.Instead,the$400millionprobewasonastraightshottowarditslandingzoneintheMeridianiPlanum,abroadplainjustsouthofMars’sequator.

Theentry,descent,andlanding(EDL)phasecalledforOpportunitytodivestraightinfromspace,sheddingspeedviafrictionwithMars’sthinatmosphere.Asupersonicparachutewouldthenblowopen,slowingthecapsulefurther.Afterthat,ifallwentaccordingtoplan,thelanderwouldspooldown,awayfromtherestofthespacecraft,viaasixty-five-foot-longtether.Asthedescentcontinued,a cocoon of giant airbags would explosively inflate around the lander.Approximately one hundred feet from the ground, retro-rockets would fire,

bringing thewhole spacecraft to a halt.The lander, surroundedby its airbags,would hang forty feet from the ground. Then the tether would be cut away,dropping the airbag-enshrouded lander to the surface, where it would bouncelike a beach ball on steroids. Eventually, after a mile or so of bouncing, thelanderwassupposedtocometoasaferestingplaceontheMartiansurface.35

Yeah,theideawasinsane.Butitwasaninsaneideathathadalreadyworked.Justthreeweeksearlier,

Opportunity’stwin,theSpirit rover,hadbouncedtosafetyontheothersideoftheplanet.Thatsix-wheeledmobilegeologylaboratorywasalreadywanderingthe Martian surface, taking data. So Squyres was not nervous. Well, not toonervous.

There was a long wait as the JPL flight team searched for signals thatOpportunity had survived its ordeal. Then theEDLmanager yelled out to theroom,“We’redown,baby!”Theroomexplodedincheers.Opportunitywassafeonthesurface.

Within the hour, Squyres switched to the rover operations room asOpportunity’s cameras came on and his team tried to see exactly where theircreationhadcometorest.“Thepicturecomes[uponthescreen]andit’sdark,”Squyresrecalledlater.“There’ssomethingtherebutit’sunderexposed.”Slowly,theimagegetscalibrated,or“stretched.”“ThestretchhitsandinstantlyIrealizewhatI’mseeing,”Squyreswrites.“It’s impossible, it’s toogoodtobetrue, it’stoogoodtobelieve.”36

Right in front of the roverwas an exposed layerofbedrock— thekindofthingyouseeonEarthwhenyou’redrivingonaroadcutthroughhills.And,justas on Earth, the layer of exposed rock Squyres was staring at represented arecord.ItwasasandwichofcompressedMartianhistorygoingbackmillionsorbillions of years. They were staring at Mars’s planetary evolution written inrock:thescientificequivalentofpuregold.

THEREDPLANETSHUFFLE

Inaworldofinstantelectronicaccesstoallhumanknowledgeandofroutinejettravel five miles above the Earth, it’s easy to miss the audacity of the Marsrovers.GettingSpiritandOpportunity (and, later,Curiosity) safelyonMartiangroundwascrazyenough.Butthegeniusembodiedintheroversisreasontobeproud of humankind. These robot scientists have trundled across miles ofMartianlandscape,drillingintorocks,sniffingforcriticalchemicalcompounds,

andimagingtheRedPlanetathighresolution.Themissionsrepresentthebestofourcollectivevisionandcapacityforsolvingthemostchallengingproblems.

But the explorationofMarsby these rovers andother international probesrepresents something else that transcends engineering.Eachwas a step on theladderofourcomingofageasaplanetaryspecies.Byliterallygivingusvisionsofanotherworldthroughtheirhigh-resolutioncameras,anewunderstandingofotherworlds—and,perhaps,otherworldswithcivilizations—couldbeborn.Butclimbingtothatunderstandingwasfraughtwithdifficulties,asrealityshatteredourexpectationsandthenshatteredthemagain.

LikeVenus,Marswasanearlytargetofourinterplanetaryexplorations.JusttwoyearsafterJackJames’sJPLteamflewMariner2 inward toward theSun,theirMariner4probemadethejourneyoutwardtoMars,aplanetwithanevenlongerandmorestoriedplaceinourextraterrestrialimaginings.

For theMariner4,missionCarlSaganwas againon thedesign team, andthis time hewon the debate about cameras.Mariner4 carried a primitive (bytoday’sstandards)analogTVcamera.ThepicturesitsentbackinstantlychangedourdreamsofwhatMarsmightbeandwhatitmightmeantous.

BecauseofVenus’seternalcloudcover,itneverappearedasanythingmorethanawhitedisk.ButforMars,thestorywasverydifferent.Bythemid-1800s,astronomers knewMars had surface features that changed over time.This ledmanynineteenth-centuryscientiststoadramaticconclusion:Marshadaclimatelikeourown.37

Most importantly, astronomers saw that Mars had that most essential ofclimaticfeatures:seasons.WhitepolarcapsontheRedPlanethadbeenseenasfarbackastheseventeenthcentury.ThepolarcapsgrewandretreatedasMarsprogressed through its 687-day orbit. It was with good reason that in 1870,ClaudeFlammarionenvisionedMarsasaworldrifewithlife.38

By the turnof the twentieth century, theMars storygained anew level ofdramaviaPercivalLowell’sobsessionwiththeRedPlanet.Lowell’sfascinationhadbegunwithearlierstudiesbytheItalianastronomerGiovanniSchiaparelli,whichappeared to show long, straight featureson the surface.Lowell claimedthese were canals, representing the work of an intelligent civilization.39 Inpopularbooks,LowellarguedforcefullythatMarswasinhabitedanditssocietywas, in effect, a victim of climate change. The planet was drying up and thecanalswere adesperate attempt tobringwater from thepolar ice caps.Whilemost astronomers dismissed Lowell’s observations as wishful thinking, in thepopularimaginationthediehadbeencast.ThroughbookslikeH.G.Wells’sWarof theWorlds,Mars became the alienworldmost people imagined to host an

aliencivilization.Bythemid-twentiethcentury,astronomershadalreadyaccumulatedenough

telescopic evidence to be confident that Mars was not home to an advancedcivilization.The atmosphere appeared tobe thin and theplanet cold.Still, thepossibility that life existed in some form on that world remained very real.Periodically, the planet experienced significant changes in color that someargued had a biological origin.40 As Mariner 4 was launched, Carl SaganremainedhopefulthatMarsmightbehometoatleastsomekindsofvegetationor,attheleast,microbes.

ButwhenMariner4sailedpasttheRedPlanetonJuly14,1965,thetwenty-twoimagesitsentbackkilledthedreamoflifeonMarsinboththepublicandscientificimaginations.

Itwasthecratersthatdidit.Mariner4 sawa lot of craters onMars, and some of themwere vast.On

Earth,cratersdon’tlastlong.Whethertheyformfromvolcanoesorfrommeteorimpacts,mostcratersonEarthgeterasedaftermanymillionsofyears. It’s thefamiliarprocessesofweatheringbywindandwaterthatwipethecratersaway.Seeing large craters on Mars meant its surface hadn’t changed in billions ofyears.Mariner 4 showed us a Mars that looked a whole lot like the empty,desiccatedMoon.41

Inthewakeofthenewpictures,aNewYorkTimeseditorialannouncedtoitsreaders,“TheastronomersofpastdecadeswhothoughttheydetectedcanalsontheMartiansurfaceandspeculatedthatitmighthavebustlingcitiesandbeingsengaged in livelycommercewerevictimsof theirownfantasies.”Concluding,“The redplanet isnotonlyaplanetwithout lifenowbutprobablyalwayshasbeen.”42

First Venus and thenMars. The main accomplishment of humanity’s firstinterplanetaryemissariesseemedtobethedeathofourinterplanetarydreamsoflifeonotherworlds.

Luckily,Marsdidn’tstaydeadforlong.In1971,Mariner9becamethefirstspacecraft to park at a planet’s doorstep. Rather than just zipping by at tenthousandmilesperhour,Mariner9went intoorbit around theRedPlanet.Bytaking up residency this way, the probe found Mars’s story to be far morecomplicatedandfarmoreinteresting.43

Mariner 9 was built to map a good deal of the planet’s surface.When itarrived,however, it foundMarscovered inaglobe-swaddlingdust storm.Thesurfacewastotallyobscured.Becausethespaceprobehadbeenbuiltwithsomeinherentsoftwareflexibility,NASAengineerswereabletodelaythemappingtill

thestormabated(twoRussianprobesthatarrivedatthesametimeasMariner9had no such flexibility and returned little useful data).WhileMariner’sworkwas delayed, the planet-encircling storm highlighted the critical role airborneparticles(thatis,dust)couldplayinshapingclimate.44Intheyearstocome,thatlinkwouldbecomeapoliticalfootballforearthboundpolicymakers.

Eventually, the storm cleared and Mariner 9 returned more than seventhousand images. In those pictureswas our first hint that,while today’sMarsmaybebone-dryandfrozen,Marsofthepastmighthavebeenaverydifferentkindofworld.Thepivotdependedentirelyonwater.

Mariner 9 revealed landscapes that looked a whole lot like they’d beencarvedbyflowingwater.Thereweredryriverbedsandbroaddeltas.Therewerefloodplains and rainfall basins. Confirmation that these features really wereshapedby torrentsof liquidwaterwouldhave towait for futuremissions.ButwhatMariner9 immediately told uswas simple and profound: the planet hadchangedinabigway.45

Mariner also revealed that our smaller neighborwas a planet as unique asour own. Mars was home to Olympus Mons, a towering volcano that risesalmostfourteenmilesfromtheplanet’ssurface.ItalsohostsVallesMarineris,afour-mile-deep canyon the size of North America that put the puny crack inArizonawecall“Grand”intoanew,cosmicperspective.46Mars, it turnedout,hadvolcanoesandvalleys,craggyhighlandsandsmooth,broadlowlands.Itwasaplaceallitsown,withtourism-worthysitesunlikeanywhereonEarth.Andallthis topography would matter as the first attempts to understand the Martianclimategotunderway.

TheviewoftheNirgalVallischannelsonMarstakenbyMariner9in1971.ImageslikethesewerethefirstindicationthatMarsoncehadwaterflowingonitssurface.

ThenextgreatstepinrevivingthepossibilityoflifeonMarscamewiththetwoViking landers that touched down via parachutes and retro-rockets in thesummer of 1976. Once again, Carl Sagan played an integral role, designinglanderexperimentsthatlookedformicrobiallifeintheMartiansoil.Thebiologyexperiments returnedambiguousresults,but theViking landers’meteorologicalstations allowed us to see, for the first time, what the weather was like onanotherplanet.47EachMartianday (calleda sol), theViking landers sent backmeasurementsoftemperature,pressure,andwind.Thedataflowedforsixyears,untilonelanderfailedandtheotherwasturnedoffbymistake.48ThroughVikingwewere on ourway toward seeingweather and climate on otherworlds as acousintoourown.

With theadventof theMartian rovers in the2000s, themantraofNASA’sMarsprogrambecame“followthewater.”IflifehadonceexistedinMars,we’dfirsthavetoprovetheplanetwasoncewetenoughandwarmenoughtosupportlife.49 But the presence of surface water can never be separated from thequestionofclimate.Sobyfollowingthewater,NASAalsocommitted itself tounpackingthestoryofMartianclimateandMartianclimatechange.LikeVenus,theRedPlanetwasactingasaguideforunderstandingourownworld.

THEGREATMARTIANCLIMATEMACHINE

Robert Haberle wasn’t planning on becoming a world expert on the Martianclimate. After serving in Vietnam, Haberle returned to civilian life in 1968,kicking around Europe for a while, “being young and anxious to explore theworld.” Finally, starting in college at San Jose State, he needed to declare amajor.“Iwaslookingthroughthecatalogueandsawmeteorology,”herecalledtomeinaninterview.“Ithoughtthatmeantthestudyofmeteors.Mywifehadtoexplain tome itwas about theweather.”50 Itwas an unlikely beginning for amanwhowouldeventuallyhelpdevelopNASA’spremierMarsGlobalClimateModel, one of the world’s most powerful tools for studying the Red Planet’shistory.

The model’s own history dates back to the late 1960s, when pioneersConwayLeovyandJimPollack tookaclimatemodeldevelopedforEarthandbegan adapting it forMars.51 Pollack was one of Carl Sagan’s first graduatestudents, and they collaborated together for years. Leovywas an atmosphericpluralist.Hewanted to build a version of climate studies that reached beyondEarthtoembraceeveryplanetwithanatmosphere.

For scientists, the word climate refers to long-term patterns of weather.While theweatherchanges fromday today (sunnyonTuesdaybut rainingonWednesday), climate represents the long-term patterns ofwinds, precipitation,ice cover, andocean flow.Tomakea climatemodel, scientistsmust solve thephysicsequationsgoverning theseprocesses.Thatmeans a climate “model” isreallyamathematicalphysicsmodel.It’sadescriptionoftheworldthatusesthehighlyspecificandveryexactinglanguageofmathematicalphysics.

Justasarchitectsmakemodelsofaskyscraperoutofpaper,balsawood,andplastic, scientists use the laws of physics, expressed in the language ofmathematics, to construct models of a physical system. If it’s a gas enginethey’re modeling, then the mathematics lets them understand and predictsomethingliketheengine’sfuelconsumption.Ifit’sabridgethey’remodeling,thenthemathematicsletsthemunderstandandpredicthowmanycarscansafelytravel from one side of the bridge to the other. And if it’s a planet’s climatethey’re modeling, then the mathematics lets them understand and predict thelong-termpatternsoftemperature,cloudcover,andsoon.

Tobeeffective,however,aclimatemodelneedsalotof“movingparts.”Itneeds to describe a lot of different kinds of physics, chemistry, and, perhaps,otherprocessesaswell.Itmustaccountfortheflowofatmosphereonaspinningplanet. It has to describe how radiation from the Sun warms the air near the

surface,causinggases to rise. Itmustdealwithhowsomeof thosegases, likewatervapororcarbondioxide,willcondense into liquidsor icewhentheygetcold(that’showthemodelstrackcloudformation,rain,andsnowfall).Buildinga climatemodel that gets the answers right (meaning itmatches observations)requiresyearsofinsanelyhardwork.

It also requires a lot of equations to describe the combined action ofatmospheric flow, condensation, and the movement of radiation. Each one ispretty complicated on its own, taking a lot of human ingenuity tomaster.Butsolvingallthecomplicatedequationstogetheratthesametimeissimplybeyondthe intellectualpowerof anyoneperson.So tomakeprogress, scientistsmustturn to digital computers that solve the equations in tiny steps, over and overagain, billions of times each second. In this way, the computers animate theequations.Theybringdetailshiddeninthemathematicalcomplexitytolife.AndthemodelsHaberleandothersbuiltdidjustthat.TheybroughtMartianclimateto life for scientists. Through the models, researchers could see the fullcomplexity of Mars’s climate. Most important, they could see both thesimilarities and the differences in how it worked relative to that of our ownworld.

JUSTLIKEEARTH,ONLYIT’SNOT

“Allplanetsaresubjecttothesamebasicforces,”saysRobertHaberle.“It’sjustthat thestrengthof thoseforceswillbedifferentondifferentplanets.”52WhileMarstodaymaybeafrozen,aridworldutterlyunlikeEarth,themechanicsofitsclimate bear essential similarities to ours. Let’s startwith its differences fromEarth.WhileVenushasa lotmoreatmosphere thanourplanet,Marshasa lotless.Thesurfacepressure readoffby theViking landersand theotherMartianweatherstationsislessthanonepercentofwhatwegetonEarth.Thatmeansthetotal weight of Mars’s blanket of gases is 99 percent less than Earth’s. LikeVenus, most of Mars’s atmosphere is made up of CO2. But with so littleatmospheretogoaround,Marsdoesn’tgetawholelotofgreenhousewarming.Typical nighttime lows go down to –128 degrees Fahrenheit, while daytimehighsonlygetashighas–24degreesFahrenheit.53Marsisdefinitelyaplacetochill.

It’salsoadesert.There’svery littlewater inMars’satmosphere—just0.01percentofwhat’s found inEarth’s.54Since theatmosphericpressure is so low,exposedliquidwaterboilsawayinseconds.Thisisthesameeffectyougetwhen

youtrytoboilwaterhighinthemountains—thewaterdoesn’tneedtogetveryhot before it turns to vapor. That’s why the water that does exist onMars iseither gaseous (water vapor) or locked up in ice at the poles. There may,however,bealotofwaterunderground,asiceoreveninliquidform.

So, depending onwhich part of your spacesuit failed, conditions onMarstodaywouldquicklykillyou,eitherfromasphyxiationorhypothermia.Andyet,forallMars’sdifferencesfromEarth,theMartianclimatemachinestilloperatesinwaysveryfamiliartoearthlings.

ImagineforamomentyouareaPortuguesesailorinthe1400s.You’retryingtogetfromWestAfrica,whereyou’vebeentrading,backtoPortugal.Ifyoutrysailingdirectlynorthward,you’llfindstormsandvariablewindsthatmoveyoualong at a sluggish pace. But if you try something crazy and sail west—outdeeper into theAtlantic and away fromPortugal—youget a pleasant surprise.Sailfarenoughwestandyouhitbeautiful,steadywindsthatwillcarryyoubackeastandnorth.You’rehomeinPortugalinnotime.Whatyou’vediscoveredarethetradewinds.55

A couple of hundred years after European sailors stumbled on the tradewinds, English lawyer and naturalist George Hadley found their explanation.The tradewindsaregiant riversofair,drivenbysolarheatingand theEarth’srotation.Hadleyrecognizedthathotairinthetropicsalwaysrisesupward,whilecold air at the poles always sinks. The air in between has to fill in the gaps,leadingtoagiantequator-to-polepatternofcirculation.56

If theplanetweren’tspinning, thatwouldbe theendof thestory:up/downand north/south motions. It’s Earth’s rotation that bends the equator-to-poleatmosphericconveyorbeltthroughwhat’scalledtheCoriolisforce,whichtwiststheflow,addinganeast/westcomponenttothecirculation.Thebigcircularflowin the North Atlantic is one of these giant rivers of air. In the southernhemisphere,there’samirror-imagetradewindpattern(theeast/westdirectionisflippedbecausethedirectionoftheCoriolisforcechangesacrosstheequator).Intotal,Earthhassixofthesevast,circulatingatmosphericflows,andthestrongestofthese,flowingjustaboveandbelowtheequator,arecalledHadleycells.

Mars, like Earth, is spinning. At 24.7 hours, the length of its day isremarkablyclose toEarth’s.57 Since the lawsof physicsdon’t carewhereyoulive,Mars’searthlikespinshouldmeanHadleycellsappearontheRedPlanet,justastheydoonourworld.“It’soneofthefirstthingsthatcomesoutofagoodMarsclimatemodel,” saysHaberle.“Youseebigcirculationpatterns from theMartianequatortopoleandbackagain.”58

TheHadleycellisnottheonlyfamiliarclimatepatternonMars.“Marshas

jet streams,” saysHaberle, referring to the rivers of fast-moving air that existhigh in Earth’s atmosphere. “Every rotating planet with an atmosphere hasthem.” And, just as on Earth, sometimes those jet streams will buckle andwander. Atmospheric scientists call these flow patterns “Rossby waves,” andtheywerethecauseofthedreaded“polarvortex”thatbroughtrecord-coldairtoinhabitantsontheEastCoastinthewinterof2014.59

Whiletheir technicaldetailscanbedaunting,Hadleycells, jetstreams,andRossby waves all show us something profoundly simple and important: thephysics of climate is universal. All worlds obey the same rules: Earth,Mars,Venus,evenanexoplanetahundredlight-yearsaway.Mostimportantly,theyarerules thatwenowunderstandbecausewe’veseen themworkingonmore thanoneplanet.

HABITABLEWORLDS,SUSTAINABLEWORLDS

IfyouwanttoknowtheweatheronMarsrightnow,thereisanappforthat.60TheCuriosityrover,whichlandedin2012,includesameteorologicalstationthatbeamsconditionsbacktoEarthforanyandalltosee.Followtheappforawholeday, and you’ll see the temperature rise and fall between very un-earthlikeextremes. You’ll also see the atmospheric pressure change in ways that aredefinitelynotwitnessedonourworld.

Onanygivenday,theamountofatmospherepressingdownontheMartiansurfacecanchangebyasmuchas10percent.That’s almost likebeing inLosAngelesinthemorningandthenclimbingthemileuptoDenver’sthinnerairafew hours later, only to return again to sea level by nightfall. For our story,what’s important about these changes is that the dramatic pressure swings arecompletelycapturedintheMartianclimatemodels.ThereissolittleaironMarsthat,oncetheSunbeginswarmingthesurfaceanddrivinghotterairupward,theentire planet’s atmosphere readjusts, sending pressurewaves from one side oftheglobetotheother.Allthemodelstrackthesereadjustmentsandnailthedailyair pressure variations. In other words, the climate models get these answersright.61Thataloneisanimportantpointforourearthboundclimatedebate.Weunderstandclimatewellenoughtopredictitonotherplanets.

But thesuccessinunderstandingMars’sclimatetodayhighlights thesinglemost important lesson Mars has taught us: climate changes, and with it,habitabilitychanges,too.

Habitability is a key concept for astrobiologists, who think of it in an

intuitive way: the ability of a planet to be inhabited by life. In the Drakeequation,theformaldefinitionofhabitabilityisthepresenceofliquidwateronaplanet’ssurface.

TherobotswesenttoMarsofferusprettydefinitiveevidencethatMarsoncehadliquidwateronitssurface.Someofthatevidenceisgeologicalandcomesviamineralogy.TheexposedMartianbedrockinfrontofOpportunitynotonlysentStevenSquyres intofitsof joy,eventually italsorevealed thepresenceofsmall, sphericalpebbles termed“blueberries.” Instrumentsembedded in the tipoftherover’smulti-jointedarmallowedSquyresandhisteamtorecognizetheseblueberries as hematite, a mineral that only forms in the presence of liquidwater.62

SomeoftheevidenceforawetversionofMarswasmoredirect.Sevenyearsafterthediscoveryoftheblueberries, theCuriosityroverfoundasetofcarvedrock features on Mars that could only have resulted from deep, fast-movingwater flows.Curiosity scientists could even estimate the nature of the flow—abouthipdeepandrushingdownstreamatthreefeetasecond.63

So,Marsoncehad liquidwateron its surface.But thatmeans itmustalsohavehadamuchthickeratmospherekeepingthatwaterfromflashingawayintovapor.Andifthewaterwasrushingonthesurface,thatthickatmospheremustalsohavebeenwarmingtheplanettotemperatureswellabovefreezing.Putitalltogether,anditseemstheRedPlanetwasonceblue,atleastforawhile.

Scientists call this warmer, wetter period of Martian climate history theNoachian, for the story of Noah and the flood. Their best estimates place itbetween4billionand3.5billionyearsago.64Thereremaindeepquestionsaboutwhat happened to thewater onMars.Getting answers to those questionsmayhavetowaituntilwecansendactualgeologiststotheRedPlanet.

Butevenaswewait for thoseanswers, the recognitionofMars’sdramaticclimatic change already offers us a critical astrobiological perspective on ourownAnthropocene era.Mars shows us that habitability—thatmost critical ofastrobiologicalconcepts—isnotforever.Aplanetcanchangeitshabitablestate.Mostimportantly,itcanloseitentirely.

When we worry over our entry into the Anthropocene, we are inherentlyconcerned with our project of civilization’s sustainability. But what issustainability but a special example of habitability? What we are reallyconcernedwithwhenwetalkabout theAnthropocene is thehabitabilityof theplanet for a particular kind of energy-intensive, globally interdependent,technologicalcivilization.Thepresentclimateepoch—theHolocene—hasbeenparticularlyhabitableforthatkindofproject.

Marsshowsusthathabitabilitycanbeamovingtarget.ThesameislikelytobetrueofsustainabilityintheAnthropocene.Planetschange,andthatisalessonMars and its history help us come to termswith. It is not, however, the onlylessontheotherworldsinoursolarsystemhavetoteachus.

REMARKABLEJOURNEYS

OnJune12,1982,CentralParkhostedaseaofhumanity.SpillingovertheGreatLawnandontoFifthAvenue,theparkwasthrongedasneverbeforeinits150-year history. The New York Times reported that there were “pacifists andanarchists, children and Buddhist monks, Roman Catholic bishops andCommunistPartyleaders,universitystudentsandunionmembers.”DelegationshadarrivedfromVermontandMontana,BangladeshandZambia.“Thesmiling,hand-clappinglineofmarchersthreadingintotheparkstretchedbackthreemilesalong Fifth Avenue.” According to the Times, it was “the largest politicaldemonstrationinUShistory.”65Allthosedelegationsandallthosepeoplewereintheparkforonereason:tosavetheworld.

Theshadowofnuclearwarfare,whichloomedsolargeasthefinalfactorinFrankDrake’s equation,hadgrown longer anddarkerby theearly1980s.TheelectionofRonaldReaganaspresident,alongwithrenewedaggressiveactionsbytheSovietUnion,seemed,onceagain,toedgetheworldclosertoanall-outnuclearexchange.By1982,thetwosuperpowershadincreasedtheirstockpiletomorethanfiftythousandnuclearweapons.66ThemassiverallyinNewYorkwasintendedtobuildsupportfora“nuclearfreeze”—anendtotheweaponsbuildupand the beginning of a nuclear drawdown. But neither the US nor Russiangovernmentwaslistening.

Inresponse,anewkindofpeacemovementgrew.Itwaslargerandbroaderthananything thecoldwarriorsof the1960shadbeen forced tocontendwith.While the Central Park rally marked the nuclear freeze movement’s rise topolitical relevance, its framing of humanity’s basic nuclear dilemma differedsignificantly from that of the ColdWar era twenty years before, when FrankDrake formulated his final factor. This shift became apparent a year after themassive rally, when a group of scientists published a study that changed thelanguageofnuclearwar.

The paper was titled “Nuclear Winter: Global Consequences of MultipleNuclearExplosions.”CarlSaganand JamesPollackwerebothon the teamofauthorswhowere collectively referred to as TTAPS (Richard P. Turco,Owen

Toon, Thomas P. Ackerman, Pollack, Sagan). The TTAPS argument wasstraightforward:evenamedium-scalenuclearexchangewouldleadtosomanyfires that soot lofted into the atmosphere would significantly cool the planet.Agricultural production would seize up and the world would be plunged intohungerandchaos.The lessonfromtheirstudywasstraightforward too:almostany nuclear exchange could transform the planet in dangerous ways. Theweaponscouldneverbeused.67

By this time,CarlSaganhadbecomeacelebrityviahisbest-sellingbooksand his TV appearances. He highlighted the TTAPS study with an extendedessayinParademagazine.68

While theReagan administration publicly dismissed the science of nuclearwinter, the majority of the scientific community took it seriously. From thatpoint,therewasnogoingback.“Nuclearwinter”enteredtheworld’svocabularyand its imagination. Years later, both Soviet and American officials wouldopenly discuss how nuclear winter’s doomsday scenario helped draw the twonationstothenegotiatingtable.69

Theentryofnuclearwinterintothepoliticallandscapewasnotablefortworeasons.First,itwasaresultbasedonaclimatemodel.Pollack,Sagan,andtheircollaboratorshadused themathematicalphysicsgoverningglobal atmosphericflowstotrackthebehaviorofparticlesblownintotheairbyglobalfires.Forthefirst time in human history, amodel of a planet’s climatewould frame globalpoliticaldebate.Butit’sasecondfeatureofthedebatethatmattersmostforourmoment.AkeyargumentfornuclearwintercamefromMars.

Theglobe-engulfingMartianduststorms,firstobservedindetailbyMariner9,providedcriticaldataforthenuclearwinterresearchers.Thebehavioroftinyparticlescarriedhighintotheatmospherewouldhavebeenmeretheorywithouttheflotillaofprobeswe’dsenttoMars.Withthedatatheysupplied,theMartianclimatemodelswereexpandedto includenewlyrealizedphysicalprinciplesofhowsolarandinfraredradiationinteractedwithdust.Thus,thespaceprobesandthe climate models revealed the powerful effect of dust on the Martianatmosphere.Thatunderstandingwasthentransferredtothedistinctlyterrestrialproblemoffiresrangingacrosstheplanetafteranuclearwar.TheTTAPSpaperwasexplicitincallingMarsoutasatestbedfornuclearwinterphysics.

ThehistoryofTTAPSandnuclearwinter showsus thatknowledgegainedfromanalienworldhasalreadyinfluencedearthbounddebatesaboutourfuture.Now,thirtyyearslaterandinthemidstofourmodernclimatedebates,wemustrecognize how deeply our understanding of climate is rooted in what we’velearned from “wheels-on-the-ground” studies of other planets. The desperate

attempt by climate-change deniers to sow doubt on climate science (and itsmodelingefforts)willfullyignoreswhatfivedecadesofspacetravelhavetaughtus: we havemore than one world, and one story, to school us in the ways aplanetcanchange.

We humans sent exemplars of our ingenuity to Venus and Mars. Later,humanity’srobotemissarieswouldreachtheouterworldsofJupiterandSaturn(and their remarkableocean-bearingmoons).By2016, everyplanet andeveryclass of solar system object had been visited at least once by our probes.Asteroids, comets, anddwarfplanets—wehad“touched” themall andwehadlearnedfromthemall.

Inmaking those remarkable journeys,wedidmore thansimplysatisfyourcuriosity or beat other nations for bragging rights.While we might not haveknown it at the time, thesemissions to other planets were also giving us theconceptual tools we now need to make fateful decisions about our own still-unknownfate.

WecouldnothaveunderstoodthegreenhouseeffectaswedonowwithoutwhatwaslearnedfromtherobotprobestoVenus.Wecouldnothaveunderstoodthe process of climate modeling as we do now without the rovers trundlingacross Mars. And the atmospheres of Jupiter, Saturn, and other solar-systemworldshaveeachprovidedtheirownlessons.Wetraveledbillionsofmilesonlytoseeourplanetandourownpredicamentcomeintohighresolution.

CHAPTER3

THEMASKSOFEARTH

AIRLESSONS

ImagineyouareatimetravelerwhojustlandedonEarth2.7billionyearsago.As you step outside on this younger version of our planet, what’s your firstexperience?Theanswerisprettysimple.

Youdie.Tobespecific,youasphyxiate.ForaboutthefirsttwobillionyearsofEarth’s

history, itsatmospherecontainedonlyminute tracesofoxygen,even though ithadlongbeenahometolife.Foralmosthalftheplanet’shistory,its“air”wascomposed almost entirely of nitrogen and CO2.1 Today, however, the Earth’satmosphereisalmostallnitrogenandoxygen,withonlyatinyfractionofCO2.Whathappenedtomakesogreatachange?

Thisoneall-importantdetailaboutEarth’shistory—theriseofitsoxygen—isalessonforustoday.Itwaslife,actingonaglobalscalebillionsofyearsago,that altered the planet’s atmosphere. In doing so, it also changed the futurehistoryoftheEarth,leadingtohumansandourprojectofcivilization.Nowlife,in the form of our civilization, is once again poised to alter the planet’satmosphere and its complex machinery of evolution. The comparison of thattime, billions of years past, with our ownmoment of climate change offers adoorwayintotheremarkablestoryofthe“masksofEarth.”Itsnarrativebearsatruthfewofusrecognize.

Ourworldhasbeenmanyplanetsinthepast.These other versions of Earth were profoundly different from the cloud-

mottled,blue-greenworldweknowtoday.Eachwasaconsequenceofplanetaryforcesshapingandthenreshapingourworld.Together,theyrevealhowdeeplyhumansandourprojectarepartofamuchlongerstory.Whenitcomes to lifechangingtheplanet,weareneitheruniquenorunusual.That’swhythestoryofourplanet’spast,astorythatisfundamentallyastrobiological,issocriticaltous.KnowingtheEarthsthatwerewillgiveus thevocabulary tocraftanewstory,onethatkeepsuspartoftheEarthsoontobe.

NOEASYDAY

The Polecat arctic transports were beasts. Designed for duty in the harshestconditions, each was the size of a minibus. The caterpillar-treaded special-purpose vehicleswere builtwide to keep them steady on uneven terrain,withpowerful diesel engines for hauling cargo and personnel across ice, snow, orevenupthesteepsideofaglacier.2

OnOctober16,1960, the sideofaglacierwasallyoungSorenGregersensaw as he looked out the window of his assigned Polecat. Just a few hoursearlier,Gregersen,aseventeen-year-oldDanishBoyScout,hadbeenstuffedintothePolecat’scabbyasmilingGI.Twodaysbeforethat,he’dlandedattheUSAirForce’sThuleAirBaseonthewesternshoreofGreenlandandbeenoutfittedwithregulationcold-weathermilitarygear.GregersenwatchedinwonderasthePolecatbeganitslongtrekupthe“ramp,”aslopingroadcarvedintotheglacialice.Hewasbeginninga150-miletrekoutontooneofEarth’smostinhospitablelocales.3

Bouncing around the Polecat’s cab, Gregersen was caught somewherebetween excitement and terror.After all his hopes, preparations, and travel, itwasreallyhappening.HewasonhiswaytoCampCentury,acitytheAmericansbuiltundertheice.

AtthesametimethatJackJameswasblastingMarinerprobestotheplanetsandFrankDrakewastuninghisradiotelescopeinsearchofaliencivilizations,theUSmilitarywas pushing audaciously across a different kind of boundary.This one,whereBoyScout SorenGregersenwas bound, lay at the top of theworld.

Greenlandisagianticeslabwheresevenhundredthousandsquaremilesofglacierriseamileandahalfabovesealevel.4Temperaturesatthecenterofitsvast ice plateau typically drop to a Marslike –70 degrees Fahrenheit. Windsroutinelysweepacrossitsbarrenplainsofsnowat125milesperhour.5Andyet,

in 1959, the US government chose to build a military base and a scientificlaboratoryrightinthemiddleofGreenland’sfrozenemptiness.

ThelogicoftheColdWarledtheUStoplantheimpossibleintheformofCampCentury.Thebaseconsistedoftwenty-onetrenchesdugintotheice,eachup to three football fields long. Each trench was twenty-six feet wide andtwenty-six feet deep,with snowpackedacross steel arches to create a ceiling.Prefab buildings, hauled across the glaciers, had been laid into each trench toserve as barracks for the camp’s two hundred servicemen and scientists.Poweringthebaserequireda$5millionportablenuclearreactorthatthemilitarydragged out across the ice sheet.6 Taken together, building Camp Centuryrequired a monumental effort, but one that would achieve a monumentalbreakthrough.

ThebarracksbuiltinsideoneoftheicetunnelsatCampCentury.

In our era, when people who know nothing of climate science makesweeping claims of sweeping ignorance, it’s important to remember the risksrequired to make that science happen. The soldiers and scientists at Camp

Century lived at the edge of the world, and their work carried considerabledangers.Transport flights and crewshad to contendwith extremesofweatherunlikealmostanywhereelseontheplanet.Inthesummerof1961,ahelicoptercrash outside the base took the lives of all six aboard.7 But those GIs, theirofficers, the scientists, and even Boy Scout Soren Gregersen had all come toGreenland’sglacialwastelandonamission.“Itwasthemostexcitingthingthatever happened to me,” recalled Gregersen, now a retired professor ofgeophysics,whenIspokewithhim.“Thatexperienceiswhatgotmestartedinscience.”

Camp Century was a joint US–Danish effort (Greenland is a Danishterritory). To create publicity for the polar mission, the Boy Scouts in bothcountries held competitions seeking “junior scientific aides.” In late 1959,Gregersen and American Boy Scout Kent Goering each won their chance tospendfivemonthson,andin,theice.

“WelivedrightalongsidetheGIs,”saysGregersen.“Everyday,wegotsometaskrequiredtomaintainthebase.Sometimesitwaschoppingawaytheicethatconstantlygrewinsidethetunnels.Sometimesitwasworkingonthepumpsthatfedanenormousfreshwaterreservoirdeepintheice.Iloveditall,andallofitwasthrilling.”

But for young Gregersen, it was the science that made the strongestimpression.ThereweremanyreasonswhytheUSmilitarybuiltCampCentury.Planshadbeendiscussedtohousenuclearmissilesintheice(theever-shiftingglacierskilledthatidea).8TherewasalsotheneedtokeepwatchontheSoviets.Gregersen remembers thevast radar arrays, pointingnorth, atThuleAirBase.Butthemilitarywasespeciallyinterestedinclimate.Thehistoryofwarfarewas,afterall,fullofmilitarycampaignsdoneinbyweather.Justas thefundingfortheexplorationofspacewasopenedbytheColdWar,theEarth’sclimateanditshistory had also become a military concern. That translated into funding forclimate science. Themoney took scientists to themost remote corners of theplanet. It was also how young Soren Gregersen first saw the drills at CampCentury.

Inroomscarvedfromcenturiesoffallensnow,CampCenturyscientistssetupdrillingderricks,likethekindyou’dseeinoilcountry.Theirgoalwastodivedownward through almost a mile of ice and thousands of years of planetaryhistory.9“Isawtheeffortbeingmadeinthoseicedrillinglabs,”saysGregersen.“And itmade a huge impression onme.What theywere trying to do—it justseemedamazing—recoveringthehistoryoftheplanetusingancientsnow.”

Transformativevisionsoftheworldusuallycomewhenwefindnewwaysto

seeit.Inscience,theabilitytogettonewkindsofdata—literallynewwaysofseeing—allowsustoreviseandrefreshourunderstanding.JackJames’sMarinermission toVenus, Carl Sagan’sMartian dust data, and the radio telescopes atFrank Drake’s Green Bank observatory all rewrote our understanding ofastronomy and planetary science. In the decades after World War II, ourunderstandingoftheEarthwasalsobeingreimaginedbynewdatathathadbeenbeyond the reachofearliergenerationsof researchers.CampCenturywasonecriticalchapterinthestoryofthatchange.

Iceageswerestillamysteryin1960.Themostcertainthingscientistscouldsayaboutthemwasthatthey’dhappened.Overthelastfewmillionyears,mile-thick slabs of ice covered much of the Northern Hemisphere. At least fourdifferent times, they ground their way south and then retreated back.10 Eachglacial epoch left the planet cold and dry. Ocean levels dropped almost fourhundred feet—the height of a forty-story building—as somuch of theEarth’swaterbecamelockedinice.Inbetweentheiceages,theplanetgotreprievesintheformofwarmer,wetterinterglacialstates.11

TheEarthenduredthelasticeageforalmostahundredthousandyears.Onlyafter the final laggard glaciers retreated did the project of human civilizationbegin. Our history of farming and cities, writing and machine building fitsentirely within the Holocene: the current ten-thousand-year-old interglacialperiod.12AndeventhoughscientistsknewthebasicsequenceofeventsleadingtotheHolocene,thedetailsofhowtheclimateslippedfromonestatetoanothereludedthem.Theysimplydidn’thavethedatatoseethedetailsofthechange.Whattheyneededwasawaytofollowtheplanet’stemperature,yearbyyear,allthe way back to when glaciers were last king. Under the auspices of the USArmy’s Cold Regions Research and Engineering Laboratory, Camp Century’sdrillingoperationgavescientiststhatrecord.

TheworkwasledbytheDanishscientistWilliDansgaardandtheAmericangeophysicistChesterLangway.Themile-thickslaboficecoveringGreenlandismaintained by yearly layers of snowfall, packed one on top of the other. Thestrataofice,builtupyearbyyearoverthemillennia,formakindoffrozenlayercake.Eachlayercomprisesarecordofthatyear’sclimate.Withineachlayerofice was a chemical marker that served as a proxy thermometer. Using it,scientists built a high-resolution recording of Greenland’s temperatures goingbackthousandsofyears.13

After six relentless years of work, Dansgaard, Langway, and their CampCenturyteamdrilledallthewaydowntobedrock,morethanfourthousandfeetbelowthetopoftheicesheet.Oncethe“icecore”dataretrievedbythedrilling

wasconvertedintotemperatures,DansgaardandhiscolleaguescouldseeEarth’spassage out of the last ice age.Moving backward, they first saw a period ofroughlyconstanttemperaturestretchingbackeightthousandyears.ThiswastheHolocene,thetimeduringwhichhumancivilizationhadbeenbornandgrowntothrive. Going farther backward, they could also see the transition from thewarmthofourcurrentclimatetothefrozenglacialagemorethantenthousandyearsago(thePleistocene).14

Alongwith thesmooth transitionfromthe last iceage to thecurrentwarminterglacialperiod, theCampCenturydataalso showeda seriesof spectacularshort-term shifts thatwould come to haunt our climate future.Around twelvethousandyearsago,inaperiodcalledtheYoungerDryas,theplanetappearedtodropfromawarmingstatebackintotheicebox.Itwasastunningdiscovery.Injustamatterofdecades,averagetemperaturesaroundtheplanethaddroppedbyfivedegreesFahrenheitinsomeplacesandasmuchastwenty-sevendegreesinothers.15Ifcomparablydramaticglobalchangesoccurredinthemodernera,it’shardtoimagineourprojectofcivilizationmakingitthroughintact.

Later drilling work in Greenland and Antarctica confirmed the CampCentury studies. One American researcher working in Antarctica recalls amoment of truth when just looking at an ice coremade the speed of climatechangeapparent.Theicechangingfromlighttodarkacrossjustafewinchesinthe core was a visceral confirmation of abrupt large-scale swings in globalclimate.

ThehistoryofGreenlandtemperaturesbasedonicecorerecords.

The recognition of rapid climate change presented researchers with awarning the importance of which they could not yet understand. At the time,human-driven, or “anthropogenic,” climate change was nothing more than a

possibilitydiscussedinthemostabstracttermsatmeetingsofscientificexperts.Almostnoonewas ready toconclude that thekindof rapidclimate shift seentwelvethousandyearsagomightbesomethingwecoulddrivethroughourownactions.

WHICHEARTH?

WhateverquietpreparationsweregoingoninhomesacrosstheEarth,WilliamAnderswas not part of them. That’s becauseAnderswas on a spaceship.OnChristmasEve1968, twohundred thousandmiles from theplanetofhisbirth,Anders and fellowApollo8 astronauts FrankBorman and JamesLowellwerebecomingthefirsthumanstoorbittheMoon.

“Ohmy God,” Anders said to his crewmates as he marveled at the viewoutside the small window of his Apollo commandmodule. “Here’s the Earthcoming up,” he said, looking out across the moon’s horizon. “Wow, is thatpretty.”

AndersaskedforarollofcolorfilmwhileBormanjoked,“Heydon’t takethat[picture],it’snotscheduled.”Loadingupthecamera,Andersstoppedforamomenttoconsiderthemagnitudeofthevisionbeforehim.Thenhesnappedanimageoftheworldthatwouldchangetheworld.16

TheiconicEarthrisephotographtakenbyWilliamAndersduringtheApollo8missionin1968.

CalledEarthrise,Anders’spictureoftheblueEarthhangingabovethegraymoonscapebecameiconic.Lifemagazinenameditoneoftheonehundredmostinfluentialimagesinhumanhistory.17Sincethen,space-basedpicturesofazureoceans, swirling white clouds, and green-brown continents have becomefamiliar. But that familiarity is undercut by a striking truth that has beenemergingsincethetimeofCampCentury:theplanetweknowtodayisnottheEarththatwas. Ifyouhadvisitedourworldonehundredmillion,fivehundredmillion,or threebillionyears ago,youwouldhave foundaplanet that lookedverydifferentfromAnders’simage.

Exhaustive work going back to the 1800s has allowed geologists andpaleontologiststoconstructatimelineofourworld’shistory.Butonlyinthelast

half centuryor so has that timeline been resolved into the details of planetarychange. There are four long eons in theEarth’s history, representing themostimportanttransitionsintheplanet’sclimateandlife.Theseeonsaresubdividedintoeras,whicharefurtherdividedintoperiodsandepochs.ThePleistoceneandHolocene, whose transitions were revealed by Camp Century ice cores, areexamplesofepochs.18

Theplanet’sstorybeginswithanunnamedcloudofinterstellargasanddust.Almost fivebillionyearsago, that slowly spinningcloud,close toa light-yearacross, collapsed under its own weight. The Sun formed at the center of theinfallingmass,andarapidlyspinningdisksurroundingtheyoungstaremergedas well. Within this dense disk, particles of dust began colliding frequentlyenoughtoformfree-floatingpebbles.Thosepebblesthencollidedtoformrock-sizedobjects.Therocksthencollidedtoformboulders,andsoon,allthewayuptoasteroid-sizedplanetesimals.Afterbetweentenmillionandahundredmillionyears,gravitydrewtheplanetesimalstogetherandassembledtheEarthandotherrockyplanets(Mercury,Venus,andMars).19

Thiswas the beginning of theHadean, Earth’s first eon. Lasting from 4.6billionto4billionyearsago,itsnamespeakstotheplanet’shellishconditions.Earthduring the earlyHadeanwas covered in aglobe-spanning seaofmoltenrock. Eventually, this magma ocean cooled and hardened, forming a solidsurface.Butasteroidsandcometscontinuedtoraindownontheplanet,endinginaperiodcalledtheLateHeavyBombardment,whenoursolarsystemcleareditself of planetary construction debris. Each of these apocalyptic impactsshattered the surface, turning some or all of it back into molten rock. Gasesreleased from the bombardment and themagma oceans it regenerated left theHadean Earth with an atmosphere composed mostly of nitrogen and carbondioxide.20

Thus,theEarthwasonceafireworldofmoltenseas.Theplanet’sfirstformsoflifemayhaveemergedbytheendoftheHadean.

Therepeatedasteroidbombardmentswould,however,havesterilizedtheworld,forcing biology to potentially start over and over again.21 Either way, by thebeginningof thenexteon—theArchean—thekindof lifeweknow todaywasalready inplace.TheArchean lasted from4billion to2.5billionyearsago. Itwas during this vast span of time that life based on the biochemistry of self-replicatingmoleculescalledDNAspreadacrosstheworld.ButintheArchean,all life consisted of simple, single-celled organisms living in the oceans. Thereasonforthiswateryfixationwassimple:thewholeplanetwasprettymuchanocean.22

Whilecontinentsnowcoverabout30percentoftheEarth’ssurface,duringtheArcheantheyhadyetto“grow.”Thegroundyoustandontodayiscomposedofgranitethatislessdensethantheblackvolcanicbasaltmakinguptheoceanfloor.Granite is formeddeepwithin theEarth’smid-layers(called themantle).Likewarmairinacoldroom,graniterisesslowlyupwardasitforms,allowingit to become separate from the more dense ocean crust.While there remainscontroversyabout theprocess,manyscientistsbelieve thatduring theArcheanthe continent making was still beginning. Rather than planet-spanningcontinents, the world hosted just one or two proto-continents called cratons.EachcratonwassmallerthanIndiaistoday.

Thus,theEarthwasonceawaterworldofalmostendlessocean.Life slowly explored new domains of structure and metabolism as the

Archean gave way to the Proterozoic eon, lasting from 2.5 billion to half abillionyearsago.TheearliestcellsonEarthhadbeenrelativelysimpleaffairs.Called prokaryotes, they include modern-day bacteria. The first prokaryoteslived by breaking down complex molecules into simpler structures (basicallyfermentation). The evolution of early forms of photosynthesis had, however,givensomeprokaryotestheabilitytodrawenergydirectlyfromsunlight.Theseweretheearliestformsofphotosynthesis,wherebycellsusesunlighttogeneratefood.23

BythebeginningoftheProterozoiceon,lifehadlearnednew,moreefficientphotosyntheticstrategies.Someofthesecamefromthedevelopmentofawiderrangeofinternalmachinery,likeacellularnucleustoholdthegeneticblueprintsof the cell. The emergence of these nucleus-bearing eukaryotic cells changedlife’strajectoryontheplanet.Withtheadditionofnewformsofphotosynthesis,more energy became available to cells, allowing them greater flexibility andadaptation.ThefirstmulticellularorganismsappearedduringtheProterozoic,aslife began to experiment with the division of labor. Cells specialized intodifferentformsthatworkedtogether.Leftwithoutthelargerorganism,however,thesespecializedcellswoulddie.24

Alongwith the changes in life, the planet itselfwas changing.During thebillion-year-plusstretchoftheProterozoic,thefirstcratonsgrewintofull-sizedcontinents. Eventually, the slowmovement of the Earth’s crustal plates (platetectonics)drewthemtogether to formasupercontinent,avast landmasscalledRodinia.Other supercontinentswould formandbreakapartover thecourseofEarth’shistory.Eachwouldchangetheplanet’sclimatebyalteringglobaloceancirculationandresettingpatternsofrockweatheringandCO2cycling.25

Perhaps the most important climate shifts to come during the Proterozoic

werethefirstperiodsofnear-totalglaciation.Atleastfourtimesduringthiseon,changes in the concentrations of atmospheric greenhouse gases plungedplanetarytemperaturesintothefreezer.Fromthepolesallthewaytotheequator,the entire planetmayhavebecome locked inmiles-thick layers of ice.26 Seenfromspace,thissnowballworldwouldhaveappearedasamottledandcrackedPing-Pongballwithnolargeexpansesofopenbluewater.

Thus,theEarthwasonceasnowballworldofendlessice.For all the changes Earth experienced, none was more remarkable or

mysterious than life’s suddenburstof creativity just after thePhanerozoic eonbegan540millionyearsago.Acrossaremarkablyshortspanofgeologicaltime,evolution threw itself a party. What began as still-simple multicellular liferapidly diversified into an orgy of new forms and new species. In just fiftymillionyears,evolutionproducedallthebasicstructuresthatmarklifeonEarthtoday.CalledtheCambrianExplosion(itoccurredduringtheCambriangeologicera),itwasanevolutionaryaccelerationonascaleneverseenbeforeorafter.27

ItwasonlyaftertheCambrianerathatallthe“prehistoric”worldsweknowfrom popular fictions arose. There was the Carboniferous era three hundredmillionyearsago,withitsvastswampforests.Thoseforestseventuallybecamethecoalbedswe’veusedtopowerourprojectofcivilization.28TherewasalsotheJurassicera,dominatedbythehugedinosaursthatliveoninmoviesandthedreamsoflittlekids.Andfinally,therewasthemorerecentcyclingoficeagesand interglacial periods, during which we humans appeared and eventuallyflourished.

TheEarthswungbackandforthbetweenmanyversionsofitselfduring thefecundeonofthePhanerozoic.Butofparticularinteresttoourownagearetheperiodswhentheplanetarythermometerrosetofeverlevels.

Fifty-five million years ago, the supercontinent called Pangaea begansplitting apart. The volcanism that accompanies plate tectonics went intooverdrive,dumpingCO2intotheatmospherefarfasterthanitcouldberemovedby natural feedbacks. Global average temperatures rose fourteen degreesFahrenheit above what we experience today. Called the Paleocene-EoceneThermalMaximum, theresultwasaplanetalmostwithout ice.29TemperaturesinGreenland,whereafutureSorenGregersenwouldendurehissubzeroglacialsummers,stayedatabalmy70degreesFahrenheit.

Thus,theEarthwasonceajungleworld,aswelteringhothouseplanetdevoidofsnow.

Given the scaleof theEarth’s changesbetweenonemaskand another, thenext questionwe should ask seems clear.What forcewaspowerful enough to

driveourworld’sdramatictransformations?

THEGREATOXIDATIONEVENT

TheengineerasksDonaldCanfieldifheisclaustrophobic.Canfield,aprofessorof ecology, has just squeezed himself into the cramped confines ofAlvin, theworld’smostfamousdeep-seasubmersible.It’safalldayin1999onaresearchshipslowlyrollingintheGulfofCalifornia’sbluewaters.

“Claustrophobic?No,notatall,”Canfieldsays,lyingenoughtomakethembothfeelbetter.

Theengineerflasheshimaknowingsmileandsays,“Good...whateveryoudo, don’t touch the red handle. It’s only for emergencies.”30 The hatch slamsclosed.

Afteranhour-longdescent,Canfield isskimmingalongon thefloorof theGuaymasBasinintheGulfofCalifornia,morethanfiftymileseastoftheBajaPeninsula andover amile below the surface.Thebasin is a “spreading zone”where twoofEarth’scontinentalplatesarepullingapart.31As theplatesmoveaway from each other, they carry the Baja Peninsula away from mainlandMexico at a rate of about one inchper year, the same rate as your fingernailsgrow.32Inbetweenthespreadingplates,newseafloorcrustisconstructedashotmagmaupwellsfromdeeperwithintheplanet,cools,andthenhardensintosolidrock.

From the circular observing port cut into Alvin’s six-foot titanium crewcapsule,Canfield gets his first view of the basin’s floor. Far from thewell-litupperocean,it’sanalienworldlaidoutbeforehim.

“All around us,” Canfield recalls in his book, Oxygen, “We see theeffervescence of hot [sulfur]-rich, hydrothermal waters percolating from theaccumulatingcrust.”Boilingwater,heatedby theEarth’s internal fury, rises indarkcolumns from thevents.High-temperaturegeology is,however,onlyonefacetoftheotherworldlyvisioninfrontofCanfield.Remarkably,lifeisthrivinghereintheheatandthedarkness.“GreatmoundsofRiftiatubewormsrisefromthe shadows swaying gently on expansive hills of gypsum crust,” hewrites.33The enormous tubeworms have no color—none is needed in this world ofperpetualdarkness.

Everywhere, Canfield makes out what appears to be fallen snow on thegypsum-crustedseafloor.Whathesees,however,isnotsnow,butbacteria.Theabundantmicroscopiccreaturesdrawtheirenergyfromtheheatandsulfur-based

compoundsspillingfromthehydrothermalvents.34Theirabilitytothriveinsuchan extreme environment is what allows the whole strange ecosystem laid outbeforeCanfieldtoexist.

Canfieldmade this trip to the ocean floor to gain insights into theEarth’spastintermsofanalternativebiochemistry.WhathefoundatthebottomoftheGuaymas Basin were hints pointing to versions of life that need no sunlight.Theseare,perhaps,vestigesofanearlyincarnationoftheplanetbeforeitsmostsignificant transformational event: the rise of oxygen in the Great OxidationEvent.

“Trytoimaginesomethingsoprofound,sofundamental,thatitchangedthewhole world,” Canfield writes. “Think of something so revolutionary, that itforever changed the chemistry of the atmosphere, the chemistry of the oceansandthenatureoflifeitself.”35

Afterposing thisquestion,Canfieldsurveys thecriticalmoments inhumanhistory: the Great Plague, the Renaissance, and World War II. “These wereimportantevents,”hewrites.“Buttheirinfluenceoutsidethehumanrealmwassmall.”Hethengoesontoconsidertheextinctioneventsixty-fivemillionyearsagothatkilledthedinosaurs,andtheone250millionyearsagothattookdownalmost95percentofallanimalspeciesontheplanet.Eventhoseeventspaleincomparison toCanfield’s target. “Each of thesemajor extinctions changed thecourseofanimalevolution,butstill,theydidnotfundamentallyalterthefabricof life or surface chemistry of Earth.”36 What, he asks, did so completelytransformtheEarth?TheanswertoCanfield’squestionturnsouttobeassimpleasdrawingabreath.

During Earth’s earliest eras,much of biologymay have been powered bychemistryakintowhatCanfieldsawonhisdive.BythemiddleoftheArchean,however,atleastsomesingle-celledorganismshadfiguredouthowtotapanewand abundant energy source: sunlight. The first emergence of photosyntheticorganisms in the form of what scientists call anoxygenic phototrophs (non-oxygen-producingsunlighteaters)wasamajorinnovationinthehistoryoflife.Through the remarkable trial and error of evolution (and lots of time), somebacteria developed molecular light receptors. These were nanoscale machinesthatabsorbedenergyfromtheSunandusedittopowerchemicalreactionsthatpopped out sugar molecules. Sugar, in whatever form, is the basic chemicalbatteryforallthemetabolicshenaniganscellsneedtostayalive.37

After abillionor soyearsofnon-oxygen-producingphotosynthesis,naturegot very creative. Sometime in the late Archean, evolution produced a newversion of photosynthesis that, for the first time, used water to drive its

chemistry.BecausewaterissuperabundantonEarth,cellsusingthisnewkindofphotosynthesis won out over the older forms. But these organisms—calledcyanobacteria, or blue-green algae—did more than just multiply. Sucking inwater,CO2,andsunlight,theyalsostartedspittingoutmoleculesofoxygenasakind of waste product of their activity.38 In this way, their innovative water-eating, light-powered, oxygen-producing metabolism led them to become themostpowerfulforceinthehistoryoftheplanet.

Overtime,theactivityofthecyanobacteriadumpedsomuchoxygenintotheoceans and atmosphere that the entire planet was forced to respond. Thegeologic record shows evidence of early “whiffs” where atmospheric oxygenlevelsincreasedbysmallamounts.Butby2.5billionyearsago,thefixwasin.Across just a few hundred million years, the concentration of atmosphericoxygenincreasedbyafactorofamillion.

ThiswastheGreatOxidationEvent,orGOE.Ironically, theriseinoxygenwaspoison to the bulk of the life that existed at the time.Oxygen’s ability tobindwithsomanychemicalsmeansitcanquicklydegradethefunctionofcellsandkillthem.Butevolutionfiguredouthowtomakelemonadeoutoflemons.Itlearned to work with oxygen’s juiced-up chemistry to create better, moreenergetic forms of life. Soon, creatures that breathed in oxygen had evolved.Theyused the element to power faster andmore complexmetabolisms.39Thebigbrainyou’reusing to readandcomprehend thesewordswouldneverhavebeenpossiblewithoutoxygen’skicktoevolution.

By the end of the GOE, the anoxygenic phototrophs, once the planet’smasters, had been forced into oxygen-free warrens, learning how to live inplaceslikethefetidsulfurpitsofYellowstoneorevendeepinourstomachs.Inthisway,thenewoxygen-breathingformsoflifeinheritedtheopenseaandopensky.

Thepresenceofoxygenintheatmospherealsoallowedlifetocolonizethelandenmasse.BeforetheGOE,cell-damagingultravioletradiationfromtheSun(the kind that gives you sunburn) streamed unremittingly through theatmosphere.Only in theoceans, below the surface,was life safe enough fromUVlighttoformrichecosystems.Butwithoxygencametheatmosphericozonelayer.Ozoneisagas,madeupofmoleculeswiththreeoxygenatoms,thatformshigh in the stratosphere. It’s a potent absorber of UV radiation. This ozonesunblock shield, which made the land safe for life, could not have formedwithouttheriseinatmosphericoxygen.40

So, what does the GOE, with all its power and reach, teach us about theAnthropocene? It demonstrates that life is not an afterthought in the planet’s

evolution. Itdidn’t just showuponEarthandgoalongfor the ride.TheGOEmakesitclearthat,atanearlierpointinEarth’shistory,lifefullyandcompletelychanged thecourseofplanetaryevolution. It showsus thatwhatwearedoingtodayindrivingtheAnthropoceneisneithernovelnorunprecedented.Butitalsotellsusthatchangingtheplanetmaynotworkoutwellforthespecificformsoflife that caused thechange.Theoxygen-producing (butnon-oxygen-breathing)bacteriawereforcedofftheEarth’ssurfacebytheirownactivityintheGOE.

So,fromtheGOEwegaininsightsthatarethemselvesaturningofthewheelinhumanity’s conceptionof itself and itsplace in thecosmos.Wecome toanidea that touches both the deepest levels of scientific consequence and thehighest forms of mythic understanding. We come to the moment where thebiosphere,andourplaceinit,canbefullyimagined.

THEBIOSPHEREBEGINS

Scientistsbecomefamousforalotofreasons.TherearethoselikeEinsteinandDarwinwhosevisionsshatteroldideas.Theirnamesgoontoliveforeverinthepantheonofgenius.ThentherearethoselikeCarlSaganandStephenHawking,both brilliant researchers, whose talents as writers allowed millions of non-scientiststounderstandthebeautyandpowerofscience.Buthowmanypeoplehave ever heard of Vladimir Ivanovich Vernadsky? His name is far from ahouseholdword outside of his nativeRussia.But that obscurity is destined tochangealongwiththeplanet.

ItwasVernadsky’s ideas—and theirgenius—thatheraldedanewscientificconception of life’s planetary context. As we enter more deeply into theAnthropocene,wewillfindVernadskyalreadythere,waitingforustocatchupwithhim.

Vernadskywas born in 1863 in the St. Petersburg of Imperial Russia.Hismothercamefromnobility;hisfatherwasaprofessorofpoliticaleconomyandstatistics.41 Vernadsky’s parents were known for their devotion to democraticandhumanisticideals.Fromthem,heinheritedafiercedeterminationtolivebythoseideals,whichwasgraftedontoaloveofscience.Acrosseighty-twoyearsofwars,revolution,andacutepoliticalturmoil,Vernadskydidnotwaverinhisdevotion to scientific inquiry.And even at thegreatest personal risk, heneverwavered inworking for the freedom to pursue scientific ideas,wherever theyled.42

Vernadsky began his scientific work in the chemical study of minerals.

TravelingacrossEuropeinthelate1800s,hewaskeentoapplythemostmodernmethodsofphysicstothestudyofrocks.Hisgoalwastobringprecisiontoolstobear on questions about the planet’s history. But even as Vernadsky wascommitted toexactingempiricalstudies,hewasalwaysmore thanaspecialist.Acrosshiscareer,hestruggledtoseehowthewholeemergesfromthenarrowerstoriesscientistscanunlockfromtheparts.

RussianscientistVladimirIvanovichVernadsky.

In thisway,Vernadskybuiltasolid,data-drivenfoundationforanewfieldcalled geochemistry, which unpacked Earth’s history by examining the

microscopic composition of its physical constituents. Then Vernadsky wentfurther. It wasn’t just geology and chemistry that were linked. In his eyes,biologyalsohadtobebroughtintotheplanet’sstoryatafundamentallevel,soheinitiatedasecondfield:biogeochemistry.43

Vernadsky was often critical of biologists for the way they treated“organismsasautonomousentities.”Inhiseyes,anyindividualspeciescarriedmore than just an imprint of the environment within which it had evolved.Instead,theenvironmentwasshapedbytheactivityoflifeasawhole.Asheputit,“Anorganismisinvolvedwiththeenvironmenttowhichitis[has]notonlyadaptedbutwhichisadaptedtoitaswell.”

ThisattentiontobothmicroscopicandmacroscopicviewsledVernadskytohismostimportantadditiontothelanguageoflifeinthecontextofitsplanetaryhost.BuildingondiscussionswiththeSwissgeologistEduardSuess,Vernadskyproposed that the study of the Earth would not be complete withoutunderstanding the central role of life as a planetary force. Earth, in his view,cannotbetrulyunderstoodwithoutunderstandingthedynamicsofitsbiosphere.

Living as we do after astronaut William Anders’s Earthrise, it’s hard toimagine that the biosphere could ever be a new or radical idea. But it wasVernadskywhogavetheconceptitsscientificbirth.ItwasVernadskywhofirstclearly articulated what later scientists, studying everything from the GreatOxidationEventtomodernclimatechange,wouldslowly—andwithgreateffort—come to verify: Life was not just a patchy green scruff holding a tenuouspositionbetweenrockandair;instead,itwasaplanetarypowerasimportantasvolcanoesandtides.Itwasanactiveforceshapingthecomplexmultibillion-yearhistoryoftheworld.AsVernadskywrotein1926:

Thematterof thebiospherecollectsand redistributes solarenergy,andconverts itultimately intofreeenergycapableofdoingworkonEarth....TheradiationsthatpourupontheEarthcausethebiospheretotakeonpropertiesunknowntolifelessplanetarysurfaces,andthustransformthefaceoftheEarth.44

Overthewholeofhiscelebratedcareer,Vernadskycontinuedtomodifyandextendedhisconceptofbiosphere.Specifically,hesawitasaregion—ashell—extendingfrombelowtheEarth’scrust(thelithosphere)allthewaytotheedgeof the atmosphere. Within this shell, the action of life dramatically changedflowsofmatterandenergy.

Mostimportantforourownmoment,Vernadskysawthattheworld-shapingpowersoflifewerebothancientandongoing.“Adjustinggraduallyandslowly,lifeseizedthebiosphere,”hewrote.“Thisprocessisnotyetover.”

It’s the scaleofhisvision thatmakesVernadskyso important toour story.

Earth’sentryintotheAnthropoceneis,atonelevel,purelyanissueofinteractingplanetaryprocesses.OurentryintotheAnthropocene,however,isdifferent.Forus,it’salsoanissueofmakingmeaning,ofmakingsenseofourplacewithintheweboflifethatisalsoaforceshapingtheplanet.Vernadskyenvisionedaglobalviewthatachievedboth.Itwasbothscientificandmythicinscale,longbeforesatellitesandspacemissionscouldmakesuchaglobalviewofEarthtangible.

Afterhisdeathin1945,thelimitsoftheColdWarmeantitwouldtakesometimeforVernadsky’sradicalviewoflifeanditsplanetaryreachtoreachbeyondRussia.45 But in time, Vernadsky’s vision did find its champions. As humanculturewas reshaped by its new space age, two scientists in particular wouldpickupVernadsky’sbiosphericvisionandgrowitintoafull-fledgedscience.

BIOSPHERERISING

James Lovelockwas always the outsider’s insider. From the first radio set hecobbled together as a boy in England after World War I, Lovelock was aninventor of prodigious talent. Eventually, that talent drew governments andcorporationstoseekhishelp.

DuringWorldWarII,Lovelock’sdegreeinchemistrytookhimintomedicalresearch, where he invented everything from precision airflow meters forstudyingthecommoncoldtospecializedwaxpencilsthatcouldwriteonwettesttubes.Thistalentasa“maker”wouldeventuallybringadegreeofindependenceas his inventions drew a steady income. In the 1950s, Lovelock designed acheap,portabledevicefordetectingminuteamountsofchemicalcontaminants.Thepatentwassovaluableitallowedhimtopursuescienceonhisownterms,independent of an academic or government affiliation. But governments werestillkeentosignhimontotheirprojects.46

In 1961,Lovelock foundhimself at the same JetPropulsionLaboratory inPasadena,where Jack James and his teamwere exhausting themselves on theMarinerVenusmission.ForLovelock,thesprawlingcampushadthelookof“ahastyairportwithprefabricatedcabinsdottedoverthehillside.”47JPLhadpaidfor his trip to its nascent campus because they needed his aid in designingsensitiveinstrumentsforthenewspacemissions.Eventually,LovelockwasputonateamproposingexperimentstosearchforlifeonMars.

Sitting throughmeetingswhere biologists laid out plans to detectMartianmicrobes, Lovelock found himself unconvinced. “The flaw in their thinking,”Lovelockrecallsinhisbiography,“wastheirassumptionthattheyalreadyknew

whatMartianlifewaslike....IgatheredthedistinctimpressionthattheysawitaslikelifeintheMojaveDesert.”48

But Lovelock, with an outsider’s perspective that would haunt him later,came at the problem from a different direction. “I think we need a generalexperiment,” he told the group, “something that looks for life itself, not thefamiliar attributes of life aswe know it onEarth.” 49 Pressed by the programmanagertoproposeexperimentsthatlookedfor“lifeitself,”Lovelockwastakendown a road that would lead him straight into the realms of Vernadsky’sbiosphere.

Lovelock’sbackgroundinphysics,chemistry,andbiologyledhimtoseetheproblemin termsofplanetaryatmospheres.Heknewthat lifewaskeeping theair oxygen-rich. Take the biosphere away, and that oxygen would chemicallycombinewithothercompoundssuchthat,ifyouwaitedlongenough,theEarth’satmospherewouldbecomeoxygen-free.Withoutlife,itwouldreturntoastateof“chemicalequilibrium”dominatedbytheCO2releasedfromvolcanoes.50

BasedonwhathesawonEarth,Lovelockreasoned that lifewouldalwayskeep a planetary atmosphere in a state far from equilibrium. That meant theactivityoflifewouldconstantlypushontheplanet’schemistry.Thebiosphere’scontinualresupplyofoxygen,anelementthatwouldotherwisereactaway,wasjustoneexampleofsuchapush.

Over thenext twoyears,Lovelockcontinued tovisit JPLandcontinued toworkout thedetailsofhisatmosphere-as-life-detectorexperiment.But then, inSeptember of 1965, a flash of insight showed him therewasmore to his ideathanjustanexperiment.

Inanofficehe sharedwithnoneother thanayoungCarlSagan,LovelockwasporingovernewdatashowingthattheMartianatmospherewasdominatedbyCO2.UnlikeEarth’sblanketofgases,Mars’satmospherewaslockedintothesame kind of dead chemical equilibrium as that of Venus. A CO2-dominatedatmosphereisexactlywhatyou’dexpectastheresultofchemicalreactionsthatwere allowed to run their own course, like mixing a bunch of compoundstogether in a box and leaving the whole thing alone forever. It was at thatmomentthatLovelocksawthelight.

“It came to me suddenly, just like a flash of enlightenment, that [for thechemistryoftheEarth’satmosphere]topersistandkeepstable,somethingmustbe regulating [it].” The identity of this “something” came toLovelock just asquicklyasthequestionhad.“Itdawnedonmethatsomehowlifewasregulatingtheclimateaswellasthechemistry.SuddenlytheimageoftheEarthasalivingorganismabletoregulateitstemperatureandchemistryatacomfortablesteady

stateemergedinmymind.”51Itwasapowerfulimage.LovelocksawtheEarthasasingleentity—“alive”

insomesense—andregulatingitselfinthesamewayourbodiesmaintaintheirtemperatures.Lovelocksoonbeganfleshingoutthedetailsofhisidea, lookingforspecificmechanismslifecouldharnesstoadjustconditionsacrossanentireplanet.As theworkprogressed,herealizedheneededanamefor the idea.Hethoughttocallitthe“Self-regulatingEarthSystemTheory,”butaconversationwith a neighbor, the novelist William Golding (author of Lord of the Flies),convincedhimotherwise.GoldingsuggestedLovelocknamethetheoryaftertheGreekgoddessoftheEarth,Gaia.52

There is some irony in the fact thatCarl Sagan,who did somuch for ourconceptofEarthinitscosmiccontext,wouldbepresentfortheinsightthatgavebirthtoGaiatheory.GiventhatSaganwasneververysupportiveofLovelock’sidea,itisevenmoreironicthathe’dserveasmidwifetothenextcrucialstepinitsdevelopment.

In the years following their divorce, the biologist Lynn Margulis almostsingle-handedlyforcedthescientificcommunitytorecognizetheimportanceofcooperation, rather than just competition, in evolution. Her theory ofendosymbiosis demonstrated how the tiny chemical-processing plants in ourcellscalledorganelles hadoncebeen independentorganisms.Margulisprovedthatorganelles—likemitochondria,forexample—hadbeenabsorbedintolargerbacteria billions of years ago to form a cooperative, symbiotic whole. Thissymbiotic evolution was likely the origin of the eukaryotic (nucleus-bearing)cellsthattransformedlife’strajectoryduringtheArcheaneon.53

In the early 1970s, Margulis had become interested in the question ofatmosphericoxygenand itsmicrobialorigin.When sheaskedher ex-husband,Carl Sagan, if he knew someone who might be good to talk with about theproblem,hesuggestedLovelock.Fromthisunlikelyintroduction,LovelockandMargulisbeganacollaborationthatfullydefinedtheGaianconceptoflifeasaself-regulatingplanetarysystem.WhereLovelockbroughtthetop-downviewofphysics and chemistry, Margulis brought the essential bottom-up view ofmicrobiallifeinallitsplentitudeandpower.54

The essence of Gaia theory, as elaborated in papers by Lovelock andMargulis, lies in the concept of feedbacks that we first encountered inconsideringthegreenhouseeffect.

JamesLovelockandbiologistLynnMargulisinfrontofastatueofGaia.

The temperature of the human body always hovers around 98.6 degreesFahrenheit. That’swhat is known as a steady state.At your death, your bodydrops back to room temperature. That’s equilibrium. The same ideas can beappliedtotheoxygenintheatmosphere.Thecurrentlevelsofoxygenareheldinasteadystatebychemicalreactionsdrivenbythepresenceoflife.Buthowdoeslife keep the oxygen levels steady? We’ve already seen how photosyntheticbacteriagaveEarthitsoxygen-richair.Butwhydidoxygenlevelsriseupto21percent, and no further? This is an important question, because if theconcentrationofoxygenintheairweretoclimbashighas30percent,theplanetwouldbecomeatinderbox.Anylightningboltwouldtriggerfiresthatwouldn’tstop.So,whatkeptoxygen levels from risingabove thisdangerous threshold?Toanswerthatquestion,LovelockandMargulisturnedtotheideaoffeedbacks.

In their Gaia theory, Lovelock and Margulis argued that life as a wholeexertedglobalnegative feedbackson theplanet.Those feedbackshadkept theplanet in a series of long-term steady states over its history that were alwaysoptimalformakingtheplanethabitableandinhabited.Inotherwords,lifekepttheplanet cozy for life. If, for example, oxygen levels rose toohigh, then theincreased oxygen would, itself, trigger blooms of microorganisms whosebiochemistrywouldleadtothoselevelsbeingdrawnbackdown.Itwasavery

bigideaindeed.LovelockandMarguliswereofferingascientificnarrativewhosetiestothe

scale of world-building myth were explicit. It was Vernadsky on steroids, avisionofplanetaryevolutionwherelifewasnotjustaforce,butaforcewithitsownkindofintention.Butjustbecauseanideaisbigandbeautifuldoesn’tmeanit’s true. In particular,with the all-important idea of intention—meaning life’sintentionwithitsGaianfeedbacks—thetwoscientistsopenedaPandora’sbox.

THEBIOSPHEREBOUND

OberonZell-Ravenheart(whosegivennameisTimothyZell)nevermetaNewAgeideahedidn’tembrace.Heisapaganandashaman.AfullyordainedpriestintheFellowshipofIsis,healsofindstimetoworkasaninitiateintheEgyptianChurch of the Eternal Source. Zell-Ravenheart is also a Gaian, and that, in anutshell,iswhysomanyscientistshatedLovelockandMargulis’sbig,beautifulidea.

Early on,Gaia found itself scorned as a scientific theory by scientists butwildlypopularinthelargerculture.AshistorianandphilosopherMichaelRuseputs it: “[Thepublic] embracedLovelock andhis hypothesiswith enthusiasm.Peoplegot intoGaiagroups.ChurcheshadGaiaservices,sometimeswithnewmusic written especially for the occasion. There was a Gaia atlas, Gaiagardening,Gaiaherbs,Gaiaretreats,Gaianetworking,andmuchmore.”55

Gaia theorycamealong justas theenvironmentalmovementandpost-’60sNewAgeismweregoingmainstream.In1979,nuclearpowerbecameanationalissue thanks to the partial coremeltdown at the ThreeMile Island generatingstationnearHarrisburg,Pennsylvania.Thepollution-drivenevacuationofLoveCanal in Upstate New York became the poster child for what environmentaldegradation looked like. Gaia theory, with its evocation of Earth as a singleliving organism—a vast planetary mother—channeled popular ecologicalconcernswithanalternativevisionofhumanity’splaceintheschemeofthings.

Many scientists pounced on Lovelock and Margulis for promoting theequivalentofsnakeoil.AsmicrobiologistJohnPostgate,afellowoftheRoyalSociety,put it:“Gaia—theGreatEarthMother!Theplanetaryorganism!AmItheonlybiologisttosufferanastytwitch,afeelingofunreality,whenthemediainvitemeyetagaintotakeitseriously?”56

The real problem with Gaia theory for many scientists was the issue ofteleology.It’sahallmarkofbiologythatevolutionthathasnopurpose,direction,

or goal (telos is theGreekword for “goal”). The idea that the biospherewassomehowmanipulatingthechemicalandphysicalconditionsontheplanetforitsowngoodseemedinherently teleological(that is,goal-oriented). Itsmackedofintention,andevolutiondoesn’thaveintention.57

LovelockandMarguliswereunbowedintheirdefenseofGaia.In responsetocriticswhoclaimedtheirproposedfeedbackswerenothingmorethanfantasy,Lovelock produced his now-famous Daisyworld model. Developed withmathematician James Watson, the Daisyworld model used a simple set ofequationstodescribeaplanetwithtwospeciesofdaisies(blackandwhite)andagradually brightening sun. The solutions to the equations showed clearly howfeedbacks from the daisies (the black ones absorbed sunlight,while thewhiteonesreflectedit)couldnaturallykeeptheplanetatasteadytemperatureevenasthe sun heated up. Itwas a tour de force of representing a complex ideawithsimplemath in the service of proving an essential point. As Lovelock put it,“Daisyworldkeepsitstemperatureclosetotheoptimumfordaisygrowth.Thereisnoteleologyorforesightinit.”58

AndLovelockandMargulismadeitclear,theywerenotclaimingtheplanetshould be considered alive in any true sense of the word. New Age GaianMother Earth ceremonies notwithstanding, Lovelock and Margulis wereultimately arguing for the central role of the biosphere in planetary evolution.TheywerepickingupwhereVernadskyleftoff,andputtinginmorescience.

With the publication of the Daisyworld model in 1983, the tide, at leastpartially, began to turn. Biospheric feedbackswere recognized as an essentialpartofplanetarylawsofoperation.Thesefeedbacksrepresentthedefinitionofhowtothinklikeaplanet,andresearchersembracedthebiosphere’scentralrolein their studies of the Earth. But in the process, the name “Gaia theory”wasdropped and replacedwith the less contentious “Earth system science.”Whiletheconceptofself-regulationremainedcontentious,researchersnowknewthatthelinkagesbetweenthebiosphere,atmosphere,andothersystemsweresotightthat theyhad tobe considered together as a single entity.The adoption of theEarth system paradigm represented its own revolution in how we think ofplanets,and today it forms thecross-disciplinaryfoundationforall researcherstryingtounderstandclimatechange.59

As these studies of Earth system science were extended to include theplanet’s past, a crucial new idea would be added to the researcher’s lexicon.BuildingonVernadsky,Lovelock,andMargulis,anewgenerationofscientistsbegan speaking of a “coevolution” between life and the planet. That word,coevolution, would help rewire astrobiology. Life could no longer be isolated

fromtheplanetthatgaveitbirth.Instead,aplanetcouldbedeeplytransformedby the life it births, includingwhen that life goes on to create its own globe-spanningcivilization.Thus,withinthatsingleterm,coevolution,laytheseedsofanewstorywaitingtobetoldabouthumanityandourAnthropocene.

CHAPTER4

WORLDSBEYONDMEASURE

HOWTORUINYOURLIFEWITHPLANETS

All of Thomas See’s fellow astronomers hated him. That was a particularlyironic position for See to find himself in, given that he was one of themostpopularof“popular”astronomersinthelatenineteenthcentury.1

See began his career full of promise.Hewas considered an expertwith atelescope, and his skill as a writer for non-scientific audiencesmade him theastronomerreportersturnedtowhentheyneededaquoteoranexplanation.Buthismeteoricascentwouldlaterbefollowedbyafallintothedepthsofscientificscorn. In the end, See was so despised by his colleagues that his experiencebecameanobjectlessoninruiningascientificreputation.

It’sastorythatbeginswithaplanet.SeewasborninruralMissouriin1866.Thoughhewasclearlyagiftedchild,

hisfamilywouldnotallowhimtoattendschoolfull-timeuntilhis teens.Oncethere, his natural aptitude for science andmathematics caught the attention ofteacherswhohelpedhimgetintothestateuniversity.Later,hisnaturaltalentledhim to work with some of the best astronomers of the era, studying pairs oforbitingsunscalledbinarystars.

See’sworkinvolvedprecisionmappingofthesiblingstarsastheychangedpositionon the sky.Hewas tireless in these “astrometric” studies.Heworkedeighteen-hour days, translating the information in photographs produced overmany nights of telescopic observation into positions on sky maps. Thisastrometricdatawasthenfedintocalculationsthatspitouttheexactshapeofthebinary stars’ orbits. Finally, estimates of the stars’ masses could be extracted

from theorbits byusing lawsofphysics.Nooneknewverymuchabouthowmuch mass stars contained back in the 1890s, and See’s work was hailed ascutting-edgescience.

SeewashiredattheUniversityofChicago,andthenattheobservatorythatPercival Lowell, the rich, Mars-crazy amateur astronomer, was building inFlagstaff,Arizona.ItwasatLowell’sobservatorythatthetroublebegan.

AstronomerThomasJeffersonJacksonSee.

In 1899, See published a letter in the prestigious Astronomical Journalclaimingthebinarysystemcalled70Ophiuchiwas“perturbedbyadarkbody.”Hemeant theorbitsof thetwostarsseemedtobedistortedbythegravityofathird, unseen object. Later, See would claim to see other binary stars withinvisible companions, reporting, “They seem to be dark . . . and apparentlyshiningbyreflectinglight.Itisunlikelythat[theunseenobjects]willprovetobeself-luminous.”Seewasbeingcoywithhislanguage,buttheimplicationsofhisstatement were unambiguous. Hewas telling the world he’d discovered otherplanetsorbitingotherstars.

ThequestionofwhetherotherstarsintheskymighthaveplanetsgoesbacktotheancientGreeks.Formillennia,astronomersandphilosophersarguedabouttheexistenceofother solar systems in thecosmos.GiordanoBruno riskedhislifearguingthatotherworldsexist.Thatiswhydirectevidencefortheexistenceof even one planet orbiting one other star would have been an epoch-makingdiscovery. See was making an extraordinary claim with his orbit-perturbing“dark body” of a planet. But in science, extraordinary claims requireextraordinaryproof.For thepracticing scientistmaking suchclaims, ahealthydose of skepticism is essential, because someone else is sure to check yourresultsvery,verycarefully.

See lacked that internal skepticism, andhewouldpay a steepprice for itsabsence. InMay 1899, a former student of See’s named Forest RayMoultonpublished a paper in the sameAstronomicalJournal, demonstrating that See’splanetaround70Ophiuchicouldn’texistbecausethelawsofphysicswouldnotallowit.

Science is a “call and response” kind of business.Much as blues or jazzmusicianswill pick up on a riff that’s played by one of their bandmates, SeecouldhavetakenMoulton’sresultsandbuiltonthem.Hecouldhaveconcededthat, with cutting-edge observations such as his, there were bound to bemisinterpretations. He could have learned from the episode and built betterscience.

Instead,hedoubleddown.InablisteringlettertotheAstronomicalJournal,SeeattackedMoultonand

triedtoweaselhiswayoutofmistakenclaimsaboutplanets.Hewrote thathealreadyknewaboutMoulton’sobjectionsand thenwaffledabout thenatureoftheorbitand theplanet.Theeditorsof the journalwereso takenabackby theacidtoneofSee’sletterthattheytooktheextraordinarystepofprintingonlyafew pieces of its text. Then they handed See the Victorian version of asmackdown: “The present is as fitting an opportunity as any to observe thatheretoforeDr. See has been permitted, in the presentation of his views in this

journal, the widest latitude that even a forced interpretation of the rules ofcatholicitywouldallow;butthathereafterhemustnotbesurprisediftheserules,whether as to soundness, pertinency, discreetness or propriety, are construedwithinwhatmayappeartohimundulyrestrictedlimits.”

TheAstronomicalJournalwasessentiallythreateningSeewithcensure.Things went downhill from there. See’s resentments and temperament led

him from the world’s greatest centers of astronomy down to the “NavalObservatory”atMareIsland,California.Thiswaslittlemorethanatimekeepingstationattached to ahugenaval shipyard.Mare Islandhadno telescopeworthmentioning.

Lacking access to a good instrument for observations, See turned hisattentiontotheory.Unfortunately,whilehehadcleartalentswithatelescope,hisinstinctsforfundamentalphysicswereterrible.Seemanagedtomisstheboatonevery major revolution happening in physics at the turn of the century. Heconsistently rejected the profound discoveries about atomic phenomena in thenewscienceofquantumphysics,andheopposedEinstein’striumphanttheoryofrelativity,claimingthathisownideasaboutcosmicstructurehadbeenprovenbyobservation.(Theyhadnot.)

The final nail in the coffin of See’s scientific reputationwas a 1913 bookcalledTheUnparalleledDiscoveries of T.J.J. See. The author called See “thegreatest astronomer in the world.” Upon further investigation, however, somesuggestedthatitwasSeehimselfwho’dwrittenthebook.Hewouldneverregaintherespectofhispeers,andhediedin1962,rejectedbyhischosenprofession.

THEPROBLEMOFPRECISION

Seewouldnotbethelastastronomerwhoseclaimsofaplanetdiscoverywouldprove tenuous or career-threatening. A number of times in the years thatfollowed,astronomersclaimedtohavedetectedaplanet,onlytoseetheirclaimsevaporate. The difficulty in finding exoplanets can be summarized in a singleword:precision.Planetsaresmall,andstarsarebig.Planetsaredim,andstarsare bright. Planets are cold, and stars are hot. Planets have smallmasses, butstarsweighinasbehemoths.TheSun,forexample,wouldappearatrilliontimesbrighter than theEarthwhen seen from the stars.Thatmeans trying to see anearthlike planet across interstellar distances would be like looking from NewYorkCity toAT&TPark inSanFrancisco,where theGiantsplay,andmakingoutafireflynexttooneofthestadiumspotlights.

So for scientists to “see” a distant planet, theymust pull the tiny signal it

producesoutoftheenormousimpactofitsstar.Thereareanumberofstrategiesastronomers can pursue to detect an exoplanet, but all demand high-precisionmeasurements.

ThebasisfortheoldestmethodsofdetectingplanetsistheastrometryT.J.J.Seewasusing,which focuseson theorbitalmotionof thestarandplanet.Weusually think that planets orbit around their stars.The truth, however, ismoreinteresting: objects always orbit each other. Binary stars of equal mass bothcirclearoundapointhalfwaybetweenthem.Butifthemassofoneoftheobjectsis less thanthatof theother—asthecasewouldbewhenatinyplanetorbitsabigstar—theorbit’scenterwillbenearertothecenteroftheheavierobject.Soeven though it looks like a planet orbits its star, the planet’s gravity is stillforcingthestartoshufflearoundinatinyorbit.Thecenterofthatlittledanceisjustslightlydisplacedfromthestar’sowncenter.

See’sastrometricstudiesweredesigned tosee that tinystellarmotion.Theideawastotrackthepositionofastarovermanyyears.Inthisway,astronomerswould see the star zigzag as it was “perturbed” by the gravity of its unseenplanet.Butchangesinthestar’spositionasitwobbledbackandforthwouldbeminuscule.Forexample,alienslookingattheSunfromfifteenlight-yearsawaywouldhavetostraintoseetheorbitalwobblecausedbyeventhemostmassiveplanetinoursolarsystem.TheprecisionneededtomeasurethesetinyshiftsinpositionwasbeyondthetechnologySeehadathisdisposal.

There is another way of tracking the gravitational dance of a star and itsplanet, one that relieson trackingchanges in the star’svelocity rather than itsposition.Asthestarexecutesitslittleorbit,thegravityoftheplanetwillcauseitto swing first toward observers onEarth and then away. If astronomers coulddetect these changes in velocity—called reflex motion—it would constitute adetection of the orbiting planet.But like the orbits themselves, the changes instellar velocity caused by orbital reflex motions are so small that takingmeasurements at the needed level of precision presented a huge technicalchallenge.

A third way of seeing an exoplanet focuses only on a star’s brightness,meaningitstotallightoutput.Duringanygivenyear,betweentwoandfivesolareclipsesarevisiblefromtheEarth’ssurface.EachoccurswhentheMoonlinesup just right for earthbound observers, passing in front of the Sun and eitherpartiallyortotallyblockingitslight.Thesameprinciplecanbeappliedtoplanethunting.

Imagineadistantstarthathostsanexoplanet.Nowimaginethattheplanet’sorbitarounditsparentlinesupperfectlywiththe“lineofsight”betweenEarthand the star. That kind of alignment means the exoplanet will briefly swing

between Earth and the star once during each of its orbits, just as the MoonswingsbetweenEarthandtheSunduringaneclipse.Eachtimetheplanetgetsbetweenusanditsstar,itwillblockafractionofthestar’slight,andfromEarthwewillseethestardimeversoslightly.

Astronomersusethetermtransit todescribeaplanetcrossingthefaceofastar.Seeing an exoplanet transit its own starwould requirehyper-precise lightdetectors.AlienslookingattheSunfrominterstellardistanceswouldseeitslightdimby just one percentwhen Jupiter crossed its face.AnEarth transitwoulddimtheSunbyjust0.01percent.Alongwiththisdemandforprecision,thereisanother complication. Stars can naturally produce light variations of the sameorder as an exoplanet transit.Dark regions on stars, called “spots,” caused bypowerful stellar magnetic fields, are just one of many sources of naturalvariation.Anysuccessfultransit-basedexoplanet-huntingmethodwouldhavetobe exact in both its measurements and its understanding of the star beingmeasured.

By the early 1970s, planets had been hiding beneath their veils ofimprecisionforsolongthatmanyscientistshadgivenupontryingtofindthem.Inaddition,throughoutthe1950sand1960s,therehadbeenenormousprogressinotherarenasofastronomy,likethestudyofdistantgalaxies.Huntingforotherworldscametoseemlikeadeadend.

“I remember how, in the early 1990s, peoplewould look down at the fewresearcherswhowerepushingforplanethunting,”recallsonescientist.“TherewereNASAadministratorswho’dwalktheotherwayjusttoavoidbeingbuggedbythem.Itwasahardtimeforthoseguys.”2

But the fortunes of the planet quest were about to change. The first stepstowardtakingexoplanetsseriouslybeganinthemid-1970s,andthemotivationcamedirectly fromFrankDrake’soriginalquestionsabout thesearch foralienintelligence.

THEPATHSTOANANSWER

FrankDrake and Carl Sagan’s very public discussions about exo-civilizationsestablished thescientificbasis for thesearchforextraterrestrial intelligence,orSETI.But thesearch itselfwould requireanewgenerationof scientists.ChiefamongtheirnumberwasJillTarter.

LikeDrake,TarterbeganherscientifictrainingatCornell,intheengineeringphysics program. But by the time she completed graduate school at the

UniversityofCalifornia,Berkeley, she’ddecided to focusherworkonSETI.3Overalonganddistinguishedcareer,Tartercarriedoutobservationalprogramsat radio observatories across theworld, served as project scientist forNASA’sSETI program, and was given the Bernard M. Oliver Chair at the SETIInstitute.4Shehasseen firsthandhowthequestionofexo-civilizationsand thequestionofexoplanetsconverged.

In the 1970s, Tarter’s dedication to SETI took her to a series ofmeetingswherequestionsofprecisionandplanetdetectionwerefirsttakenoninearnest.“Technology for findingplanets just didn’t exist back in the early1970s,” shesays. “That means astronomers needed to get together and figure out exactlywhatthebarrierswereandhowwecouldbeatthem.”5Withthisgoalinmind,in1975aworkshopwasorganizedatNASA’sAmesResearchCenterinSanJose,at which the general problem of SETI technologies was first laid out. Thisworkshopfocusedonsearchstrategiesforsignalsfromexo-civilizations,buttheattendeesagreedthatthefactorsintheDrakeequationneededtobeexploredontheirownaswell.Themostimportantofthesesub-questionswasthefractionofstarswithplanetsandthefractionofplanetsinthehabitablezone.6

“Theoriginalworkshop led to twoothers that focusedexplicitlyonplanet-hunting methods,” Tarter told me in an interview. “There was a meeting at[Ames]in1978.Thiswasthefirsttimethedifferentmethodsofplanethuntingweredrilleddownintotoseewhichonehadthehighestchanceofsuccess.”

Records from that meeting show that most of the discussion focused onastrometric sky mapping, the approach that See used. Searches based ondetectingreflexmotionswerediscussedindetail,too.Directdetection—actuallyseeing the light fromaplanet—wasalsoon the table.7But the transitmethod,basedon thedimmingofstarlightdue toapassingplanet,didn’tevenmake itintothereport.Thefuturewouldshowtheironyofthisexclusion.

Thoughtheproblemswithallthemethodswereacknowledgedtobevast,thereport ended on a positive note. “The prospects of increasing our confidenceconcerningthefrequencyanddistributionofotherplanetarysystemsaregood,”theauthorsconcluded.8Later,anotherSETI-inspiredNASAworkshopwasheldattheUniversityofMarylandtoexploretechnicaldetailsinmoredetail.

“People came away from that [second]meetingwith a sense ofwhatwaspossible,”Tartertoldme.“Thereflexmotionapproachwasseenasparticularlypromisingifthetechnologycouldbehammeredout.Ithinkalotoffolkswerereallyexcited.”9

AstronomerandSETIresearchleaderJillTarter.

Noteveryonewassohappy,however.WhilethetransitmethodwasraisedattheMarylandmeeting, its prospectswere deemed to be dim. The final reportconcluded, “TheWorkshop considered the role of photometric [transit-based]studiesinanefforttodetectotherplanetarysystemsandupheldtheconclusionsofearlierstudies,namely,thatphotometricstudiesarenotpractical.”10

That conclusion didn’t go down well with one tenacious scientist. “Therewas a youngNASA researcher namedBill Borucki,” Tarter said. “He felt thetransit method had a lot of promise, even if everyone else thought it washopeless.Ithinkhewasdeterminedtoprovethemwrong.”

THEFALLOFATHREE-THOUSAND-YEAR-OLDQUESTION

In1995,atanastronomyconferenceinFlorence,SwissscientistMichelMayorwalked up through the audience and took his place at the podium. The otherastronomerspresentlookedaroundtheroomandwonderedwhyafilmcrewhadjustappeared.That’swhenMayordroppedhisepoch-makingbombshell.HeandhispartnerDidierQuelozhadfirmevidencefortheexistenceofanotherplanetorbiting another star.11 When it came to solar systems, at least, we were not

alone.In the decade and a half following theAmes andMarylandmeetings, the

hurdles blocking the way to reflex motion–based planet searches had beenovercome. In the US, astronomers Geoff Marcy and Paul Butler had built aseriesofevermoresensitiveinstrumentstomonitora longlistofnearbystars.Theirswastheworld’smostcompleteplanet-huntingprogram.

ButMarcyandButlerwereexpectingothersolarsystemstolooklikeours.They thought they’dneedyearsof trackingbefore thesignalofaJupiter-sizedplanet in a Jupiter-sized orbitwould appear in their data (Jupiter takes twelveyearstomakeoneswingaroundtheSun).TheEuropeanresearchersMayorandQuelozhadanobservationalprogramorientedtowardfindingclosebinarystars.They’d gotten lucky with their planet detection, but also had the insight torecognizewhatthey’dfound.12

MayorandQuelozdiscoveredtheirplanetorbitingthestar51Pegasi,whichis fifty light-years from Earth (one light-year equals six trillion miles). Theplanet, called 51 Pegasi b, was Jupiter-sized, but swung around its star onceevery fourdays.Thatmeant itwasalmost ten timescloser to its star thanourinnermostplanet,Mercury,istotheSun.13Agiantplanetonatinyorbitwasnotwhatastronomerswereexpectingwhenitcametosolarsystems.

BackintheUS,MarcyandButlerquicklybeganlookingforplanetsonsuchshortorbits.Itdidn’ttakelongforresultstoappear.Atapressconferencejustafewmonths afterMayor’s talk inFlorence,Marcy andButler announced theirowndiscoveryoftwomoreJupiter-sizedworlds.14

Newplanetsbegantopileupafterthediscoveryof51Pegasib.Astheshockof discovering exoplanets wore off, astronomers got down to the work ofbuildingacensusofthenewworlds.

But therealprizestill lay in thesearchforEarth-sizedworlds livingin thestar’s habitable zone,wherewater andperhaps life could exist on the surface.TheEarth’smassisonethree-hundredththatoftheSun,afactthatmeantevenmoreprecisionwasneeded todetectEarth-sizedworlds.Thatneed forgreaterprecisionwasalsocoupledwiththeproblemthatreflexmotionsonlyworkedonone star at a time.What astronomers desperate for data neededwas a preciseway to discover planets wholesale. That threshold would be broken by thestubbornresilienceofamanwhosimplyrefusedtoacceptrejection.

BillBoruckiwas a longtimeNASA scientistwho had cut his teeth on thephysicsofspacecraftheatshields.Inthelate1970s,hedecidedtoswitchfields.The problem of planet detection offered the kind of technical challenges heloved, and after the Maryland meeting where transit-based planet-hunting

methods had been dismissed, Borucki became determined to show thesemethodscouldwork.Inanow-famous1984paper,heandacoauthorlaidoutthebasic framework for how tobuild a precise device to detect tiny changes in astar’slightoutput.Then,in1992,heproposedaspace-basedtelescopeusingthesametechnologyforplanethunting.15

While NASA thought the idea was interesting, it didn’t believe Borucki’sdetectors would work. Unperturbed by the proposal’s failure, Borucki begansystematicallyaddressingNASA’sconcerns.Hebuiltprototypesonthecheaptodemonstrate that his system could hit the needed goals. After months ofexhaustivework, Borucki’s designsworked exactly as he said theywould. In1994,hespentmonthsputtingtogetherthedocumentationneededtoproposehistransit-basedtelescopeagain.Theproposalwasrejectedasecondtime.

A different set of concerns was raised in the second rejection. The newquestionsfocusedonBorucki’sclaimthathecoulddotransit-detectiononmanystars atonce.Onceagain,Borucki cobbled funds togetherandcarried forwardtheextraordinaryeffortsneededtoaddresseachandeveryobjection.Fouryearslater,heandhisteamsentintheirnewversionoftheproposal.Theproposalwasshotdownathirdtime.16

A reasonable personmight have given up at that point.But in this regard,Boruckiwasnotreasonable.Heknewhewasright.Heknewthetransitmethodwouldbe agamechanger.Theonlydirectionhe could allowhimself tomovewasforward.

Eventually,Boruckiprevailed.Aftermore than twodecadesofworkingonthesameideaandhavingthatidearejectedasscientificallyunsound,Borucki’sproposal was finally accepted.What would come to be known as the KeplerMissionwasgiventhegreenlight.17

Keplerwasdesignedtostareatasingleportionofthesky.Inthatsmallpatchof cosmic real estate, about 156,000 individual stars had been identified asworthyofattention.18The satellitewouldpatientlywatch the same starsweekafter week, year after year. The patience was needed to accumulate enoughtransits—enoughdips in lightoutput—toprovideanunambiguoussignalofanorbitingexoplanet.

On March 6, 2009, the Kepler telescope rode into space on a Delta IIrocket.19Thelaunchwasflawless.Aftersomanyyearsofrejection,Boruckiandhis team were staring across the frontier, ready to see how well his decade-spanningvisionwouldwork.Theydidn’thavetowaitlong.

“As soon as the data started coming in from the spacecraft, we could seetransits,” recallsNatalieBatalha, aNASAastronomerwho joinedBorucki ten

years earlier. “You could see the dips as clear as day. We were literally justsitting there inouroffice,watching asnewplanetswerediscoveredwith eachtransit.”20

The first confirmed detections of exoplanets by Kepler came in January2010,buttheyweren’ttherealnews.Alongwiththesedetections,thousandsofKepler“candidates”wereidentified.Thesewerestarsshowingdipsinlightthathadn’t yet been confirmed as real planet detections.With so many exoplanetcandidates,theKeplerteamwassittingontheequivalentofacosmicpiñata.By2014, that piñata had been busted wide open. That year, the Kepler teamannouncedthediscoveryof715exoplanetsinasinglenewsrelease.21Wholesaleplanet hunting was the new reality. By 2015, the combination of Kepler andother methods had given astronomers 1,800 new worlds that were ready fordetailedinvestigation.22

As the list of exoplanets grew, one of the first and most importantconclusionswashowdifferentthearchitecturesofothersolarsystemscouldbefromourown.

Here on Earth, we all grew up learning about our solar system’s tidyarrangementofsmall,rockyworldstuckedclosetotheSunandlargergasgiantssplayedoutatever-greaterorbitaldistances.Theveryfirstexoplanetdiscovered,51 Pegasi b, showed that this arrangementwas anything but universal. It’s anexampleofwhat iscalleda“hotJupiter”—agasgiant thatsomehowendedupon an outrageously tight orbit. Big planets on small orbits are easy to find inreflex-motionsearchers,somanymoreofthesehotJupiterswerequicklyaddedtotheexoplanettally.LotsofstarswerealsofoundtohaveJupiter-sizedworldsonorbitsthesizeofEarth’s,ratherthanoutatthefartherreachesoftheirsolarsystems.

Eventually,otherkindsofplanetslivingclosetotheirparentstarswouldbefound—“hotNeptunes”andeven“hotEarths.”Innerrockyworldsandoutergasgiants were clearly not the only way nature laid out her planetary families.Systems with hot Jupiters were the most dramatic examples of “weird” solarsystems,butthereweremanyothersurprises.Systemsconsistingofonlysmallerrockyworldswerefound,andeventheylookedweirdbyourstandards.

“One of the big surprises was our discovery of what we call ‘compactmultis,’ ” says Batalha. “These are planetary systems with a bunch of smallplanets clustered very close to each other.”23 In our solar system, Earth andVenus are the nearest neighbors, coming as close to each other as twenty-fivemillionmiles.That’swhyittakesmanymonthsforustoreachtheseworlds.Butin the compact multiplanet system Kepler 42, for example, there are three

planetsstuffedintoremarkablytightorbits.Theseworldsgetonehundredtimesas close to each other as Venus ever gets to Earth.24 If you lived on one ofKepler42’sworlds,youcouldtraveltoyourneighborplanetinjustaweekorso,usingthekindofspacecraftthatgotustotheMoonbackin1969.

Thearchitectureofplanetarysystemswasn’ttheonlysurprise.“Wefoundawholeclassofplanetouttherethatdon’tevenoccurinoursolarsystem,”saysBatalha. There are no planets orbiting our Sunwith amass between those ofEarth andNeptune.That represents a considerable gap sinceNeptune is a bigmixofgasandiceandweighsinatfourteentimesthemassofEarth.EarthandNeptuneare,inotherwords,verydifferentkindsofplanets.Butastheexoplanetrevolution matured, astronomers soon found worlds—a lot of them—withmasses right in that gap between one and fourteen Earthmasses. They calledthese “super-Earths,” and it soon became clear that this new kind of planet,whichdoesn’tevenoccurinoursolarsystem,mightbethemostcommonintheuniverse.25

“Wedon’tevenunderstandwhattheseworldswilllooklike,”saysBatalha.“Some of them could be rocky. But some could be water worlds with deepoceanssurroundedbythickwater-vaporatmospheres.Otherscouldbeamixofrockandiceandgas.Thepossibilitiesareprettybroad.”

Beyondthegeneralfindings,thereweretheincrediblyweirdspecificcases.For example, there’s J1407B, the “super-Saturn” located 434 light-years fromEarth.Theringsorbitingthisgasgiantstretchtwohundredtimesfartherthanthegossamer disk surrounding Saturn.26 Then there’s 55 Cancri e, which is fortylight-yearsaway.ItsdiameterisonlytwiceasgreatasEarth’s,butithasamassalmosteighttimeshigher,resultinginadensitysogreatthatitmaybeaplanetmadeofdiamond.27AndnottobemissedistheominouslynamedWASP-12b.It’sahotJupiterwithatemperatureofnearly4,100degreesFahrenheit,makingitoneof thehottestexoplanetseverdiscovered.Astronomerscanseea trailofdebris surrounding the planet as WASP-12b boils away in a torrent ofevaporatinggas.28

Intheend,though,whatmattersmostarenothotJupiters,super-Saturns,orsuper-Earths.Thenumbersasawholearewhatmaketheexoplanetrevolutionsoimportant for us. At the beginning of the second decade of the secondmillenniumoftheCommonEra,humanityfinallylearnedthat,inoneveryrealsense,wewerenotalone.Therewereotherworldsoutthere.Justasimportant,with a full census of planets being built, the first three terms in the Drakeequationwere now fully known.With that advance, questions not only aboutplanets, but even about civilizations other than our own, could be seen in an

entirelynewlight.

DRAKEANDTHEEXOPLANETREVOLUTION

ThefirstterminDrake’sequationdescribestherateofmakingstars(calledN*).It has been known with some accuracy since the late 1950s, and subsequentstudieshaveonlyrefinedthatvalue(aboutonestarperyear).29ButwhenDrakefirstwrotehisequationin1961,thesecondterm,describingthefractionofstarswithplanets(calledfp),andthethirdterm,describingthenumberofplanetsinastar’shabitablezone(callednp),wereanyone’sguess.By2014,inthewakeofKepler and other exoplanet studies, there was enough data in hand to givescientists meaningful—that is, statistically significant—values for thosenumbers.

The implications of this advance are stunning enough to change ourexperienceofthenightsky.Let’sconsiderthefractionofstarswithplanetsfirst.Remember that, during the early part of the twentieth century, astronomersbelievedplanet formationwas a rare event,meaning the fraction of starswithplanetswouldbeverylow.Butby2014,theagreed-uponvalueforfpwasabout1.30Inotherwords,prettymucheverystaryouseeinthenightskyhostsatleastoneplanet.

Thenexttimeyoufindyourselfoutsideatnight,takeamomenttostopandconsidertheimplicationsofthisresultasyougazeatallthosepinpricksoflight.Everyoneofthemhostsatleastoneworld,andmoststarswillhavemorethanone planet. Solar systems are the rule and not the exception. They’reeverywhere.

The advent ofKepler also allowedastronomers to reach a firmconclusionabout the average number of habitable-zone planets orbiting each star.RememberthatthehabitableorGoldilockszoneisabandoforbitsaroundastarwhere liquidwatercanexistonaplanet’s surface.Thatmeansanyplanet inastar’shabitablezonemightbeaworldof rainand riversandoceans—aworldpotentially capable of supporting life. There are currently two planets in theSun’s habitable zone—Earth and Mars—and both have had water running intorrentsacrosstheirsurfaces.

Fromtheexoplanetdata,astronomerscannowsaywithconfidencethatoneoutofeveryfivestarshostsaworldwherelifeasweknowitcouldform.31So,whenyou’re standingout thereunder thenight sky, choose five randomstars.Chancesare,oneofthemhasaworldinitsGoldilockszonewhereliquidwater

couldbeflowingacrossitssurfaceandlifemightalreadyexist.The importance of the achievement represented by nailing these two

numberscannotbeoverstated.Throughthehard-woneffortsofagenerationofastronomers,we increased thenumberofknown terms inDrake’sequationby200 percent.Where there was darkness, there now is light.Where there wasignorance,therenowisknowledge.

YES,THEREPROBABLYHAVEBEENALIENS

Butwhat,ifanything,couldthetroveofdataleadingustothesenumbersrevealabout the possibility of other worlds inhabited by technology-deploying,civilization-buildingspecies?Westillhavezeroevidencethatsuchcivilizationsexist.Isthereanywaytoleveragetheachievementoftheexoplanetrevolutiontosay something—anything—about exo-civilizations? Addressing exactly thatquestionwasthetaskWoodySullivanandItookonatthebeginningof2015.

IfirstmetWoodySullivaninthelate1980s,whenIwasaphysicsgraduatestudentattheUniversityofWashington.He’stallandslenderwithawrysenseof humor and a passion for sundials and baseball (the Seattle Mariners, inparticular).Mostimportantly,WoodyisaradioastronomerwithanunwaveringinterestinSETI.WhenIwasagraduatestudent,hewastheonlypersononthefaculty at the University ofWashington who worked on the question of exo-civilizations. This was well before NASA began serious funding forastrobiology.Theexoplanetrevolutionwasstilladecadefromits inception. Inthe1980s,SETIanditsastrobiologicalsurroundingswerestillconsideredabit“out there” formany folks.ButWoodydidn’t care.Hewas interested, andhethoughttherewassciencetobedone.Sohepressedonandwroteanumberofimportantpapersonthesubject.

IoncehelpedWoodyteachacoursecalled“LifeintheUniverse.”Hesettheclassuptodealwitheverythingfromthenatureofphysicallawtotheprospectsfor life on other worlds. His perspective was broad and imaginative. I lovedbeing involved with that course, and its perspectives shaped my thinking fordecades. It was also the first time Woody and I started talking about exo-civilizations.Thoseconversationshavebeengoingoneversince,evenbeforeIdidanydirectworkinastrobiology.

In2014,WoodyandI foundourselvesasking ifall thenewexoplanetdatacouldbeusedtoinferadefiniteconclusionabouttechnologicalcivilizationsonotherworlds.Theastonishingprogressmadesincethefirstexoplanetdiscoveryhad to be good for something.Wasn’t there some way to use it with an eye

toward answering Drake’s original question about our uniqueness in theuniverse?We soon saw therewas apath forward, but to take it,we’dhave toturnDrakeonhishead.

Drake built his famous equation on a simple question: How many exo-civilizations exist now? He chose that focus because his real interest was infinding signals from alien civilizations. For his equation tomake sense, thosealienshadtobeoutthere,emittingradiosignalsrightnow(relativelyspeaking).ButtomakethekindofprogressWoodyandIwereinterestedin,werealizedwehadtochangethefocus.Wehadtoaskadifferentquestion—onethatcouldbeansweredby the exoplanetdata.Ournewquestionwasonly slightlydifferent,but thesmallchangewemadewouldmeaneverythingin termsofresults.Ourquestionwasthis:Howmanyexo-civilizationshavethereeverbeenacross theentirehistoryoftheuniverse?

Taking this approach gave us a strategy for getting an empirically basednumberconcerningtheexistenceofexo-civilizations.First,wecombinedalltheastronomicaltermsinDrake’sequationintoone.Thiswaseasy,sincetheywereall known. Then we began thinking differently about three unknownprobabilitiesinvolvinglifeinDrake’sequation(fl,fi,andft).Ratherthandealingwith them separately, our approach lumped them all together, too. We wereinterestedintheprocessasawhole,goingfromtheoriginoflifeallthewayupto an advanced civilization. We called our new term the “bio-technicalprobability,” fbt, and it is the product of multiplying all the usual life-centrictermsintheDrakeequationtogether.Inthelanguageofmath:

fbt=flfifc

Finally,byaskingabout the totalnumberofexo-civilizations thathadeverexisted,ratherthanlimitingourinteresttothoseexistingnow,wetooktheissueoftheaveragelifespanofacivilizationoutoftheproblem.Wedidn’tcareiftheexo-civilization overlapped with our own. It didn’t matter.We just cared thattheyhadexistedatsomepointincosmichistory.Effectively,thatallowedustoignorethefinalfactor—thepeskylifetimeterm,L—inDrake’sequation.

Our approach gave us a new form of Drake’s equation that looked a lotsimpler:

A=fafbt

In thisversionof theequation,Awas just the totalnumberofcivilizations

thathadeverexisted.WethoughtofAasstandingfor“archaeology”because,inaweirdway, that’swhatwewere interested in.Becausewe took thewholeofcosmichistoryasourplayingfield,mostofthecivilizationswe’dbedescribinginourapproachwouldprobablybe longgone.Butall thatmattered touswasthat they had existed at some point in cosmic space and time. That was thearcheologicalbentourapproachtook.WesawthattheKeplerdatacouldtellusmoreaboutwhathadhappenedthanwhatwashappeningrightnow.

Meanwhile,farepresentedalltheastronomicaltermsintheoriginalequation.Theimportantpointwasthat,sinceallofthosetermswerenowknown,fawasalsoknown.Thatleftjustthebiotechnicalprobability(fbt).Itrepresentedalltheunknown,life-orientedprobabilitiesinDrake’sequation.Thiswaswhatwewereafter.

By rewriting theequationwithoutLandusing thenewexoplanetdata,wethen saw thatwe could recast thequestionof theprobability of alien life in away that turned our new form of the equation into a very specific andscientificallymeaningful formulation.Our new question, therefore,was:Whatwould the biotechnical probability per habitable zone planet have to be forhumans to be the only civilization nature had ever produced over the entirehistoryoftheuniverse?

Inotherwords,whatwerethechancesthatoursistheonlycivilizationever?Putting in the exoplanet data,we found the answer to be 10–22, or one in tenbilliontrillion.32Wecalledthisnumberthe“pessimismline,”forreasonswe’llunpackbelow.Tome,theimplicationsofthisnumberarestaggering.

To understand how to think about the pessimism line, imagine you werehandedaverybigbagofGoldilocks-zoneplanets.Ourresultssaytheonlywayhuman beings are unique as a civilization-building species would be if youpulled out ten billion trillion planets and not one of them had a civilization.That’s because Kepler has shown us that there must be ten billion trillionGoldilocks-zoneplanetsintheuniverse.Sothepessimismlineisreallytellingushowbadtheprobabilityofacivilizationformingwouldhavetobeinorderforourstobeonlyonethathaseverexisted.

Tenbillion trillionplanets is a lot ofworlds togo throughwithout findinganything.Thesheersizeof thatnumberisenoughtomakeitseemlikewearenot the first time nature has ever created a civilization-building species. Bycomparison,thinkaboutgettingkilledbylightning,aneventmostofusthinkofas unlikely. The probability that you’ll be killed by a lightning strike in anygivenyear isaboutone in tenmillion.But,basedon thepessimism line,yourlightning-induceddeath is a thousand trillion timesmore likely thanhumanity

being the only civilization in cosmic history. Surely nature is not that biasedagainstevolvingcivilizations?Itcan’tbethatperverse.Orcanit?

Drake’s question—How many civilizations exist now?—still can’t beanswered.Butourquestion—Whatlimitcanbeplacedontheoddsthatit’severhappened?—couldbe.Wecouldputastakeinthegroundandsaythatifnature’sprocessesofevolutionledtooddslessthanthepessimismline,thenyes,oursisthe only energy-intensive, technological civilization that’s ever existed. But ifnature’s value for thebiotechnical probability is higher thanone in tenbilliontrillion,thenwearenotthefirst.

AfterourpaperwaspublishedinthejournalAstrobiology,Iwroteanop-edfor the New York Times about our result. The Times ran the piece with theheadline “Yes, There Have Been Aliens.”Within days, I was inundated withrequestsforinterviewsfromoutletsrangingfromthelargeandestablished,likeCBS,tosmallwebsitesrunbyavidUFOlogists.Someofthosefolksmighthavebeendiscouragedfromcontactingmeiftheheadlinehadbeenclosertowhatwereallymeant, which was “Yes, Aliens Probably Existed.” But either way, ourresult was bound to generate controversy. The critiques are worth looking atclosely,sinceinterpretingthepessimismlinecorrectlyiscritical.

Ourgoal,afterall,istoseehowastrobiologyandthestudyoflifeonotherplanetscanhelpusunderstandclimatechangeandtheprojectofcivilizationonour own world. In that pursuit, the pessimism line marks a critical boundarywhere wemight see our project set against the stars. But to truly understandwhatthepessimismlinecandoforusinthatendeavor,wemustfirstunderstandwhatitcannot.

THECRITIQUE

Oneoftheprincipalobjectionsraisedtoourpaper(andtheNewYorkTimesop-ed) was straightforward. Just because the probability that we’re the onlycivilizationincosmichistoryislow(10–22), thatdoesn’tconstituteaproofthatexo-civilizationshaveexistedbeforeus.Thiswas theargumentmadebyRossAndersen,thescienceeditorfortheAtlantic,andEthanSiegel,anastrophysicistwhowrites forForbes.33AndersenandSiegelareexcellent thinkers, and theircriticismscontainedalotofinsight.TheiressayscuttotheheartofkeyissuesinwhatWoodyandIweretryingtoexplore.Mostofall,theirskepticismmademethinkevenharderabouttheideasinourpaper,andIwasgratefulforthat.

TherewasonepointinparticularthatAndersentookissuewith,anditwas

embodied in this sentence from my Times op-ed: “The degree of pessimismrequired to doubt the existence, at some point in time, of an advancedextraterrestrialcivilizationbordersontheirrational.”34Hewasrighttocriticizethatline.Inspiteofthebarsetbythepessimismline,it’snot“irrational”tothinkweareuniqueincosmichistory.Infact,theonlyempiricallyvalidclaimWoodyandIcanmakeisthis:wecansaywithcertaintywherethepessimismlinelies.Intheabsenceofmoredata,itisrationallypossibletoconstructanargumentthatnature’svalueforthebiotechnicalprobabilityliesbelow10–22.

Questions were also raised about the values for the individual pieces thatmake up our biotechnical probability. Some argued that the probability ofmakingjustsimpleformsoflifewouldbetoolowtoallowcivilizationstoeverform.Orperhapsitwastheprobabilityoflifeevolvingitswayuptointelligencethat was really low. But these considerations don’t change our result. Ourbiotechnical probability, fbt, does not hide the fact that each of the life-centrictermsintheDrakeequationmightbesmallonitsown.Wedidn’testablishourpessimismlineby ignoringpossiblysmallvalues for the individual life-centricterms.Instead,ourreworkingof theDrakeequationbundled themall together.Ourapproachletusgoforthewholeenchiladaatonce:theentireevolutionaryprocess,fromabiogenesisuptothecreationofa technologicalcivilization.Nomatterhowimprobableyouthinkeachindividualstepis,it’sthetotalprobabilityofothercivilizationsexistingthatmatters.That’swhatyouhavetopayattentionto,andthat’swhatthepessimismlinerepresents.

Wecalledourresultthepessimismlineforgoodreason.Thewholehistoryof the debate about life beyond Earth is an argument between optimists andpessimists. It’s a debate that beganwith the opposition betweenAristotle andEpicurus,extendedthroughthe1800stoFlammarionversusWhewell,andtookits modern turn with the Drake equation, through which the battle betweenpessimismandoptimismbecamequantitative.

Sincethe1961GreenBankmeeting,manyscientistshavearguedthatexo-civilizationsare rare.What is rarely specified,however, isexactlywhat“rare”reallymeans.Scratchbelowthesurface,andyou’llseethatmanyself-describedpessimists’ version of rare is way above our pessimism line. That’s why thehistoryofthedebatecan’tbeignored.

Lookingacross thatdebate since theDrakeequationappeared,we see thatoptimismalwayshasaclearupperlimit.Youcan’tgetmoreoptimisticaboutthepossibilityoflifeevolvingonanexoplanetthanifyousayitalwaysoccurs(thatwouldmeansettingthevalueofflat1).Thesameholdstruefortheotherlife-centric terms in theDrakeequation.Youcan’tassignavaluegreater than1 to

theprobabilityofintelligence—orhightechnology—evolving.Makingallthesechoices implies that every exoplanet in the habitable zonewill create life thatgoesontoformanintelligenttechnologicalcivilization.

But pessimism is another story.How low is low?Howpessimistic do youhave tobe—expressed in termsof theDrakeequation—tobe trulypessimisticaboutexo-civilizations?ThatwaswhatWoodyand Iwereasking.Ouranswerprovidedalinemarkingthelimitoftruepessimism.Ifnaturehadabiotechnicalprobability that was lower than our limit—one in ten billion trillion—thenhuman beings had to be the only example of a high-tech civilization in thehistoryoftheobservableuniverse.Inthatcase,we’dbetrulyanddeeplyaloneinthemostabsolutelycosmicsenseoftheword.Butiftheforcesofevolutionledto a number higher than the pessimism line, thenwhat’s happenedwith us onEarthhashappenedbefore.

Ofcourse,westilldon’tknowwhatnaturehaschosen.Buttoseewhatournextstepsmightbeinthinkingaboutexo-civilizationsandourownfate,wecanlook at how our pessimism line compares with what actual pessimists haveproposedforthebiotechnicalprobability.

Pessimist#1:ErnstMayr.Chiefamong theexo-civilizationpessimistswasthe renownedGermanevolutionarybiologistErnstMayr.Mayrwas a brilliantscholar who was instrumental in linking classical ideas from Darwin to therevolutioningeneticsthatoccurredafterthediscoveryofDNA.ButMayrneverboughtCarlSagan’soptimismaboutSETIor the existenceofother intelligentformsoflife.In1995,thePlanetarySocietygavebothmenthechancetovoicetheiropinionsonthesubjectandrespondtoeachother’scriticism.WhileMayrneverprovidedanexplicitvalueforthebiotechnicalprobability,fromhisessay35wecanextractanestimateofhispessimism.

Mayrhadnodoubtsabout lifeformingonotherplanets.Of theprobabilitythat lifeexistselsewhere in theuniverse,hewrote,“Evenmostskepticsof theSETI project will answer this question optimistically.” Because moleculesnecessaryfortheformationoflifehadbeenfoundincosmicdust,heconcededthatitwasverypossibletherewaslifeelsewhere.

Thedevelopmentofintelligence,however,iswhereMayr’spessimismkicksin. Looking at the history of Earth, Mayr wrote, “Only one of these[approximatelyfiftybillionspeciesthathaveexistedonEarth]achievedthekindof intelligence needed to establish a civilization.” And on the subject ofintelligence leading to a civilization,Mayrwrote, “Onlyoneof [the twentyormorecivilizationsthathaveriseninthepasttenthousandyears] . . .reacheda

level of technology that has enabled them to send signals into space and toreceivethem.”

FromMayr’s statements, we can estimate what he thinks the biotechnicalprobabilitymightbe.Giventhathearguesthattheformationoflifeisnotahardstep,let’sassumehewouldbehappywithavalueofoneinahundredforthatfactor(10–2).Afterall,somethingthathappensonceeveryhundredtimesisnotreallyveryrare.

Given his statement about the total number of species evolved on Earthversusthesingleonethatbecameintelligent,wecaninferthatMayrmightsaythat the odds of evolving intelligence on anygiven exoplanetwith simple lifewould be one in fifty billion (or about 10–11). That certainly seems prettypessimistic.Finally,fromhisstatementsaboutcivilizationsbecominghightech,wemight inferhe’dconsider theprobability for that term tobeone in twenty.Let’serronthesideofpessimismandcallthisoneinahundred(10–2).

Ifweputallofthesetogether,wewouldfindthatMayrseemstobearguingthatthevalueofthebiotechnicalprobabilityisaroundoneinathousandtrillion(10–15). That is certainly pretty small.Recall that ifMayr is right, youwouldhave to sort through a bag of one thousand trillion planets to find a singletechnologicalcivilization.Giventhatthereare“only”onehundredbillionstarsin our galaxy,Mayr’s brand of pessimismwouldmeanwewere alone in ourgalaxy.

Butbeingaloneinthegalaxyandbeingtheonlycivilizationtheuniversehaseverproducedare twodifferent things.ComparingMayr’spessimismwith thelimit expressed by the pessimism lineWoody and I derived shows somethingremarkable.

EvenifcivilizationswereasrareasMayrproposes,thereisstillavastgulfbetweenMayr’s“oneinathousandtrillion”andthepessimismline’s“oneintenbillion trillion.”Tobeexact,even ifMayr iscorrect, therewill stillhavebeentenmillionhigh-techcivilizationsappearingacrossspaceandtime.Thatmeansten million individual stories of a species waking up to itself. Ten milliondifferentversionsofsciencebeingharnessedtoharvestaplanet’sresourcesandbuildacivilization.Tenmilliondifferenthistoriesofcivilizationseithergoingontobecomelong-lastingorcollapsingundertheweightoftheirownchoices.

Ifyoutriedtoimaginethehistoryofeachofthesecivilizations,givingeachone an hour of your time, it would take 1,140 years to get through them all.That’showmanyexo-civilizationswouldhaveexistedinwhatMayrthoughttobeapessimisticuniverse.

Pessimist #2: Brandon Carter. In 1983, the physicist Brandon Carterdeveloped an absolutely ingenious argument against exo-civilizations. Carterwas famous forusingsimpleobservations to infer immenselyvastconclusionsabouttheuniverseandourplaceinit.

His thinkingaboutexo-civilizationsbeganwith thesimpleobservation thatthetimerequiredforintelligencetoariseonEarthwasclosetothetotalageoftheSun.Inparticular,whiletheEarthhasbeenhabitableforfourbillionyears,itwillonlyremainsoforanotherbillionorsoyearsbecausetheSuniscontinuallyheatingup.ItwilleventuallygrowsohotthattheEarth’sorbitwillnolongerbeinthehabitablezone.Thus,atechnologicalcivilization(ours)hasonlyappearedonEarthclosetotheendofitsperiodofhabitability.Usingthisonefact,Cartermade thecase that intelligencemusthaverequiredevolution topass throughaseriesof“hardsteps.”Fulfillingeachofthesehardstepswoulditselfbehighlyimprobable.36

Looking at Earth’s evolutionary history, Carter argued that there were tenevolutionary hard steps. These included the evolution of oxygenicphotosynthesis or of multicelled animals. Based on these ten hard steps, hedeviseda calculation topredict theprobabilityof exo-civilizations,whichwasjustourbiotechnicalprobabilitybyadifferentname.Carter’svaluecameouttobe10–20.Heclaimedthiswas“more thansufficient toensure thatourstageofdevelopmentisuniqueinthevisibleuniverse.”

What iswonderful aboutCarter’s calculation is that it leads to an explicitnumberforthebiotechnicalprobability.Thenumberhecalculatedwassosmall,itimpliedtohimthatnotechnologicalcivilizationotherthanourowncouldeverhaveexistedacrossallcosmicspaceandtime.

Butthat’snotwhatCarter’snumberimplies!AcomparisonofthepessimismlineWoodyandIfoundwithCarter’sresultshowsthathis1983calculationstillallows for one hundred exo-civilizations.Carter intended his calculation to behyper-pessimistic, but it turns out to be optimistic instead. Carter’s originalargumentstillleavesuswiththeremarkableconclusionthatwearenotthefirst.If Carter is correct, a hundred other civilizations had passed through theprocessesofcivilizationbuildingthatwefindourselvesinnow.

It shouldalsobenoted that researcherswhohave followedCarter’s lineofreasoning now believe only five hard steps exist, if any exist at all.37 Thisconsideration,combinedwiththeothervaluesinCarter’soriginalpaper,impliesabiotechnicalprobabilityof10–10.Compare thatwithourpessimismline,andyouendupwithatrillionexo-civilizationsacrosscosmichistory.Allowingfortheexistenceofatrillionothercivilizationsisanythingbutpessimistic.

Pessimist#3:HubertYockey.Ofcourse,onecanfindways toargueforahyper-hyper-pessimisticviewpoint.ThisisexactlywhatHubertYockeydidina1977 paper. Yockey was a physicist and information theorist. His argumentfocused on the first life-oriented term inDrake’s equation—the probability oflifeformingonanexoplanet.Whataretheodds,heasked,thatrandomchemicalcombinations would produce the right kind of self-reproducing molecule forgetting life started? His answer was less than an astonishing one in a trilliontrillion trillion trillion trillion (his actual value was 10–65).38 This number iscertainlybelowourpessimismline,andifYockeyisright,thenwerepresenttheonlytimeincosmichistorythatlifeofanykindhasemerged.

But this kind of argument is balanced by the fact that there are strongcounterargumentsthatlife’semergencemaynotbesohardtoachieve.Manyoftheseresponsescomefromadvancesinbiology.Forexample,biologistWentaoMaandcollaboratorsusedcomputersimulationstoshowthatthefirstreplicatingmoleculescouldhavebeenshortstrandsofRNA(amoleculecloselyrelatedtoDNAandanintegralpartofcellularmachinery).ThesearemucheasiertoformthanwhatYockeywasthinkingabout.39Manyresearchersalsotakethefactthatlife appeared so quickly after the Earth’s formation as an indication thatabiogenesismaynotbeextremelyhardtoachieve.Eitherway,Yockey’shyper-hyper-pessimismseemstobeanoutlierinthedebateaboutalienlife.

THEBIGSTEP

Thepessimism linedoesn’tprove thatother civilizationsonotherworlds everexisted.Itdoesn’thelpusinoursearchforsignalsfromothercivilizationsthatmayoverlapwithourown.Sowhat,exactly,doesitallowustosayortodo?

More than anything, what Woody Sullivan and I did was use exoplanetscience to raise a key philosophical point that drew its potency from realobservations. Itwas an opening into away of thinking about our place in theuniverseandthechallengesofourAnthropocenemomentinaradicallydifferentwayfromwhatwe’redoingnow.

While the Drake equation was all about making contact with othercivilizations,ourperspectivewasstraightforward:theexoplanetdatanowletsusmakeareasonableargumentthattherehavebeenmanyothercivilizationsbeforeours. If you agree that the pessimism line is low enough tomake those othercivilizationsfarmoreprobablethanwecouldhaveknownbefore,thenyoucanalsotakethestepofconsideringthemworthyofseriousconsideration.Withthat

step, something remarkable can follow in facing the challenge of theAnthropocene.

Beforewegoanyfurther, letmebeclear thatyoudon’thave tomake thatstep. Remaining deeply agnostic about the existence of other civilizations incosmichistoryiscertainlyastancethatsciencecannotargueagainst.So,ifyoudon’tthinkit’sworthconsideringthoseothercivilizationsseriously,that’sfine.EverythingwehavealreadyexploredaboutastrobiologyandtheAnthropocenewillstillholdtrue.Ourunderstandingoftheclimatechangewearedrivingtodaymuststillbeseenasgroundedinwhatwelearnedbystudyingotherplanets inthesolarsystem.Andourquestionsaboutwhattodonextmuststillbeinformedby the understandingwe gained by studying our own planet’s long history ofcoevolutionbetweenthebiosphereandEarth’sothercoupledsystems.Weknowwhatwe know becausewe have already learned a lot aboutwhat itmeans tothinklikeaplanet.ThatmeanstherearerulestothegamewhenitcomestotheevolutionoftheEarth,includingtheEarthwithusonit.Thatperspectivealoneundercutstheargumentsofclimatedenialistsandrepresentsafundamentalshiftinhowweunderstandourselvesandthechallengesbeforeus.

Butifyouarewillingtoseethepessimismlineastheuniverse’sinvitationtoconsider other civilizations seriously, then we can begin to ask what othercivilizationsmeanforus.Thepurposehereisnottoconsiderthemasthesourceof science fiction stories, but to recognize that we are probably not the firstexperimentincivilizationbuildingtheuniversehasrun.

Throughoutallofhistory,ourmythologieshavetolduswhoweare,whatweare,andwherewestandinrelationtothecosmos.Butthosestoriesignoredthepossibilitythatweareoneofmany.Ourstoriesdidnot—becausetheycouldnot—includethepossibility thatourcivilizationwasaplanetaryphenomenonthatwasnotunique.Thatiswhytheexoplanetrevolutionandalltheastrobiologywehaveexploredsofarcanbeakindofwake-upcallforus.Itcanbepartofourcomingofageasacivilization.

The discovery that the universe is teeming with habitable-zone planetsconnectsthechallengeswefaceintheAnthropocenedirectlywiththequestionsFermi,Drake,andSaganaskedfiftyyearsago.Thepessimismlinetellsusthattheuniversehashadlotsofopportunities todowhat itdidonEarth.Withthatinformation,wecanbegintoseriouslyconsiderthattherehavebeenmanyotherstories, meaning many other histories, beyond our own. It’s an invitation tobeginputtingourselvesandourchoices intoamoreaccurateand fullycosmiccontext. Ifwe take that step, then everythingwe’ve learned about planets andclimateandbiospheresbecomesrelevanttothoseothercivilizations,too.Wecantreatthoseothercivilizationsasobjectsofstudy.Thatiswhyascienceofthose

civilizations—atheoreticalarchaeologyofexo-civilizations—istheterritorywemustexplorenext.

CHAPTER5

THEFINALFACTOR

MANYWORLDS,MANYFATES

Two decades into the twenty-first century, we find ourselves facing theexistentialchallengeofcreatingasustainableversionofhumancivilization.ThescaleofhumanactivitiesispushinghardonthetightlylinkedplanetarysystemsthatmakeupEarth’sclimate.As theplanetbegins tomoveoff intoadifferentclimatestate,ourprojectofcivilizationwill,at thevery least, find itselfunderstress.Atworst,Earth’schangesmaymakeourprojectimpossibletomaintain.

We urgently need to adapt civilization so that it can continue for the longterm, so that it can become fully and globally sustainable.But beforewe canstartworkingtowardthatgoal,there’sanequallyurgentquestionthatoftengoesunstated:Howdoweknowthat’sevenpossible?Howdoweknowthereissucha thingasa long-termversionofourkindofcivilization?Mostdiscussionsofthesustainabilitycrisisfocusonstrategiesfordevelopingnewformsofenergyortheprojectedbenefitsofdifferentsocioeconomicpolicies.Butbecausewe’restucklookingatwhat’shappeningtousasasingularphenomenon—aone-timestory—we don’t think to step back and ask this kind of broader question. Toeveninposeitseemsdefeatist.Butitmustbeaddressedifwearetomakethemostinformed,intelligentbetsonthefuture.

Let’s be clear about what our question implies. Maybe the universe justdoesn’tdolong-term,sustainableversionsofcivilizationslikeours.Maybeit’snotsomethingthat’severworkedout,evenacrossalltheplanetsorbitingallthestars throughout all of space and time.Maybe every technological civilizationlike ours has been just a flash in the pan, lighting up the cosmos with its

brilliance for a few centuries, or even a fewmillennia, before fading back todarkness.

ThisquestionspeaksdirectlytoFermi’sParadox.PerhapsthebottleneckwefacetodayexplainstheGreatSilenceofthestars.Ourquestionpointstothefinalfactor inDrake’sequation—theaverage lifetimeofcivilizations.Even ifeveryplanetorbitingeverystarintheuniverseevolvedacivilization,itwouldstillbepossible that none lasted very long. That kind of fate might be universal forexactlythesamereasonswefindourownfuturechallenged.

So,doesanyonemakeitpastthechallengewenowface?Staringdownthatquestioniswheretherubberreallymeets theroadin the

astrobiology of theAnthropocene.The pessimism line tells us that, unless theuniverse ishighlybiasedagainst theappearanceandevolutionofcivilizations,otherscamebeforeus.Eachofthosecivilizationswillhavehadatrajectoryofdevelopment in termsof theirgrowthandtheir impactsontheirplanets.Thosetrajectoriesarewhatwewanttounderstand.Givenwhatwehavelearnedaboutplanetsandclimate,therearegoodreasonstoarguethatmanyplanetsevolvingayoung, energy-intensive civilization will be driven into an Anthropocene-liketransition.If therehavebeenexo-civilizationsbeforeus,we’vealreadylearnedenough about “thinking like a planet” to see if the conditions leading toAnthropocenesarecommonorrare.So,howcanweusethescienceweknow,gainedfromtheplanetswehaveseen,tobeginascienceofthecivilizationswehaven’t?

HOWNOTTODOASCIENCEOFEXO-CIVILIZATIONS

Prostheticforeheads.That’swhatyouwanttoavoid—theKlingons,theVulcans,theUFOalienswiththebigheads.Sciencefictionhasgivenusenduringimagesofalienraces.Notsurprisingly,mostofthemlookalotlikeus,butwithdifferentkindsofforeheadsorearsoradifferentnumberoffingersontheirhands.

Indevelopingourscienceofexo-civilizations,we’renot interested inwhataliens might look like or how they might behave. We’re going to avoid thespecificsoftheirbiologyandtheirsociologybecausescienceprovidesuslittletoworkwithonthoseissues.

So,whatissuescansciencehelpuswith?TherearethreetermsfromDrake’sequation thatmakeup thebiotechnicalprobability.They involvebasicbiology(theoriginoflife),evolutionarybiology(theriseofintelligence),andsociology(thedevelopmentof societies).When it comes towhatmighthappenonotherplanets,weareonmurkygroundforeachoftheseterms.Butifweasktheright

questions,thereareprinciplesthatconstrainourtheoreticalexplorations.Theseconstraints are like guide rails keeping our theoretical bowling balls fromplungingintothegutter.

For the basics of life, for example, we are going to have to rely on ourknowledge of chemistry.Butwe already know that chemistryworks the samewayindistantregionsofthecosmosasitdoesonEarth.Fromobservationsofinterstellar clouds, planet-forming disks, and even exoplanet atmospheres, wecan see physics and chemistry playing out exactly as they do down here onEarth.So,nomatterwhatsurpriseslifeonotherworldsmayhaveforus,itmuststill utilize the samebasic laws of physics and chemistry that apply onEarth.Basedonthiscosmicuniformity,scientistsarealreadyexploringwhatalternativebiochemistriesmight look like.1There are even studiesofhowphotosynthesismightworkonplanetswithverydifferentkindsofsuns.2

Onthequestionof intelligence, thingsgetshakier.That’sbecausetherearesomanystepsneeded for itsdevelopment.Worse,wedon’tknowwhichstepsareessentialandwhichhappenedtobespecifictohowintelligenceworkedoutonEarth.Indealingwiththeevolutionofintelligence,however,wedoat leasthaveaprinciplewebelieveshouldbegeneralacrossallplanets.ThegeniusofDarwinianevolution is itsubiquity.Darwinproposedthatall lifeonEarthwasshapedbythesamesetofsimpleprocesses:mutation,adaptation,andsurvivalofthe fittest. Simply put, whatever organism is best adapted to the environmentwill outlast its competition. It’s aprinciple that applies to everything from thefirstself-replicatingmoleculestomodern,fully-formedbiologicalorganisms.Itshouldevenapplytofutureself-replicatingrobotsifweevermakethem.

So, when it comes to evolution on other worlds, this kind of uniformityshouldproveuseful,particularlywhenwethinkgloballyintermsofbiospheres.Darwinian evolution, in terms of population growth and competition inecosystems, gives us a constraint for our ideas as we follow them to theirconsequences.

The scienceof sociologyand thequestionof the formationof civilizationsseem to be another story entirely. We cannot assume that sociological truthswe’ve observed on our world will hold true across time and space. Do othercivilizationshavepoliticalparties?Dotheyworshipagodorgods?Wecantellstoriesabouthowanexo-civilizationmightorganizeitself,butourdescriptionswouldalwaysbejustthat—astory.Here,Iamspecificallyreferringtoquestionsof their morality or economics or religion. Have they, for example, createdinstitutionsthatvaluealtruismoverconflict,orconflictoveraltruism?Doestheideaofinstitutionsevenmakesenseintheircivilizations?

Unlike the foundational laws of physics and chemistry or the potential forDarwinian evolution to be cosmically general, it’s hard to see what kind ofuniversalprinciplesexist thatwouldallowus toconstrainsomething likealieneconomics.Whenitcomestosociology,Idon’tbelievesuchconstraintsexist.

So, while we are now in a position to begin building a science of exo-civilizations, the questionswe canmeaningfully take onmust be limited.Weneed to avoid science fiction stories. That means speculation about whethercivilizations are warlike or peaceful, or whether cultures focus on empirebuilding or are content to stay at home, is out of bounds. Trying to answerquestionsaboutanyofthesedichotomiesisclosetoahopelesstask.Extendingour knowledge from the seen to the unseen requires something that keeps ourtheorybuildingwithinnature’spossiblebounds.Nomatterhowfarwewanttoreach,therehastobesomegroundforourfeettostandupon.Forthetimebeing,that means sticking with the physics and chemistry of planets (things likeclimate)andthepartsofbiologyonecanreasonablyargueshouldbecommon.Indevelopingourscienceofexo-civilizations,weshouldtrytoavoidquestionsabout culture. That will be the challenge in building our astrobiology of theAnthropocene.

Ofcourse, this strategycutsoutawhole lotofquestions thatmanypeoplewant to know about exo-civilizations. For example:What are aliens like?Dotheyhave twosexes,or twenty-three?Have theybuiltasocietyon logicoronlove?Aretheytradersorwarriors?Andofcourse:Dotheylooklikeus?Ifthoseare the questions youwant to ask, I’m afraid you’re out of luck as far as ourscientificallyboundedtheorizingisconcerned.

But there isonespecifickindofquestionabout thosecivilizations thatourscience of exo-civilizations can address directly. By sticking to the laws ofplanets we learned through Carl Sagan, Jack James, Lynn Margulis, JamesLovelock, and thousands of others, we can now ask the question thatmattersmost to our project of civilization: How common is the Anthropocene? Howoften do civilizations trigger climate change on their planets? And, mostimportant,howeasyisitforacivilizationtomakeitthroughitsAnthropocenebottleneck?

OFPREDATORSANDPREY

TheAdriaticSeahas fed Italy’s eastern shores for eight thousandyears.FromVenice in the north toBrindisi in the south, itswarmwaters have provided alivelihood for more than a hundred generations of fishermen. There are 450

differentspeciesoffishswimmingintheAdriatic,manyofwhichendupasfoodon Italian tables.3 But those tables have always been demanding. Humanfishermen are the Adriatic’s top predator, and many of the sea’s species arecurrentlyindangerofcollapsefromoverharvesting.

But thebeatoffishermen’soarsor thebuzzof theirmotors in theAdriatichas not been uniform across history.Conflict can slow the pace of fishing, asfleets ofwarships patrolling coastsmake thework evenmore dangerous thannormal.InWorldWarI,theAdriaticbecameabattlezone.Thenewefficiencyofmechanized navies gave Italy’s enemies a long enough reach that commercialfishingintheAdriaticalmostgroundtoahalt.

For all its hardship, that lull in fishing proved to be an unlikely gift toscience.ByslowingthehumandrawontheAdriatic’sfishingstocks,aparadoxsurfaced that reshaped how biologists thought about animal populations,ecology,andthenatureoftheirownwork.

In the years immediately after the war, a young marine biologist namedUmberto D’Ancona was working himself to exhaustion studying fishpopulations and their evolution. Through long, diligent work, D’Anconaamassed statistics on sales at fish markets in cities like Trieste, Fiume, andVenice, across the length of Italy’sAdriatic coast.His data bracketed thewaryears, beginning with 1910 and ending in 1923. Poring over the numbers,D’Anconasawsomethingthatdefiedexplanation.

During the war years, when fishing had been reduced, the number ofpredators such as sharks seemed to soar. This might have made sense if thenumbers of prey fish, like mackerel, had also climbed, as D’Ancona hadexpected.Morepreyshouldmeanmorepredators.Butthenumbersofpreyfishdidn’t rise during the war. Instead, they dropped. The statistics in front ofD’Anconatoldhimthat lessfishingledtofewerpreyfishandmorepredators.Theyoung scientist puzzledoverhis paradoxuntil, in desperation, hebroughthis biology problem to an unlikely consultant: the great mathematician andphysicistVitoVolterra.4

Volterrawasaworldleaderinsolvinghardphysicsproblems.Hisworkhadtouchedoneverything fromthestructureofcrystals to thebehaviorof fluids.5ButVolterra’s reputationwas not themain reason fate andD’Ancona broughthim into the domain of biology.D’Anconawas alsomarrying the professor’sdaughter.LuisaVolterrawasherselfascientist,withaspecializationinecology—thebiologicalstudyofpopulationsandtheirenvironment.

PhysicistVitoVolterra (third from left)developed thepredator-preymodelofpopulationecologyforhisson-in-law,marinebiologistUmbertoD’Ancona(farright).D’Ancona'swife,ecologistLuisaVolterra(daughterofVito),standsnexttohim(circa1930).

At the timeVolterra tookup theproblem,mathematical “modeling”of thekindfoundinphysicswasnotyetinthebiologists’toolkit.Biologistscertainlydealtwithstatistics,butmodelingissomethingdifferent.Modelingisessentiallya theoretical enterprise. It’s a process that begins by choosing a set ofassumptionsabouthowtheworldworks.Thoseassumptionsthengetturnedintomathematicalequations,andthoseequationsarewhatscientistsmeanwhattheytalkaboutamodel.

AswehaveseeninbuildingclimatemodelsforEarthorMars,theessentialstep in mathematical modeling is solving equations. Those solutions aredescriptionsoftheworld’sbehaviorovertime.Theyare,therefore,predictions.So, whatever equations Volterra came up with for D’Ancona’s fish problem,theirsolutionsneededtopredicthowthepredatorandpreypopulationschangedwithtime.

Physicists have been making mathematical models ever since Newtondevised his laws of mechanics back in the 1600s, giving physics its deeplytheoreticalemphasis.Butbiologistsintheearlytwentiethcenturysawtheirworkinadifferentway.Thekindofmodelingphysicistsroutinelycarriedoutdidn’t

seem up to the task of explaining the complexity of living systems and theirinteractions.Ascomplicatedastheorbitsofplanetsmightbe,thecomplexityofa single cell, or even a simple food chain, puts astronomers to shame. Forbiologists,fieldworkalwaysledtheway.6

By the time Volterra began thinking about his son-in-law’s problem,however,thingswerechanging.Amovementhadalreadybeguntobringtheory,in the formofmathematicalmodels, intobiology.Thatworkhadbegun in the1800s,whenPierreVerhulstofBrusselsdiscoveredwhatheclaimedwasalawof populations.7 Consider, for example, a few bacteria introduced to a pond.Theirnumberswillclimbrapidlyaseachcelldivides into twonew“daughter”cells. The two daughter cells then divide, leading to four granddaughter cells.Theprocesscontinues,yieldingeightcells, and then sixteen, and soon.Soon,the bacteria population is skyrocketing. But it’s a process that can’t continueforever. Limitations on food and space mean that at some point the bacteriapopulationreachesanenvironmentallimit.Thatlimitiscalledtheenvironments’carryingcapacity.Thepopulationstartslow,risesquickly,andthenflattensoutattheenvironment’scarryingcapacity.

A century later, Volterra (and others) took theoretical biology further bycreatingwhatisnowknownastheclassicpredator-preymodel.8Itbeginswithtwoequations.One tracks thepreypopulation,whichcouldbe something likethenumberofbunnies inaforest.Thesecondfollowsthepredatorpopulation,which we could imagine as the number of wolves in the same forest. Theimportant point for modelers to capture is that the two populations are tiedtogether.Thewolveseatthebunnies,andthatchangesthebunnypopulation.Buteating bunnies lets the wolves reproduce, adding to their population. So, thebunny population affects the wolf population, too. In these linked equations,there’sapart(a“term”)thatdescribeshowthebunniesgeteatenbywolves,andanotherthatdescribeshowthewolveshavemorebabiesbyeatingbunnies.

In the language of math, the predator (wolf) and the prey (bunny)populationsarecoupled.Theydependoneachother.Thetwoequationsmustbealsosolvedtogether,whichmakestheproblemtrickyfromatechnicalpointofview.Volterraworkedoutthissolution,anditshowedhimthatthewolvesandbunnies can end up cycling back and forth from high to low populations andbackagain.Whatwastrulysurprising,though,wasthetiming.

Inanenvironmentwhereboththebunnyandwolfpopulationsstartlow,themodel predicted that only the prey begin increasing rapidly. The bunnies startreproducing first, and their numbers climb. The wolf population only beginsincreasingafterenoughbunniesarearoundtomakethemeasytofindandcatch.

Eventually, the bunny population peaks as the rapidly growing number ofwolves starts having its impact. After that, the bunny numbers drop and theystarttogrowscarce.Thewolfpopulation,however,takessometimetofeelthechange.Onlylaterdotheirnumberspeakandthenstartdropping.Eventually,thewolfpopulationgetslowenoughforthebunniestorecover,andthecyclebeginsanew.

WhatD’Anconasawduringthewarwasthatthesharks(thepredators)werestillontheupswing,whilethemackerel(theprey)werealreadypasttheirpeakandindecline.Volterra’smodelpredictedthelagbetweenpopulationpeaks,soitexplainedwhythesharknumberswouldbeseenincreasingwhilethemackerelwerefalling.Inthisway,Volterra’stheory—meaninghismathematicalmodel—letD’Anconaget to therootofhisapparentparadox.9The theoryrevealed theessentialbiologyofpredator-preyinteractions.

WhatemergedfromtheworkofVolterraandotherpioneerswasatrueformof theoreticalbiology. In this setting, theory doesn’tmean a hypothesis, like adetective’snotionofwhocommittedamurder.Rather,inscience,theorymeansa large body of knowledge resting onmathematical principles that have beenthoroughlyverified throughexperience.Thetheoryofpopulationbiology(alsocalled population ecology) that Volterra and others founded was powerfulenough that it could be applied to an ever-growing range of problems.Today,populationbiologists,ecologists,andtheircompatriotsusemathematicalmodelsto study everything from the spread of disease to the propagation of invasivespecies.10Their approachwould, eventually, even find itsway to the study ofhumancivilizations.

EASTERISLAND,EASTEREARTH

Easter Island is a longway from anywhere. Locatedmore than two thousandmileswestofChileandfourthousandmilessoutheastofHawaii,it’sanisolatedoutpost of land surrounded by seemingly boundless expanses of ocean. Theskilled Polynesian sailors who colonized the Pacific thousands of years agodidn’t reachEaster Island in their longcanoesuntil sometimearound400CE.When they did, they found an island rich in fertile soil, as well as plant andanimallife.Itwasapromisingbeginningtoastorythatwouldendinruin.

WhenDutchexplorersdiscoveredEaster IslandonEasterSunday in1722,theyfounda“barrenplacewithafewthousandpeoplelivinginabjectpovertyandfightingovermeagerresources.”11Theislandwasdevoidof trees,andthe

ground was covered only with unproductive scrub. But the huge stonemonuments dotting the island and fashioned in the shape of human sentinelsspokeofaverydifferentpast.Manyofthe“stoneheads”werethirtyfeethighandweighedmorethanfiftytons.ThesilentfacesofthemonumentsreflectedatimewhenEasterIslandhadhostedavibrantcivilizationwithapopulationthatmayhavepeakedatmorethantenthousandpeople.12Whatevercultureexistedbefore theDutcharrived, itwas technologically advancedenough to carve themonuments from rock located at the volcanic core of the island and transportthemacrossmilesofruggedterrain.

Themystery of what happened to Easter Island’s civilization has hauntedgenerationsofwritersandscientists.ErichvonDäniken, inhis1973bestsellerChariotsoftheGods?wentasfarastosuggestanaliencivilizationwastheonlyexplanation.13How,heasked,couldtheislandershavemovedthemassivestonemonumentswhentherewerenotreesaroundtouseasrollers?Butancientalienswere not required.The answer toEaster Island’smystery turned out to be farsimpler,andfarmoredepressing.

TherearenotreesonEasterIslandbecausetheEasterIslanderscutthemalldown. They deforested their island in the building and transportation of thosegiant stone heads. In the process of deforesting the island, they also started adownwardspiralthatdrovetheircivilizationtocollapse.

TheiconicstatuesonEasterIslandareevidenceofathrivingcivilizationthatcollapsedbeforetheDutchlandedtherein1722.

While there remains debate about the exact trigger forEaster Island’s fall,environmental degradation driven by the inhabitants’ own activity played anessentialrole.EasterIslandservesasanobjectlessonfortheinteractionbetweenan isolated, habitable environment and a civilization using that environment’sresources:theydidittothemselves.TheparalleltoourcurrentsituationonEarthseemsclear.

Inhis2007bestseller,Collapse,anthropologistJaredDiamondunpackedthatparallel.14Hisworkexploredthetrajectoriesofanumberofhumancivilizationsthatdisappearedattheheightoftheirvibrancyandpower.Diamond’sexamplesincluded the Anasazi of the American southwest, the Maya, and the Norsecolony on Greenland. In each case, the civilization overshot the carryingcapacityofitsenvironment.Theirpopulationsgrewasthesocietybecameevermore ingenious at extracting resources from its surroundings. Eventually, thelimits to growth were hit. A short time after running into those limits, eachcivilizationfellapart.EasterIslandwastheposterchildforDiamond’sstory.

BythetimeDiamondbroughthistoricalexamplesofenvironmentalcollapsetothepublic’sattention,scientistshadalreadybegunthemathematicalmodelingofEasterIsland’sfall.UsingthesamekindsofbiologicalpopulationmodelsasthosepioneeredbyVolterraandothers,theseresearchersdevelopedequationstoexploretheislanders’trajectoryfromvibrancytocollapse.

It began in 1995 with a paper by environmental economists James A.BranderandM.ScottTaylor.15BranderandTaylor setout twoequations.Thefirst described the change in the human population over time. The seconddescribedthechangeintheavailabilityoftheisland’sresourcesovertime.Justas inVolterra’s predator-preymodels, the two equationswere coupled.As theislanders used the island’s resources for food and technology, their numbersgrew.Theresources,liketrees,wererenewable,andtheequationscoulddescribethemgrowingbackatnaturalrates,evenastheywereharvestedbytheislanders.ButwhenBranderandTaylorsolvedtheirequationsforthecoupledtrajectoriesofboththehumanpopulationandtheisland’sresources,theirmodeltrackedtheislanders’fatewithagrimcertainty.

As the population grew, the resources could not keep up. Overharvestingpulledresourcesdown,andeventually,theisland’sinhabitantswentwiththem.Peakingsometimearound1200CE,thehumanpopulationofEasterIslandthenexperiencedagradualdie-off,endingwithjustafewthousandinhabitantsleftbythetimetheDutcharrived.Themathematicalmodelgotthegeneraltrendinthe

historyright.Other researchers soon followed up on Brander and Taylor’s work. They

changedtheassumptionsinthemodelbyaddingnewtermstotheequationsorchangingtheformofthetermstoreflectdifferentkindsofinteractions.A2005study by Bill Basener and David S. Ross16 looked at the problem slightlydifferently.Theyassumedthattheislandhadacarryingcapacityforhumans,aswell as for the island’s resources (like trees or animals). In theirmodels, theythenmadethehumancarryingcapacityexplicitlydependentontheresources.Astheresourcelevelsdeclined,theabilityoftheislandtohostahumanpopulationwould drop as well.When Basener and Ross solved these new equations forEaster Island’s history, they saw something different from the gradual die-offBranderandTaylorfound.Thepopulationclimbedtoitspeakandthendroppedlikeastone—atruecollapse.

Theorybuilding regarding thehistoryofEaster Islandcontinues,withnewstudiesappearingeachyear.Therearemanyopen issues that researchersmuststrugglewith, since someof thedataabout the islandbefore theDutcharrivalremains open to interpretation.But the basic path of the islanders’ fate seemswellcapturedinthemodels.

Thatsuccessshowsusthewayforwardforthinkingaboutourownplanetaryfate in its proper cosmic context. What is true for an isolated island, itsecosystems,anditsinhabitantsshouldalsobetrueforplanetsintheisolationofspace.

ATHEORETICALARCHAEOLOGYOFEXO-CIVILIZATIONS

In1959,CarlSagantookthegreenhouseeffect,atheorydevelopedsixtyyearsearlier for theEarth,andapplied it to thedistantplanetVenus. In1983,JamesPollackandhiscollaboratorstookdetailedmodelsofduststormsonthedistantplanetMarsandappliedthemtoEarth’sownclimateafteranuclearwar.Inthemidst of the current exoplanet revolution, astronomers are taking knowledgegainedfromstudyingVenus,Mars,andEarthandapplyingittothehabitabilityofdistantworldsorbitingdistantsuns.

For the last five decades, our knowledge of planets as generic cosmicphenomena has exploded. Data from these different worlds has been cross-pollinated with our understanding of Earth, helping us to understand otherworlds,bothintheirownrightandinrelationtoourown.Thiscross-pollinationis so robust that scientists are now creating detailed models of possible

biospheresonexoplanets.Theywanttobereadywithpredictionswhensoon-to-be-completed telescopes give them next-generation views of exoplanetatmospheres.

But if we are already creating theoretical models of biosphere-harboringexoplanets, what keeps us from carrying out the same process for worldsharboringcivilizations?Ifweasktherightkindofquestions,nothingstandsinourway;wecangetstartednow.Byunitingourunderstandingofplanetswithpopulationecology—in thespiritofVolterraand thosewhofollowed—wecantake a first stab at exploring the coupled trajectories of civilizations and theirplanetsasgenericcosmicphenomena.

It’s a project that might be called a theoretical archaeology of exo-civilizations.17 Anything we do concerning exo-civilizations will have to betheoretical.This is truenotonlybecausewedon’thavedata,butalsobecauseourmethodwillstartfrombasic ideasabout lifeandenvironments,asVolterradid in developing his predator-preymodel.Wewant to let physics, chemistry,and population ecology guide us in unpacking the possible histories of exo-civilizations.Ourgoalwiththistheoreticalarchaeologyofexo-civilizationsistoseewhat could have happened to them, so thatwe can get a better handle onwhatmighthappentous.

Given both the audacity and possible absurdity of anything calling itself atheoreticalarchaeologyofexo-civilizations, let’sboil the ideadownto itscoreelements.

Step 1: Other Civilizations, Other Histories. As the pessimism lineindicates, unless the universe has a really strong evolutionary bias againstcreatingcivilizations,wearenotthefirst.Ifwearewillingtotaketheexistenceofthoseothercivilizationsseriously,thenwewillrecognizethateachwillhaveitsownhistoryintermsofinteractionswithitshostplanet.

Step2: It’sAllabout theAverages.We’re really interested in things likeDrake’s final factor: How long, on average, does a technological civilizationlast? That means the results of a single theoretical model don’t really tell usmuch.Whatweneedarestatisticscompiledbymodelingalargenumberofexo-civilizations.Thankstothepessimismline,weknowwhatthatmeans.

Scientistsusuallyliketohavemorethanathousanddatapointsforwhateverthey’re studying (this is true even in political polling). With that much data,quantities like averages make sense. So long as nature’s choice for thebiotechnicalprobabilityisonethousandtimesgreaterthanthepessimismline,a

thousand exo-civilizations will have already lived out their histories acrosscosmicspaceandtime.Giventhealreadytinyvalueofthepessimismline, it’snotmuchofaleaptoimaginethatathousandcivilizationshavealreadyruntheircourse.Thiswouldrequireabiotechnicalprobabilityofjustoneintenthousandtrillion(10–19),whichisstillmuchsmallerthanmosthistoricalpessimistshavefeared.

Step 3: There Is No Free Lunch. Now we enter the territory where ourastrobiologicalviewofplanetaryscienceandclimatestudiescomesintoplay.Inthe public debate about sustainability, the focus is often on switching ourcivilization’s energy source from fossil fuels to something with less of aplanetary impact. There is nothing wrong with such a goal, but the messageoftengetsmangledinpublicdebatefrom“lessimpact”into“noimpact.”

Ifwe take the astrobiologicalviewand start thinking like aplanet,we seethere’snosuchthingas“noimpact.”Civilizationsarebuiltbyharvestingenergyand using that energy to do work. The work can be anything from buildingbuildingstotransportingmaterialstoharvestingmoreenergy.

Without technology, each human being gets one human being’s worth ofenergy each day. But with technology, we vastly expand the energy at ourdisposal.TheaverageAmericanusestheequivalentofaboutfiftyservants justtopower theirhome.18 Ifweadd in theenergyneeded fordriving, flying,andother activities, the number of virtual servants getsmuch,much higher. Sincethisisjustamatterofphysics,what’strueforusintermsofenergy,power,andworkmust be true for any civilization-building species. Thewhole process ofbuilding a technological civilization is really an exercise in harvesting energyfromthesurroundings—inotherwords,fromtheplanet.

So you can’t build the kind globe-spanning, energy-intensive civilizationwe’reinterestedinwithouthavingsomeimpactonyourplanet.Infact,thelawsof physics demand that you have an impact. Specifically, the Second Law ofThermodynamicsistheculprit.

TheSecondLawtellsusthatenergycan’tbeperfectlyconvertedintousefulwork.Thereisalwayssomewaste.Soanycivilization-buildingspeciesonanyplanet,usinganyformofenergy,mustproducewaste.Asthatwastebuildsup,itturns into feedbacks on the planetary systems.From this perspective, theCO2producedbyourburningfossilfuelscanbeseenasakindofwasteproductofourcivilizationbuilding.So,whilethewastecantakemanyforms,allofitwillaffect the planet. The states of the atmosphere, oceans, ice, and land will allchange as the waste accumulates. That’s the real scientific story of climate

changeandtheAnthropocene.Now,youmightcounterwiththeargumentthatcivilizationsmoreadvanced

thanourswillfindwaysaroundtheSecondLaw.Mostphysicistswill tellyou,“Good luck with that.” The Second Law is baked into the structure of theuniverse,andbeingabletoskirtitentirelyisveryunlikely.

But what capacities a highly advanced civilization might possess is anextremely important question for our theoretical archaeology project. It’s soimportant,infact,thatourarchaeologyofexo-civilizationsisdesignedexplicitlytoavoidspeculatingaboutit.Andthatleadsustothenextstep.

Step4:PlanetsComewithaLimitedNumberofEnergySources.Inbuildingourarchaeologyofexo-civilizations,wearegoingtofocusexplicitlyonyoung technological civilizations. That means civilizations at our stage ofdevelopment.Thisfocusmakessensefortworeasons.First,thewholepointofthis enterprise is to see what we can learn by treating our predicament as ageneral and generic phenomenon. The challenge humanity faces in theAnthropocenewouldnotbesocompellingandexistentialifwealreadyhadwarpdrives and other super-technology. Understanding our immediate fate is onegood reason to keep our thinking focused on young civilizations. But theemphasisonyouth isalsoessential forcreatingaprojectwithstrongscientificconstraints.

Oneof thegreatest impediments to thinkingaboutexo-civilizations(orourown deeper future, for that matter) is technological progress. How can weanticipate what kind of technology a civilization that’s a million years oldermighthaveatitsdisposal?Societiesthatmaturemighthavefoundentirelynewformsofenergy thatcome from thinair.Howcanour theoreticalmodelingofexo-civilizationsaccountforunknownsourcesofenergywehaven’tdiscovered?

Theansweris,itcan’t.Butluckily,itdoesn’thaveto.Thedevelopmentoftechnologyislikeclimbingaladder.Youcan’tmakea

steelbladeuntilyouknowhowtomakeanironblade.TheBabylonianssimplydidn’thavethecapacitytobuildthemetal-alloycomponentsofamodernwindturbine. Each civilization must climb up the ladder of technologicalsophisticationas itdiscovers thephysicalandchemicalprinciplesof theworldaroundit.

Forourproject,thatmeansayoungcivilizationwillhavealimitednumberofenergysourcesavailable.Crucially,weknowwhatthoseformsare.Thelawsofphysics,chemistry,andplanetaryevolutiontelluswhatresourcesmightbeatthereadyforanintelligentspeciesbuildingitswayupthetechnologicalladder.

Hereisaprettycompletelistoftheenergyresourcesaplanetmightoffer:

• Combustion.Thismeansburningstuff.Itcouldbefossilfuelsthatareburnediftheplanetwentthroughtherightkindofgeologicepoch,oritcouldjustbebiomaterials,likewoodonourworld.

• Hydro/Wind/Tides. If the planet has fluids or gases flowing on itssurface,thenthosemovementscanbetappedtogenerateenergy.

• Geothermal.Heat from the planet’s interior can also be captured andusedtodotheworkofcivilizationbuilding.

• Solar. Sunlight can be trapped in both low-tech (heat) and high-tech(electriccurrent)ways.

• Nuclear.Theenergylockedupinatomicnucleicanbeusedaslongasthereare reservesof radioactiveelements likeuraniumaround.Nuclearenergyisobviouslyhigheronthetechnologicalladderthanothermodesof energyharvesting,butgiven thatour societyhasmadeuseof it, it’sfairtothinkthatothersmightaswell.

Thespecificconditionsoneachplanetwillultimatelydeterminethemixofenergy modes available to a civilization evolving there. Geothermal may bemore favorable on some worlds, while wind may be more easily tapped onothers.Themainpointfornowisthatthelistabovehitsalmostallthechoices.Other than imaginingexoticplanetswithspecialmagnetic fieldsorcontinuouslightning conditions, what’s on the list above is all that exists. Adding newenergy sources other than thosewe’ve listed requires inventing science fictionstoriesaboutdiscovering“newphysics.”

Step 5: Know the Impact. Since we can list the different energy sourcesavailable toayoungcivilization,wecanalsocalculate theplanetary impactoftheiruse. If this sounds like science fiction, remember thatwayback in1903,SvanteArrhenius carriedout exactly thiskindof calculation for theEarthandcombustion(thatis,theburningoffossilfuels).ArrheniusknewthecompositionoftheEarth’satmosphere,andhecouldcalculatetheimpactofusingcoal.ThisimpactwastheproductionofCO2,andthechangeitproducedwasanenhancedgreenhouseeffect.19

So,forcivilizationspoweredbycombustion,wealreadyknowhowtomodeltheir impacts on their planets.All that’s needed is to account for the potentialdifferences in their host planets’ properties,whichwill include things like thecompositionoftheatmosphereandorbitallocationinthehabitablezone.

What about the impact of other energy sources? For some cases, thecalculationshavealreadybeenstarted.Astudybyscientistsat theMaxPlanckInstituteinGermanylookedattheglobaleffectsofwindpower.Windturbinesworkbypullingenergyoutofsmooth,large-scaleflowsofairandturningitintoelectricity.Butintheprocess,theyleavechoppy,turbulentairflowsdownstream.TheGerman group found that extracting energy fromwind power on a scalemassiveenough topowerourcurrentcivilizationwould leaveaglobal imprintakin to mild global warming. Even wind, the darling of renewable-energyharvestingmodalities,hasaplanetarycost(thoughfarlowerthanfossilfuels).20

Becausewehaveadeepunderstandingofthephysicsandchemistryofeachoftheenergysourceslistedabove,itdoesn’ttakeaquantumleapinsciencetocalculatehowtheirusewillproducefeedbacksonaplanetotherthanourown.For each energy source a civilizationmight harvest, we have the informationnecessarytocalculatetheassociatedplanetarycost.Withthatcapacity,wereachthefinalstepinthepathtoourtheoreticalarchaeologyofexo-civilizations.

Step6:TurntheCrank.Givensteps1through5,wenowhavearecipeforcalculating exo-civilization histories. We begin by creating a model for theinteraction of a young civilizationwith its planetary environment.Thismodelwillcomeintheformofequationspredictinghowthecivilization’spopulationanditshostplanetarysystemschangewithtime.Asinthepredator-preymodel,theequationswillbecoupled.Therewillbeanequationdescribingthechangeinthe planetary systems (such as atmosphere) and an equation describing thechanges in the civilization-building population.Each equationwill have termsthatdescribethefeedbackfromplanet tocivilizationandcivilizationtoplanet.It’s worthmentioning that to do this jobwell, we’ll needmore than just twoequations,becausewe’llprobablyneedtotrackdifferentresourcesandtheiruse,alongwiththeireffectonthedifferentplanetarysystemslikeoceans,ice,andsoforth.Butfornow,wecanstickwithjust“theplanet”and“thecivilization.”

In general, the civilizationwill use its energy sources, and thewaste fromthose energy sources will push on the state of the planetary systems. As theplanetarysystemsshiftbasedonthefeedback,thecivilizationwilleitherthriveor be stressed, as reflected in how their population changes. Because thecouplingwillbecomplicated,wewon’tknowwhattoexpectuntilwe’vesolvedtheequationsmakingupthemodel.

Doing this once doesn’t tell us very much. What we are interested in isDrake’sfinalfactor:theaveragelifetimeofcivilizations.Inordertocalculateanaverage,wewillhavetorunourmodelsmanytimesformanydifferentkindsof

planets.Inasense,byrunningtheexperimentincivilizationbuildingoverandover,wewill createourownmini-versionof theuniverse.Someof themodelruns will begin with planets that are close to the inner edge of their stars’habitable zones, where they’ll be particularly susceptible to enhancedgreenhousewarming. Somewill be farther out. Some of our model runs willhaveplanetswithatmospheresthathavelessoxygenthanours,whileotherswillhavemore oxygen. Somewill beginwith civilizations usingwind power, andotherswillbeginwithcivilizationsusinggeothermal.Yougetthepicture.

In the end,wewill have to “turn the crank” and run tens of thousands ofmodels, eachwith different starting conditions. Thatmight seem like a lot ofwork,butmoderncomputersarefast.

PATHSTOPROGRESS,ROADSTOHELL

Carrying out a theoretical archaeology of exo-civilizations correctly will bedemanding. Itwill require input fromfieldsasdiverseasatmospheric science,geology,energyscience,andecology.Tocreate realisticmodels,we’llhave togetthephysics,chemistry,planetaryscience,andecologicalinteractionsrightinterms of what we build into the models. That is going to be a long andinterestingproject.

But even as we build our way toward that goal, we can take some initialsteps now. These first explorations can give scientists the lay of theastrobiologicalexo-civilizationlandscape.Inthefallof2016,ateamofuswenton this kind of scouting mission. The result was simultaneously thrilling,hopeful,andpossiblyalittledepressing.

OurteamincludedMarinaAlberti,anurbanecologistfromtheUniversityofWashington.AnativeofItaly,herpassionishowevolutionisalreadyrespondingto theAnthropocene.Marinastudiesurbanenvironmentsandhownewspeciesare being created in themidst of our vast project of planetwide city building.AxelKleidonwasalsopartoftheeffort.AxelisalsoaninnovativethinkerwhoworksattheMaxPlanckInstituteforBiogeochemistry,developingnewwaystolook at the Earth as a single thermodynamic system, like a giant, planetwidesteamengine.Finally,therewasJonathanCarroll-Nellenback.Jonathanwasmygraduate student years ago andnowworkswithme as a senior computationalscientistattheUniversityofRochester.Histalentfortheoreticalworkisprettyremarkable.SometimesI’llbringJonathanaprobleminthemorning,andbythenextdayhe’llbringitback,fullysolvedanddisplayedinbeautifulgraphics.

Together,weformulatedamodelfor theevolutionofacivilizationwith its

planet. The equations were pretty simple. We weren’t trying to capture thedetailsofEarthorofanyotherspecificplanet.Instead,ouraimwastodescribethe interaction of civilizations and planets in the most general way possible,which would serve as a first step toward doing somethingmore detailed andrealistic.

In our approach, the population and the environment were linked via anenergy resource. The planet supplied the energy resource, and the civilizationused theenergy resource.Greaterenergyusemeanta largerpopulationon theonehand,andgreaterenvironmentalchangeontheother.Greaterenvironmentalchangeloweredtheplanet’scarryingcapacityforthecivilization,whichshouldleadtolowerpopulations.

Alongwiththesefeatures,wealsoincludedaspecificmechanismtodescribehowthecivilizationmightrespondtochangingconditionsonitsplanet.Forthesake of simplicity, we imagined that the planet had just two kinds of energyresources.Oneresourcehadahighplanetary impact (as fossil fuelsdo),whiletheother had a low impact (as solar energydoes).Here, high and low impactreflected the degree to which using the energy source forced the planetaryenvironmenttochange.

Oncetheplanetaryenvironmentwaspushedpastsomepredefinedpoint,thecivilization switched energy resources. You can think of this in terms of theplanet’stemperature.Oncetheplanetarytemperaturerosetothespecifiedvalue,thecivilizationstoppedusingthehigh-impactenergysourceandswitchedtothelow-impactsource.

Usingthisstrategyinthemodelsgaveusaspecificandsimplewaytoboildownthecivilization’ssociology.Wedidn’twanttotryandmodelhowthey’drecognizeandactontheirAnthropocene.Instead,itcamedowntotheplanetarytemperaturethatfinallygetsthecivilizationtodosomething.Sincethatwasjustaninput,wecouldchangeitfromoneruntoanotherandseehowhistoryplayedoutfor“smart”civilizationsand“dumb”ones.Eitherthecivilizationactedearly,whentheirplanet’stemperaturehadjuststartedtorise,ortheyactedlate.Whilewecouldn’tmodelthesociologyofhowtheymadethatchoice,wecouldmodelthe choice’s physical consequences. Would acting early save them? Wouldanythingsavethem?

So,whatdidthemodeltellus?Our exploration of the exo-civilization/planet system yielded three distinct

trajectories.Thefirst—and,alarmingly,mostcommon—waswhatwecalled“thedie-off.” As the civilization used its energy resource, its numbers grew asexpected (see page 196, graph A). But the use of the resource pushed theplanetary environment away from its initial state. As the evolution of the

coupledcivilization/planetsystemcontinued,thepopulationrosesharplybeyondwhat the environment could sustain. The population, in otherwords, overshotwhat the planet could support.A big reduction in the civilization’s populationfollowed, until both the planet and the civilization had reached a steady state.After that point, neither the population nor the planet changed anymore. Asustainableplanetarycivilizationwasachieved,butataconsiderablecost.

Inmanyofthemodels,wesawasmuchas70percentofthepopulationdiebefore a steady statewas reached. Imagine sevenout of every tenpeople youknowperishingbecauseofglobalclimatecatastrophes.It’snotclearhowlargeofadie-offacomplextechnologicalsocietycouldhandlewithoutfallingapart.During the period of the Black Death in the fourteenth century, Europe lostbetween30and50percentof itspopulation,butmanaged to revive.MedievalEurope, of course, wasn’t highly technological in the modern sense, nor asisolatedasaplanetinspacewouldbe.

Thesecond trajectoryclasswefoundwasonewecalled the“soft landing”(seepage196,graphB).Thepopulationgrewand theplanet changed,but themodelsshowedasmoothtransitiontoasteadystateafteranearlyswitchtothelow-impact energy resource.Eventually, the civilization came into equilibriumwithitsplanetwithoutamassivedie-off.

Four kinds of trajectories for exo-civilizations and their planets discovered from mathematicalmodels.

Thefinalclassoftrajectorywasthemostworrisome:full-blowncollapse.Asinthedie-offmodels,thepopulationinitiallygrewswiftly.Inthiscase,however,thespeedofplanetarychangepulledtheplanet’scarryingcapacitydownsofastthatthepopulationplummetedallthewaytoextinction.

Oneof themost remarkableaspectsof thisclasswas that thecollapsewasinevitable.Onewouldthinkthatswitchingfromthehigh-tolow-impactenergysourcewouldmakethingsbetter.Butforsometrajectories,itdidn’tmatter.Ifweused only the high-impact resource, the population reached a peak and thenquicklydroppedtozero(graphC).Ifweallowedthecivilizationtoswitchtothelow-impact versionof an energy resource, the collapsewas only delayed.Thepopulationwouldstarttofall,thenappeartostabilize,andfinally,suddenly,rushdownwardtoextinction(graphD).

The collapses that occurred evenwhen the civilization did the smart thingdemonstrateanessentialpointabout themodelingprocess: itcansurpriseyou.Becausetheequationsrepresentingthemodelarecomplex,unexpectedbehavior

canhappen.Theseareconsequencesyouwouldn’thavethoughtofifyouhadn’tdonetheworkofcrankingoutthesolutions.

Onlyafteryoustudythebehaviorseeninthemodelsdoyouunderstandwhathappened.Remember thatoursimplifiedmodelswere tracing thedevelopmentof a civilization and its planet together. In the case of the delayed-collapsetrajectories, we were finding scenarios that showed us that switching from ahigh-tolow-impactenergyresourcewon’tmatterifthechangeismadetoolate.Even though the civilization in our model recognized its entry into anAnthropocene-liketransitionandswitchedenergysourcestomakethingsbetter,the planet was already heading into new climate territory. Once the ball gotrolling,theplanet’sowninternalmachinerytookover.Itwasn’tcomingbacktotheoriginalclimatestate,andittookthecivilizationdownwithitasitranawayintoanewstate.

In thesecases, theplanetaryenvironment’sowndynamicswere theculprit.Pushaplanettoohard,anditwon’treturntowhereitbegan.Weknowthiscanhappen, even without a civilization present, because of what happened withVenusanditsrunawaygreenhouseeffect.Ourmodelswereshowing,ingenericterms, howa civilization couldpush aplanet into a different kindof runawaythroughitsownactivity.

TheworkthatJonathan,Marina,Axel,andIdidshowedussomeofthebasicwaysacivilizationanditsplanetmightchangetogether.Itwasgoodthatwesawthat long-term sustainable versions of the planet-plus-civilization systemwerepossible.But thewarningswere thereaswell.Theself-perpetuating feedbacksthatdrovesomecivilizationstocollapse,evenaftertheymadethesmartchoices,wereparticularlysobering.

THEFINALFACTOR

It’s reasonable to askwhat this archaeology of exo-civilizations really tells usabout reality.Aren’t thesemodels justmathematical toys? Isn’t it true thatwehave not a single instance of a civilization other than our own to makecomparisons with? Answering these questions will help us see what can begained by taking exo-civilizations seriously as subjects of scientific inquiry. Itwillalsohelpusseewhat’satstakeforusaswetrytousethisastrobiologicalperspectivetounderstandourchoicesaboutourownprojectofcivilization.

Models andReality: The trajectory of theAnthropocene shownwith real data forworld energyconsumption,CO2concentration,andglobalpopulationforthelast10,000years.

Itisabsolutelytruethatmodelsandrealityaretwodifferentthings.Amodelisasimplification, likeaskeletonwithout themuscleandskin.But lookingatjustaskeletonwilltellyoualotabouttheanimal.Thatishowweknowaboutdinosaurs.Moretothepoint,aswemoveforward,ourmodelswillbebasedonevermore sophisticated versions ofwhatwe already know about howplanetswork.Theyare, andwill be, built on ever-stronger skeleton framesofphysicsand chemistry—in other words, the laws of planets. In that way, they are farmorethanmereimaginativetoys.

Themodelsallowustogobeyondfiction.Byrelyingonthelawsofplanets,theycapturekeyaspectsofreality.Thatmeanstheyhavetheirownlogic.Theyhave their own stories to tell us thatwewould not seewithout them. It’s onethingtoargueoverwhatyouthinkwillhappenwhenacivilizationonadistantplanetbecomestechnologicallysophisticated.Yourfriendmighthaveadifferentopinion,andthat’sanall-nightargumentwaitingtohappen.But it’ssomethingentirely different to spin up themath and let it see into the complexities thatelude us. Instead of mere opinion, we can let the model show us how theuniverse might behave. The realistic constraints models place on their storiesgivethosestoriesscientificvalue.Itgroundsthemintherealmofthepossible.

All the researchwe’veexplored in this chapter constitutes just a first step.It’sanoutlineofwhat thiskindofenterprisewill looklikeaswedevotemoretimeandefforttotheendeavor.Thestorieswe’vetoldherearejustthefirstof

many,andtheywillgrowmorepreciseasourunderstandingincreases.The next step will be to build far more realistic models and use them to

explore amuchwider range of realistic cases.After running thesemodels forhundreds of thousands of different situations, we will have the simulatedtrajectories—thehistories—ofhundredsofthousandsofinhabitedworlds.

Aplanetthatlivesclosetotheinneredgeofitshabitablezonemightbesohighlysensitivetorunawaygreenhousewarmingthatitscivilizationbarelyhastime to progress before it faces its own version of the Anthropocene andcollapses. Another world, farther out from its star, may be less sensitive toplanetary change but have a civilization that refuses to recognize the changeuntilthedie-offhasalreadybegun.Adifferentspeciesonadifferentworldcouldmanage to build its project of civilization using only lower-impact forms ofenergyandmakeagentlesoftlandingtoasustainablestatethatlaststhousandsofmillennia.

What part of these stories matters to us? The answer to that question issimple:Drake’s final factor.With trajectories formillionsof simulatedplanetsand civilizations in hand,we can calculate an average lifetime.How long, onaverage,doesacivilizationlast?

Consider,foramoment,whatthatsinglenumberwouldtellus.Iftheaveragelifetimeofexo-civilizationsistwohundredyears,thenweare

in big trouble. If we find most model civilizations collapse after just a fewcenturies, the implicationwould be that civilizations like ours just don’tworkwellonaplanetaryscale.Ashortaveragelifetimewouldmeanthattheuniversedoesn’t do sustainable civilizations. The lessonwould be that we humans arethreadingtheeyeofaneedlewiththeAnthropoceneanddon’thavemuchroomforerrorinourchoices.Inthatcase,itmayalreadybetoolate.

If theaveragelifetimeofcivilizationsemergingfromourmodelsweretensofthousandsofyears,thatwouldbegoodnews.Itwouldmeanit’snottoohardfor any civilization to make it through the bottleneck of an Anthropocene.21There would be lots of different strategies for reducing our impact on theplanetary systems thatwork. Itwouldmeanwehave lotsofwiggle room.Wecouldmakemistakesandstillrecover.

Inthisway,asinglenumberfromourarchaeologyofexo-civilizations—theaverage civilization lifetime—would have profound implications for our ownfutureandouractionsinthepresent.Itwouldletusseewhatmightbecoming.And with that knowledge, our understanding of the choices we face wouldbecomedeeper,richer,andbebasedonsomewisdom.

Beyondthequestionoftheaveragelifetime,wecouldalsousethemodelstoseeexactlywhatchoicesaremostlikelytosaveus.Oncewehaveafullsuiteof

trajectories, we can ask what explicitly led some to civilizations to achieveplanetarysustainabilityandotherstocollapse.Likeadoctorlookingforacurebystudyingthemostpathologicalcasesofadisease,wecanseewhatcommonfactorsdrovethecivilizationsthatdiedtotheirfate.Themodelswillhavealotto teachus thatwecan’tseenowwith the tunnelvisionof justourplanetandjustourownuncertainfuture.

CHAPTER6

THEAWAKENEDWORLDS

THEPLANETWENEED

If,overthecourseofbillionsofyearsofcosmicevolution,somespeciesmakeitthroughtheirAnthropoceneinto long-term,sustainableversionsofcivilization,whatdotheyendupwith?Whatdotheirplanetslooklike?Howdotheseworldsfunctionintermsoftheircoupledsystemsofair,water,rock,life,andthenewaddition of a planet-spanning, technology-intensive, energy-hungry society?Thesearethequestionswecareaboutmost,becausethis is thetargetwemustaimfor.

Thereisagreatdealofwishfulthinkinginvolvedwhenthetermsplanetaryand sustainability are parked next to each other. These are visions of “greenutopias,” with sleek, electric-powered trains gliding into elegant eco-cities ofverticalfarmsandbuildingsmimickingnaturalforms.Whileit’seasytoimaginewhatasinglesustainablecitymightlooklike,imaginingasustainableplanetisanother thing entirely.Citieshave alwaysbeen thedomainsofhumancontrol.They are spaces our project of civilization carves out of nature. A planet,however,isadifferentbeast.1

Planets are their own masters. That’s what the astrobiological perspectiveshows us. The processes shaping worlds are powerful, complex, and subtle.Planetschannelvastenergiesthroughevermorerefinednetworksofcauseandeffect.Thesenetworksareembodiedinwindsthatpickupfinedustgrainsandcarrythemacrossthousandsofmiles,orchemicalcompoundsblownintotheairbyvolcanoes,only toendup,millionsofyears later,embedded in rocks lyingdeepbeneathoceans.Addlifetothemix,andplanetsbecomealmostinfinitely

more complex, as the planetary systems can now include a coevolvingbiosphere.

So, how does a healthy planet with a healthy, long-term project ofcivilizationwork?Toanswerthisquestion,wemusttakeourinvestigationtothefinallevel.CrossingtothesafetyofafullysustainableprojectofcivilizationonEarth requires not just thinking like a planet, but understanding the profoundconsequences of planets that have themselves learned to think through theircivilizations.What,inotherwords,doesitmeanforaplanetasawholetowakeup?

THERUSSIANMEETING

TenyearsaftertheirfamousencounteratGreenBank,FrankDrake,CarlSagan,andtwoothermembersoftheoriginalmeetingfoundthemselvestogetheragain.This time, the setting wasn’t the forests ofWest Virginia, but a mountain inArmenia.Drakeandhis compatriots, alongwitha squadofRussian scientists,had come to the Byurakan Observatory for the first true intraplanetary (orinternational) meeting on interplanetary civilizations.2While Green Bank hadbeenanintimateaffair,withjustninemembers,the1971ByurakanObservatorymeeting had more than forty participants, including luminaries from both theSovietandAmericanscientificestablishments.TherewereNobellaureateslikeFrancis Crick (co-discoverer of DNA) and Charles Townes (inventor of thelaser).Othernotablesincludedartificial-intelligencepioneerMarvinMinskyandCanadian neurophysiologist David Hubel, who would go on to win a NobelPrizeforhisbrainstudies.3

CarlSaganplayedacentralroleinorganizingtheByurakanmeeting.AttheheightoftheColdWar,Saganunderstoodthesymbolicvalueofaninternationalconference devoted to our place among other, hopefully more mature,civilizations.GettingthemeetingplacedintheSovietUnion,theUnitedStates’bitter enemy, hadbeenno small task, however.Topull it off, Saganneeded apartnerontheRussiansidewhowasjustascharismaticandpassionateaboutlifein the universe as he was. He found that counterpart in Nikolai SemenovichKardashev.

CarlSagan(right)withArmenianastronomerHrantTovmassian(center)andotherparticipantsatthefirstinternationalSETImeetingattheByurakanObservatoryinArmeniain1971.

Justayearandahalfolder thanSagan,Kardashevwasaradioastronomerwhohadalreadymadeimportantcontributions to thestudyofgalaxiesandtheinterstellarmedium.4He had been the force behind the first Soviet search forexo-civilizations,carriedoutjustafewyearsafterProjectOzma.He’dalsoledtheSoviets’first internalSETIconferencein1964,afewyearsaftertheGreenBank meeting. The report he had written on that meeting established hisinternationalreputationasaleadingthinkeraboutlifeandtheevolutionofotherworlds.

Inthatpaper,Kardashevlaidoutaschemeforthetechnologicalprogressofexo-civilizations.His ideasplayedan important role in theByurakanmeeting,where freewheeling discussions of the long-term fate of civilizations went onuntilthesmallhoursofthenight.ButtheimpactofwhatcametobecalledtheKardashevscalewouldlastbeyondtheByurakanmeetingandwould,initsway,provetobeasenduringastheDrakeequation.5

THEKARDASHEVSCALE

When Nikolai Kardashev proposed his scale for measuring the progress ofcivilizations,hewasprimarilyinterestedinfindingthem.Kardashev’squestioncanberephrasedinastraightforwardway:Whatarethemilestonesthatmarkacivilization’s advancement up the ladder of technological sophistication? Thebasic idea that civilizationsevolve throughdistinct,quantifiable stagesas theyprogressofferedKardashevalevertoliftthediscussionaboutexo-civilizationsabove pure speculation by providing a means to quantify their advancement.While his main interest was finding radio signals from exo-civilizations, hisscale gave us away to think about their evolution. Butwithin theKardashevscale lay an essential and mistaken bias concerning the relationship betweencivilizations and their host planets. Correcting that bias is an essential step infindingawiseastrobiology-basedpaththroughtheAnthropocene.

Kardashevbasedhisclassificationschemeontheenergyacivilizationhadatitsdisposal.Thescalehadthreelevels.

• Type 1: These civilizations can harvest the entire energy resources oftheirhomeplanet. Inpractice, thismeanscapturingall the light energythatfallsontheworldfromitshoststar,sincestellarenergywilllikelybethelargestsourceavailableonahabitable-zoneplanet.TheEarthreceivestheequivalentof thousandsofatomicbombs’worthofenergyfromtheSun every second.6 A Type 1 species would have all this power at itsdisposalforcivilizationbuilding.

• Type 2: These civilizations can harvest the entire energy resources oftheir home stars. The total output of the Sun every second is a billiontimes larger than the sunlight that falls just on Earth. The physicistFreeman Dyson anticipated some of Kardashev’s thinking in a paperwritten in 1960, in which he imagined an advanced civilizationconstructing a vast sphere around its star.7 This solar system–sizedmachinewouldcapturestellarlightenergy,perhapsviaaninnersurfacecovered in solar cells. Such “Dyson spheres” became the archetype forscientistsimagininghowaKardashevType2civilizationwouldgoaboutitsenergy-harvestingbusiness.

• Type3:Thesecivilizationsharvest theentire energy resourcesof theirhome galaxy. A typical galaxy contains a few hundred billion stars.PerhapsType3civilizationsenvelopalltheirgalaxy’sstarswithinDysonspheres, or perhaps they have even more exotic technologies at theirdisposal.

The Kardashev scale represents the scientific imagination working at the

grandest, and thereforemostmythic, scale.A singleDyson spherewouldbeamachineof staggering capacity and size.The inner surfaceof aDyson spherebuiltaround thesun,witha radius thesizeofEarth’sorbit,wouldcovermorethantenthousandtrillionsquaremiles(theequivalentofalmostabillionEarths).Building a machine of this size would require grinding up whole planets forconstructionmaterials.Wewon’tbebuildingoneoftheseanytimesoon.Dysonspheresaretrulythestuffofsciencefiction.

Butbyfocusingonstellar-energycaptureastheyardstickforacivilization’sevolution,Kardashev’s science fiction–sounding evolutionary schemecouldbeset firmly in the real world of real physics. That is what gave theKardashevscale its reach,andwhyithasendured.Forexample,anumberof researchers,such as Jason Wright at Penn State University, have conducted astronomicalsearches for the radiation signatures of Type 2 civilizations via their Dysonspheres.8Thus,asastronomerMilanM.Cirkovicwrote in2015,“Kardashev’sscale remains the most popular and cited tool for thinking about advancedextraterrestrialcivilization.”9

Alargepartof theappealof theKardashevscale lies in itscombinationofscienceandmythic-scaleoptimismviaatechnologicallycoherentroadmapforthe progress of civilizations. Its implications are undeniably hopeful. If wecontinuetoadvanceasatechnology-buildingspecies,weshouldnaturallypassthrougheachKardashevtypeonourwaytoafutureofunimaginablepowerandreach.AcivilizationthatcouldbuildaDysonspherewouldbetheequivalentoftechnologicaldemigods tous. Inphysics,power isdefinedas energyusedpertime. Since the Kardashev scale is explicitly based on energy use, the linksbetween a civilization’s physical power and its metaphorical power—betweenthescienceand themythic—arebaked into thescale’sapplication.Make it farenough,thescaletellsus,andyouwillbecomeasthegods.

More than one author has tried to calculatewhere on theKardashev scalehuman civilization falls today. In 1976, Carl Sagan suggested a way ofcalculating “fractional” values of Kardashev status based on world energyproduction.10 In Sagan’s calculation, we end up at about Type 0.7. FreemanDysonwent further, suggesting that human civilizationwill reach full Type 1status in approximately two hundred years (with Type 2 requiring anotherhundredthousandtoonemillionyears).11

That sounds pretty good. In just a couple of centuries, we are going tobecome a true Type 1 cosmic civilization. The problem, of course, is thatwemaynevergetthere.Ourprojectofcivilizationhasabottlenecktonavigaterightnow,andourprogressthroughitisanythingbutassured.

The Kardashev scale originated from a particular historical moment inthinking about exo-civilizations.LikeSagan andDrake,Kardashevwas raisedona techno-utopianvisionof thefuture.Technologywasimaginedin termsofsleek, gleaming machines that were destined to be humanity’s salvation. Wecouldexpectthattechnology’sgrowthandpowerwouldbeunconstrained.Thatwas why the Kardashev scale focused solely on energy. Civilizations wereexpectedtoriseuptheladderofenergyharvestingtoever-greaterheightsuntiltheentiregalaxywouldbecomearesourcetobemined.Andateachstage(eachKardashevtype),thefeedbackfromallthisenergyuseonthephysicalsystemsfromwhichtheenergywasdrawncouldbeignored.Planets,stars,andgalaxieswouldallsimplybebroughttoheel.

Whileitispossiblethatstarsandgalaxiesmightnotcarewhatyoudowiththeir energy, planets are another story. That is the painful lesson of theAnthropocene.

Theengineeringofentiresolarsystemsorgalaxiesissofarintotherealmofspeculationthatit’simpossibletoknowwhatchallengesitwillrequire.Butforplanets—thefocusofType1civilizations—wealreadyunderstandenoughtoseehow theKardashevscale representsakindofplanetarybrutalism. It inherits avision of advanced civilizations living in perfect, world-girdling cities wherenature is fully controlled.Science fiction is full of thiskindof thing.There isTrantor, the home world of the galactic empire in Isaac Asimov’s classicFoundation trilogy. Trantor’s surface lies hundreds of miles below the manyshells ofmachinery thatmake up its single planet-scale city.12 Amore recentexampleisCoruscant,thehomeworldofthegalacticrepublicinStarWars,withits continuous stream of “air cars” traveling amid the city’s towering spires.Thesearevisionsofplanetsconqueredbythemightyenergy-wieldingcapacitiesoftheircivilizations.

ButintheyearssinceKardashevproposedhisclassificationsystem,wehavelearned thehardway thatplanetarybiospheresarenotsoeasily ignored.FromtheworkofLovelock,Margulis, andothers, a new scientific understandingofplanets and life emerged.Evenwhen they lack life,wenowknowplanets arecomplexsystems.Andifavibrantbiosphereispresent, itbecomespartof thatcomplexwhole.The livingandnon-livingpartsof thesystemcoevolveacrosstime. In this way, the coupled systems that make up a planet have their owninternal dynamics—their own logic. That logicmust be fully embraced whenmappingoutthetrajectoriesofcivilizations,asKardashevhopedtodo.

Once again, we are forced to stop seeing civilizations like our own asstandingapart fromtheworld thatgave thembirth.Allcivilizations, includingthose that might occur on other worlds, are expressions of their planet’s

evolutionaryhistory.Fromthisperspective,ourprojectofcivilizationisjustoneconsequenceoftheEarth’shistory,notitsfuturemaster.Everycivilizationmustbeseenasanewformofbiosphericactivityarisingwithinaplanet’shistoryoftransformationandevolutionaryinnovation.

So it is not simply energy consumption (the focusof theKardashev scale)that must be considered. Instead, we must learn to think in terms of energytransformations.Weneedtolookatthosephysicallawsthatconstrainenergyasit flows through a planetary system. This means we must take a fullythermodynamicperspectiveaswefollowtheenergyofsunlightbeingturnedintotheenergyofrisingaircolumns,whichturnsintotheenergyoffallingrain,andsoon,allthewaytotheenergyoflivingcells.

RecognizingthelimitsonenergytransformationisthefundamentallessonoftheAnthropocene.Youcan’tjustbringaplanettoheel,meaningyoucan’tuseenergytobuildacivilizationwithoutexpectingfeedback.Instead,wemustbeginwitharicherunderstandingofbiospheresandcivilizationsaspartofthecoupledplanetarysystems.Thatmeansanewkindofmapforhowcivilizationsrise totheType1stageand,possibly,survivelongenoughtobecomesomethingmore.The development of long-term, sustainable versions of an energy-intensivecivilizationmustbeseenonacontinuumofinteractionsbetweenlifeanditshostplanet.

Sustainablecivilizationsdon’t“riseabove”thebiosphere,butmust,insomeway, enter into a long, cooperative relationship with their coupled planetarysystems.Butwhatdoesthatlooklike?

EARTHASAHYBRIDPLANET

Planets are nature’s way of turning starlight into something interesting. Theevolutionofaplanetacrossbillionsofyearsdependsonwhichprocessesitcanharness to absorb starlight and, by doing work, transform that energy intosomethingelse.Fromrainstormstoforeststocivilizations,thestoryofplanetaryevolutionacrosscosmictimeisthestoryoftheseenergytransformations.

Energyflowsarethedomainofthermodynamics.Theengineinyourcarisathermodynamic system. It’s a “heat engine.” Gasoline gets ignited in thecylinders, converting chemical molecular-bond energy into hot gas, or heatenergy. The hot, expanding gas pushes on the pistons, converting heat intomotionviakineticenergy.Themotionofthepistonsgetstransferredthoughthegearsintothemotionofthewheels.

So,it’snotjusttheenergyinthegastankthatmatters.It’sthetransformation

of that energy from chemical form into kinetic form that you need to payattentionto.Someofthatoriginalchemicalenergygetsdissipated(meaningit’slostandcan’thelpindoingtheworkofmovingthecar)throughtheheatingoftheengineblockorthefrictionofthetiresontheroad.

The science of thermodynamics tells us about the limits of thosetransformations.Ittellsusthatnotalltheinitialenergy(containedinthefuelinthe gas tank, for example) can be used to do useful work. Some of it, bynecessity, must turn into “waste.” Nature has built these limitations into theuniversethroughthelawsofthermodynamics.13Thatiswhythermodynamicsistherightwaytothinkaboutplanetsandcivilizationsandtheircombinedfate.

For a planet with no atmosphere, like Mercury, the available energytransformationsareprettylimited.SunlighthitsMercury’ssurface.Thesurfacewarms up and emits heat radiation back into space.Once the planet’s surfacereaches its equilibrium temperature, there isnot awhole lotmore to the story,whichiswhyMercuryhaslookedprettymuchthesamefromonedaytothenextforthelastthreebillionyearsorso.14

Add an atmosphere to the planet, however, and the story gets a lot moreinteresting.Whensunlightwarmsthesurfaceofaplanetwithanatmosphere,theair near the ground is also warmed. Then the air rises, creating large-scale“convective” circulation.Atmospheric gas rises, and then cools and falls backtowardthesurfacetostartthecirculationoveragain.Atmosphericconvectionisakindofplanetaryheatengine,convertingsunlightintomotion.

If the atmosphere also includes molecules like water, CO2, and other“volatiles,” then evaporation and condensation can occur in the circulation.15Water, forexample,willevaporatenear theplanet’ssurface, turning intoagasthatriseswiththerestoftheair.Whentheaircoolsathigheraltitudes,thewatercondensesback into a liquid (in the formofdroplets).This is how interestingthings like rainorsnowcanoccur—things thatcouldnothappenonanairlessworld.

Just these ingredients—an atmosphere with stuff that can evaporate andcondense—are enough to give a planet climate and weather. It’s why even arelatively “dead”world likeMars can still look different from one day to thenext,asduststorms,fog,orfrostrollinandout.

Thepresenceofliquidsflowingacrossthesurface,intheformofrainrunoffandrivers,addsanewlayerof“interesting,”asthestrongweatheringofrockscanbegin.Elementsoncelockedupinmineralsgetexchangedwiththeairandthesurfaceliquids,launching“cycles”ofthesematerialsbetweentheplanetarysystems.16Thebranchingpathwaysofthesecyclesandtheirfeedbacksbestowa

newrichnesstotheplanet,allowingittoevolveinevenmorecomplexways.The point here is that all of these processes are fundamentally

transformationsofenergy.Thepresenceofanatmosphereturnssolarenergyintomotionenergyasair risesand falls.WithwaterorCO2 in theatmosphere, theenergy of motion feeds into the energy associated with evaporation andcondensation.Weathering and the breaking of chemical bonds in rocks is yetanotherformofenergytransformation.So,evenwithout life,aplanetcantakeits sunlightanduse it forevermorecomplexwork,drivingchange,evolution,andinnovation.

Thinking this way about evolution and energy led Marina Alberti, AxelKleidon, and me to propose a new classification for planets.17 While theKardashev scale focused on the total energy falling onto a planet, we wereinterested in what happens to that energy once it gets within a planet. Here,“within”doesn’tmeanunderneaththesurfaceofaworld,butwithinthecoupledplanetarysystems.Whathappenswhenasun’senergy,intheformofincominglight,feedsthroughthelinkednetworksofatmosphere,hydrosphere,andsoon,includingabiosphere?

UnlikeKardashev,ourgoalinmakinganewplanetaryclassificationschemewasnotdetection(thoughitprovesusefulforthis).Instead,wewantedtousethelaws of physics, chemistry, and biology on planetary scales to see whereplanetary evolutionmight lead. In particular,wewanted to use theplanetswealreadyunderstoodtomapoutthepropertiesoftheoneswedon’tunderstand—theplanetswithsustainablecivilizations.

Working together, the three of us saw that the universe’s vast census ofplanetsmightbegroupedintoaspectrumoffivemainclasses.

An airless world like Mercury is a Class 1 planet in our scheme. Thetransformationsof sunlight are simple, and so thedegreeofwork that is doneandthecomplexitygeneratedarelimited.Class1planetsaretrulydeadworlds.

Aworldwith an atmosphere but no life, likeMars orVenus, is a Class 2planet.The flowof gases and liquids drivenby sunlight representworkbeingdone within the planetary systems. That work can make things happen on arangeoftimescales,likethedailyappearanceoffogortheyearlyappearanceofduststorms.

Class3planetsarethosewithwhatwecalleda“thin”biosphere.Theseareworlds where life has gotten started. It’s affecting the rest of the coupledsystems,butdoesnotyetdominatethesesystems.Onewaytoquantifythisistolookatwhat’scalledthenetproductivityofaplanet,meaninghowmuchenergyitsbiosphereharvests.DonaldCanfieldhasestimatedthenetproductivityofthe

Earth’searlyArcheanbiosphereandfoundthat itwasahundred timessmallerthantoday.18SoEarthduringtheArcheanwasaClass3world.IfMarshadlifeduring itswetNoachian period, four billion years ago, then it toomight havebeenaClass3world.

Class 4 planets, on the other hand, have been hijacked by life. They have“thick” biospheres that are deep networks of animal, plants, andmicrobes, allfeedingoneachotherandallfeedingbackontotheotherplanetarysystems.Theexistenceofouroxygenatmosphere,createdintheGreatOxidationEvent,tellsus that we are living on a biosphere-dominated planet where life plays anoutsized role inplanetary evolution.So theEarth, before civilization appearedtenthousandyearsago,wasaClass4world.

Ourschemewasbasedonthefactthatwehadrealexamplesofthefirstfourplanetaryclasses.Throughtheseknownworlds,wecouldunderstandhowsolarenergy feeds through the planetary systems and drives evolution. Thatknowledgegaveuspurchasetoseesomethingessentialaboutourhypothesizedfifthclass:aworldhostingasustainablecivilization.

GoingfromClass1toClass4worlds,weseeanincreaseinthecomplexityoftheirenergyflowsandtransformations.Class1worldscoulddolittleintermsof turning solar energy intowork and change. Class 4worlds comprised richnetworksofprocesseschannelingsolarenergyintoworkandchange.Fromtheperspective of thermodynamics, we could see how planets in each successiveclasshad“found”newwaystotransformtheirincidentstarlightintoevolution.On a world without life, this evolution can be rich, but the pathways areconstrained purely by physics and chemistry. In a sense, its details are fairlypredictable.OncelifeappearsinClasses3and4,biologicalevolutiontakesover.Lifefiguresoutentirelynewwaystodowork,yieldingnewprocessesthatfeedbackontherestoftheplanet.

Therelationshipbetweencomplexity,work,andenergyflowsgaveusakeytounderstandingwhatourfifthclassofplanetmightlooklike.AthickbiosphereonaClass4worldchannelsmoreenergyintoworkthanathinbiosphereonaClass3world,which itself channelsmoreenergy intowork thanonaClass2world.Thatmeansaplanetwitha sustainablecivilization—aClass5world—mightbe evenmore adept atwringingwork and changeoutof sunlight.OnaClass5planet,thebiosphere—whichnowincludesaglobe-spanningcivilization—becomes even more productive than Class 3 and Class 4 worlds. Thecivilization not only harvests more energy, as Kardashev imagined, but alsofiguresouthowtoput thisenergytoworkinways thatdonotpushtheplanetinto dangerous territory. The civilization, as part of the biosphere, adds whatphilosopherscall“agency.”Thecivilizationmakeschoiceswithgoals inmind.

Thus,Class5planetshaveagency-dominatedbiospheres.Thecivilizationisnowdeliberately working with the rest of the natural systems to increase theflourishingandproductivityofbothitselfandthebiosphereasawhole.

Perhaps the civilization converts its planet’s deserts into productiveecosystems.Such“desertgreening,”ifdonecorrectly,couldstabilizeachangingclimate.Oritmightengineerplants thatcanbothphotosynthesizeandproduceelectricity(thereareresearchersstudyingthisnow).19Oritmightcoverregionswith solar cells inways that also increase (or at leastdon’tdecrease) the totalbiosphericproductivityandhealthof theplanet.Thepossibilities are rich, andour study was meant only to suggest the right direction a Class 5 agency-dominated biosphere might take. There is much fruitful work to be done inturningthebasicconceptofClass5worldsintostrategiesforthefuture.

So,where does Earth fit into our classification scheme right now?AsweentertheAnthropocene,weareclearlyleavingtheClass4state.Ouractivityandchoices are stronglymodifying the state of the biosphere and other planetarysystems.Butwearemakingthesechangeswithoutalong-termplan,asplanetaryscientistDavidGrinspoon and others have pointed out.20We are evolving theplanettowardsomethingnew,butwecan’tsayifthatnovelstatewillincludeusinthelongterm.So,EarthatthebeginningoftheAnthropoceneisnolongeraClass4worldbutisnotyet,andmayneverbe,aClass5planet.Asofnow,it’sahybridworld.It’sevolvingtowardsomethingotherthanitwas,andit’sdoingsoinawaythat’sdangerousforourprojectofcivilization.

Thekeypointindevelopingthesefiveclassesofplanetwasthenecessityofputtingcivilizationsbackintothecontextofthebiosphere,ratherthanaboveit.From this perspective, sustainable civilizations are extensions of the longprocess of planetary evolution. Biospheres without civilizations are alreadyagentsofnovelty.Fromoxygen-producingmicrobestograsslandstomegafauna(likewoolymammoths),theyproducenewthingsthatthenenterintothewebofpositiveandnegative feedbackson theplanetarysystemasawhole.ThegreatlessonofLovelock,Margulis,andtheirGaiatheorywasthatthebiospherecouldevolve feedbacks that kept the system stable.A sustainable agency-dominatedbiosphereshouldbenodifferent.

Afterhispioneeringworkonthebiosphere,VladimirVernadskywentontoconsiderthepossibilityofplanets“wakingup”viawhathecalleda“noosphere.”CoinedfromtheGreeknoos,forintelligence,anoospherewasashellofthoughtsurrounding theplanet. Itwas the resultofabiosphereevolvingcreatures thatcould think and develop technology. From geology to life to mind, theemergence of the noosphere was, for Vernadsky, a next stage in planetary

evolution.21Class5planetsmightbeseenasworldsthathaveevolvedanoosphere.The

pervasive wireless mesh of connections that constitute today’s internet hasalready been held up as an initial version of a noosphere for Earth. Thus,wemightalreadymakeoutthecontoursofwhatasustainableworldwilllooklike.Totrulycomeintoacooperativecoevolutionwithabiosphere,a technologicalcivilizationmustmaketechnology—thefruitof itscollectivemind—serveasawebofawarenessfortheflourishingofbothitselfandtheplanetasawhole.

BeyondtheKardashevscale’sfocusonenergyas thecurrencyofplanetarydominance,wenowencounteranessentiallessonthestarsmightteachusaboutournextmoves.Planetsareenginesofinnovation.But,fromClass1toClass4,those innovations are blind. They are the result of pure chance and puremechanics—the laws of physics, chemistry, and biological evolution.They donothaveanendinmind.Thereisnoteleology.

RecallthatoneoftheloudestcriticismsofGaiatheorywasthatitcouldbeinterpreted to imply that life on Earth “wanted” to steer the planet in somedirection.ItwasinresponsetothesecriticismsthatGaiamorphedintothelesscontroversialEarthsystems theory.There,evolutionwasonceagainblind.Butwhenacivilizationemergesand triggers itsownversionof theAnthropocene,theageofblindnessmustcometoanend.

Inthedeepestsense,Class5planetswouldrepresentthecompletionofGaia.Theywouldbeworldswheretheplanetasawholehasanevolutionarydirection,a goal. That is what an agency-dominated biosphere means. The civilization,workingforitsowncontinuedexistence,recognizesitselfasanexpressionofthebiosphereandchoosesadirection.

So,wecannotbringtheworldtoheel.Instead,wemustbringitaplan.Ourprojectofcivilizationmustbecomeawayfortheplanettothink,todecide,andto guide its own future. Thus,wemust become the agent bywhich theEarthwakesuptoitself.

THEWAYFORWARD

Ultimately, theproblemweface isconfrontinga twenty-third-centurydilemmaarmedonlywithathirteenth-centurymind.Ourprojectofcivilizationhasbeensuccessfulonscaleswecouldnothaveimaginedwhenwebeganittenmillenniaago.Butwiththatsuccesshascomeconsequencesthatwilllastforcenturies.

Acrossthelonghistoryofourproject,wedidn’tknowourtrueplaceinthe

universe and could not, therefore, know our place within the planet’s ownevolution.Butnow, throughscience,wecanseeanew truth.TheEarth isbutoneworldamongtrillions,andwearenotaone-timestory.Nowwecan—andmust—make this our story. We must make it the human story, one that cutsacrosscultures,nations,andpolitics.22

Weare,mostcertainly,notthefirstspeciesthathasdramaticallychangedtheEarth’sclimate. Ithashappenedbefore, andwecanseehow that storyplayedout in the past. Earth is possibly, and even likely, not the first planet that hasevolvedacivilization.Usingallwehavelearnedaboutplanets,wecanseehowthatstory,includingclimatechange,mightalsohaveplayedoutinthepast.

ButwhattheAnthropocenemeansfortheplanet,andwhatitmeansforus,aredifferentthings.IfwecontinuetodonothingaboutouruseoffossilfuelsandtheotherdriversoftheAnthropocene,itismorethanconceivablethatwe’llpushthe planet into domains that prove difficult for our kind of complex globalcivilization. If our project of civilization collapses for a time, or evenpermanently, the Earth will happily move on without us. In that sense, oururgencyindealingwithclimatechangeandtheAnthropocenehasnothingtodowith“savingtheplanet.”OurentryintotheAnthropoceneshowsthatourprojectofcivilizationhasnowbecomeitsownkindofplanetarypower.It’sanewstorywe have to tell about ourselves, and everything now depends on learning andactinguponit.

Across thepagesof thisbook,we’veassembled thisnewnarrative throughsmaller stories of that story’s own evolution. We have encountered heroicscientistswho tookus up to themountain so thatwemight see farther.TherewereFrankDrake,JillTarter,andNikolaiKardashev,whobravedthescornoftheircolleaguestotaketheexistenceofexo-civilizationseriouslyasatopicforscientificinquiry.Throughtheirefforts,wecouldbegintoseelifeandthestarsin a new light. There were explorers like Jack James and Steven Squyers,blasting robots across space to the otherworlds in our solar system. Throughtheir work and the studies of researchers like Robert Haberle, we learned thelawsofclimateandevolutionforallplanets.ArmycorpsmenandscientistslikeWilliDansgaardbravedCampCenturyonGreenland’s icesheet tohelpusseemore deeply into the transitions of Earth’s climate. Then came people likeDonald Canfield, who traveled the world to unpack the deep history of ourplanet and its life. Putting all this together were visionaries like VladimirVernadsky,JamesLovelock,andLynnMargulis,wholiftedoursightstoseehowthatlifecanpartnerwithitsplanettoevolveintosomethinggreater,somethingmore. Finding other planets was the job of scientists likeMichelMayor, BillBorucki, Natalie Batalha, and others. Their work answered a millennia-old

questionand,indoingso,filledthenightskywithatrilliontrillionworldsandpossibilities.Andfinally,appearingatalmosteveryturn,therewasCarlSagan.More thanalmost anyoneelse,weowe thepossibilityof thisnewstory tohisgenius.

Sciencehasgivenusanewperspective,anewvision,andanewstorythatcanhelpus findawayforwardasweface thechallengeof theAnthropocene.Butthiscanonlyhappenifwelistencarefullyandtrulymakethisnewstoryourown.

Itistimetogrowup.The central argument of this book, and one that Carl Sagan already

understood, is that humanity and its project of civilization represent a kind of“cosmicteenager.”Wearelikelyjustoneworldamongmanythathasgrownacivilizationtothepointwhereithasgainedpoweroveritselfanditsplanet.But,likeateenager,welackthematuritytotakefullresponsibilityforourourselvesandourfuture.

Gaining the astrobiological perspective is the first, essential step in ourmaturationandourability to face theAnthropocene. Itmeansrecognizing thatwe and our project of civilization are nothing more than the fruit of Earth’songoing evolutionary experiments. Any civilization on any planet will benothing more than an expression of its home world’s creativity. We are nodifferentfromthosewewouldcall“alien.”

Soourfocushastoshift.It’stimetoleavethetiredquestion,“Didwecreateclimate change?” behind. In its place we must take up our bracing newastrobiological truth: “Of course we changed the climate.”We built a planet-spanningcivilization.Whatelsewouldweexpecttohappen?

Butweshouldalsorecognizethatcreatingclimatechangewasn’tdonewithmalevolence.Wearenotaplagueontheplanet.Instead,weare theplanet.Weare, at least,what the planet is doing right now.But that is no guarantee thatwe’llstillbewhattheplanetisdoingonethousandortenthousandyearsfromnow.

AschildrenoftheEarth,wearealsochildrenofthestars.Ifnothingelse,theAnthropocenecanmakethatfactasrealtousastheshriekofahowlingstorm,theoppressiveheat of adesert landscape, or the cool silenceof adeep forest.Throughthelightofthestars,throughwhattheycanteachusaboutotherworldsand the possibilities of other civilizations, we can learn what path throughadolescencewemusttake.Andinthatway,wecanreachourmaturity.Wecanreach our full promise and possibility.We canmake theAnthropocene into aneweraforbothourcivilizationandtheEarth.In theend,ourstory isnotyetwritten.Westandatacrossroadsunderthelightofthestars,readytojointhem

orreadytofail.Thechoicewillbeourown.

ILLUSTRATIONCREDITS

22 AlanDunnTheNewYorkerCollection/TheCartoonBank

40 ©FrankDrake

57 ©NASA

62 ©Bettmann/GettyImages

82 ©NASA

102 ©W.RobertMoore/NationalGeographic/GettyImages

108 ©NASA

119 Sputnik/SciencePhotoLibrary

126 PhotocourtesyoftheEstateofLynnMargulis

142 ©AceyHarper/theLIFEImagesCollection/GettyImages

176 CourtesyoftheArchives,CaliforniaInstituteofTechnology

181 DavidNunuk/SciencePhotoLibrary

207 ©ByurakanObservatory

ACKNOWLEDGMENTS

Thisworkwould not have been possiblewithout the support and, sometimes,direct intervention of some very smart and kind people. I must first thankHoward Yoon of Ross Yoon Agency for his working so closely with me todeveloptheearlyversionsoftheideaintoacoherentformandhismanyyearsofhelp in so many, many ways. From the very beginning, my editor at W.W.Norton,MattWeiland, sawhow tomake the idea and its incarnation into thisbookcleanerandmoresharplydefined.Itwasagreatpleasuretoworkwithhimand I am deeply grateful that his skills were brought to bear on this project.Simplyput,heisagreateditor.IamalsogratefultohavehadRemyCawleyonthe W. W. Norton team. In editing, copyediting, and managing the imageprocess,herprecisionandthoroughnesswereessential.Iwasalsoluckytohavetwo wonderful University of Rochester undergraduates working with me asassistants on the book. Molly Finn worked tirelessly on fact checking andaccumulating proper references and reference forms. EliseMorgan endured acrazyautumntrackingdownimagesandpermissions.Bothshowedremarkableskillsforyoungscholars,andIwasluckytohavefoundtheirhelp.

Forascientist, it isalwaysalittlefrighteningtowriteabout topicsthatarenotsquarelyinthedomainofyourresearchspecialty.Forme,thisincludednotonly sciences like atmospheric chemistry, but also the amazing history of thediscoveries I wanted to explore in the book. I bear full responsibility for anymistakesmadeinthetext.Intryingtogetthestoryright,though,Iwashelpedby many scientists who were generous with their time. In particular, mycollaboratorWoody Sullivan at the University of Washington provided manyfine insights into the manuscript. My gratitude to him runs very deep. JasonWright of Penn State gave an early version of the book a thorough and deepreading.Notonlydidhemakethebookmoreaccurate,buthealsodrovemeto

thinkmoredeeplyaboutanumberoftopicsonexo-civilizations.JillTarternotonlygavemeanumberofgreatinterviews,butalsoprovidedexcellentfeedbackonthemanuscript.IamequallygratefultoDonaldCanfield,bothforinterviewsonatmosphericchemistryandhisreviewofthechapteronEarthscience.JamesKastingofPennStateandLeeMurrayoftheUniversityofRochesterprovidedexcellent feedback on the climate and Earth science sections as well. RobertHaberlewas generouswith his time in explaining the history ofMars climatemodelingandreviewingthechapteronsolarsystemexploration.

IamalsogratefultoSorenGregersen,whomIbotheredanumberoftimestotellmehisstoriesofbeingaBoyScoutandlivingoutontheGreenlandicesheetwiththeUSmilitaryatCampCentury.IamequallygratefultoNatalieBatalhaandBillBoruckiforgivingmetheirtimeforinterviews.

There are also many people I have to thank just for their intellectualcompanionship.RobertPincusandPaulGreenarealwaysatthetopofthelistonanytopicforme.MyPhDadvisorandcontinuingcollaboratorBruceBalickhasalways been a source of good advice and ideas. I also had many excellentconversationswithmycolleaguesat theUniversityofRochester:DanWatson,Eric Blackman, Alice Quillen, Eric Mamejek, Judy Pipher, and Bill Forrest.Also,Imustthankmycollaboratorsontheworkdescribedinthisbook:WoodySullivan, Marina Alberti, Axel Kleidon, and Jonathan Carroll-Nellenback.OngoingdiscussionswithEvanThompsonwerealsofunandhelpful.WritingonthesetopicsforbothNPRandtheNewYorkTimesgavemeafirstchancetocasttheideasinnon-scientificlanguage.IamverygratefultoJamieReyersonattheTimes,aswellasMeghanSullivanandJustineKeninatNPR.

Finally,IamparticularlygratefulformyNPRblogco-founder,collaborator,andfriendMarceloGleiser,whoprovided theopportunity tospend timeat theInstitute forCrossDisciplinaryEngagement atDartmouth,where someof thisbookwaswritten.Thankyou,Marcelo.

Finally,Imustthankmychildren,SadieandHarrison,aswellasmybrother-in-law,HendrikHelmer, formakingme laugh . . . a lot.And always, always,always,Ithankthestarsformywife,AlanaCahoon,withoutwhomnoneofthiswouldmatter.

NOTES

INTRODUCTION:THEPROJECTANDTHEPLANET

1. John D. Durand, “Historical Estimates ofWorld Population: An Evaluation,”PSC Analytical andTechnicalReports,no.10(1974):table2.

2. Department of Economic and Social Affairs, Population Division, The World at Six Billion (NewYork: United Nations Secretariat, 1999),http://www.un.org/esa/population/publications/sixbillion/sixbillion.htm.

3. PaulMann,LisaGahagan,andMarkB.Gordon,“TectonicSettingoftheWorld’sGiantOilandGasFields,”inGiantOilandGasFieldsoftheDecade,1990–1999,ed.MichelT.Halbouty(Tulsa,OK:AmericanAssociationofPetroleumGeologists,2014).

4. DepartmentofEconomicAffairs,PopulationDivision,WorldPopulationProspects:KeyFindingsandAdvance Tables, 2015 Revision (New York: United Nations, 2015),https://esa.un.org/unpd/wpp/Publications/Files/Key_Findings_WPP_2015.pdf.

5. InternationalAirTransportAssociation,2012AnnualReview,June2012.6. LynnMargulis,“GaiaIsaToughBitch,”inTheThirdCulture:BeyondtheScientificRevolution,ed.

JohnBrockman(NewYork:SimonandSchuster,1995).7. KimStanleyRobinson,Aurora(NewYork:Orbit,2015).8. UniversityofZurich,“GreatOxidationEvent:MoreOxygenthroughMulticellularity,”ScienceDaily,

January17,2013,www.sciencedaily.com/releases/2013/01/130117084856.htm.9. European Space Agency, “Greenhouse Effect, Clouds and Winds,” Venus Express,

http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effect_clouds_and_winds.10. V.-P.Kostama,M.A.Kreslavsky,andJ.W.Head,“RecentHigh-LatitudeIcyMantleintheNorthern

PlainsofMars:CharacteristicsandAgesofEmplacement,”GeophysicalResearchLetters33,no.11(2006),doi:10.1029/2006GL025946,andNASAJetPropulsionLaboratory,“MarsIceDepositHoldsas Much Water as Lake Superior,” news release, November 22, 2016,https://www.jpl.nasa.gov/news/news.php?release=2016-299.

11. JoeMasonandMichaelBuckley,“CassiniFindsHydrocarbonRainsMayFillTitanLakes,”CassiniImaging Central Laboratory for Operations, January 29, 2009, http://ciclops.org/view.php?id=5471&js=1.TheliquidsonTitanincludecomponentsofgasoline.

12. Colin N.Waters et al. “The Anthropocene Is Functionally and Stratigraphically Distinct from theHolocene,” Science 351, no. 6269 (January 8, 2016),http://science.sciencemag.org/content/351/6269/aad2622.

13. DaleJamieson,ReasoninaDarkTime(NewYork:OxfordUniversityPress,2014).14. NASAExoplanetScience Institute, “Exoplanet andCandidateStatistics,”NASAExoplanetArchive,

https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html.

CHAPTER1:THEALIENEQUATION

1. C.P.Snow,ThePhysicists(Boston:LittleBrown,1981).2. AlanLightman,ASenseoftheMysterious:ScienceandtheHumanSpirit(NewYork:Vintage,2006).3. EricM.Jones,WhereIsEverybody?:AnAccountofFermi’sQuestion(LosAlamos,NM:LosAlamos

NationalLaboratory,1985),https://www.osti.gov/accomplishments/documents/fullText/ACC0055.pdf.4. Jones,WhereIsEverybody?:3.5. EnricoFermi,“MyObservationsDuringtheExplosionatTrinityonJuly16,1945,”Fermat’sLibrary,

http://www.atomicarchive.com/Docs/Trinity/Fermi.shtml.6. As astronomer JasonWrightputs it, “Astronomers stare at the skyprofessionallywith someof the

most sensitive equipment in theworld. IfUFOswere common,wewould see themall the time. ItstrainscredulitythatarmiesofamateurswithcamerasregularlyseeUFOswhentheprofessionalswithgiant telescopesdonot.” JasonWright, “Astronomers andUFOs,”AstroWright,December1,2013,https://sites.psu.edu/astrowright/2013/12/01/astronomers-and-ufos/.

7. MichaelHart,“AnExplanation for theAbsenceofExtraterrestrialsonEarth,”QuarterlyJournaloftheRoyalAstronomicalSociety16(June1975):128.AlsoseeRobertH.Gray,“TheFermiParadoxIsNeitherFermi’sNoraParadox,”Astrobiology15,no.3(March2015):195–99.

8. GlenDavidBrin,“The‘GreatSilence’:TheControversyConcerningExtraterrestrialIntelligentLife,”Quarterly Journalof theRoyalAstronomical Society 24, no. 3 (1983): 283–309, and JamesAnnis,“AnAstrophysicalExplanationforthe‘GreatSilence,’”JournaloftheBritishInterplanetarySociety52(1999):19–22.

9. Robin Hansen, The Great Filter—Are We Almost Past It?, September 15, 1998,http://mason.gmu.edu/~rhanson/greatfilter.html.

10. Heike Langenberg, “Slow Gulf Stream During Ice Ages?,” Nature News, December 9, 1999,http://www.nature.com/news/1999/991209/full/news991209-10.html.

11. Thiscouldhappeninmanyways,buttheeasiesttoimagineisasignificantpopulationreduction—a“die-off”—that keeps the population’s capacities below the level for a technological/industrial re-emergence.Notethatdramaticclimatechangecouldresultinaspeciesthatoncehadatechnologicalcivilization living for hundreds of thousands of years on aworldwhere large-scale agriculture hasbecome impossible. It isverydifficult topredictwhat theevolutionary/sociologicaloutcomeof thisscenariowouldbe.

12. MatthewF.Dowd,“FractionofStarswithPlanetarySystems,fp,pre-1961,”inTheDrakeEquation,ed.DouglasA.VakochandMatthewF.Dowd(NewYork:CambridgeUniversityPress,2015),56.

13. Steven J. Dick, Plurality of Worlds: The Extraterrestrial Life Debate from Democritus to Kant(Cambridge:CambridgeUniversityPress,1984),6.

14. Dick,PluralityofWorlds,26–27.15. Dick,PluralityofWorlds,62.16. There remains some dispute over what exactly Bruno was convicted for in the heresy charge.

Evidencepointstomorearcaneissuesofdoctrine,ratherthanastronomy.HisviewsonCopernicanismand otherworlds, however, contributed to his career of conflictwith theChurch.Dorothea Singer,GiordanoBruno:HisLifeandThought(1950;repr.,NewYork:GreenwoodPress,1968).

17. Bernard de Fontenelle,Conversations on the Plurality ofWorlds (1686; repr., London: J. Cundee,1803),112.

18. Dowd, “Fraction of Stars,” 67, and Steven J. Dick, Life on Other Worlds: The 20th-CenturyExtraterrestrialLifeDebate(Cambridge:CambridgeUniversityPress,1998).

19. DouglasA.Vakoch, ed.,Astrobiology, History, and Society: Life Beyond Earth and the Impact ofDiscovery(Berlin:Springer,2013),108.

20. PercivalLowell,“ObservationsattheLowellObservatory,”Nature76(1907):446.

21. WilliamWhewell,Of the Plurality of Worlds (1853; repr., Chicago: University of Chicago Press,2001),207.

22. Whewell,PluralityofWorlds,204–5.23. AlfredRusselWallace,Man’sPlaceintheUniverse:AStudyoftheResultsofScientificResearchin

RelationtotheUnityorPluralityofWorlds(London:ChapmanandHall,1904).24. Dowd,“FractionofStars,”67.25. FlorenceRaulinCerceau,“NumberofPlanetswithanEnvironmentSuitableforLife,ne,Pre-1961,”

in The Drake Equation, eds. Douglas A. Vakoch and Matthew F. Dowd (New York: CambridgeUniversityPress,2015),98.

26. NaturalResourcesDefenseCouncil,“GlobalNuclearStockpiles,1945–2006,”BulletinoftheAtomicScientists 62, no. 4 (July/August 2006): 64–66,http://media.hoover.org/sites/default/files/documents/GlobalNuclearStockpiles.pdf.

27. Stephanie Pappas, “Hydrogen Bomb vs. Atomic Bomb: What’s the Difference?,” Live Science,January6,2016,https://www.livescience.com/53280-hydrogen-bomb-vs-atomic-bomb.html.

28. DonP.Mitchell,“TheR-7Missile,”http://mentallandscape.com/S_R7.htm.29. Steve Garber, “Sputnik and the Dawn of the Space Age,” National Aeronautics and Space

Administration,lastmodifiedOctober10,2007,https://history.nasa.gov/sputnik/.30. FrankDrakeandDavaSobel,IsAnyoneOutThere?(NewYork:DelacortePress,1992),5.31. DrakeandSobel,IsAnyoneOutThere?,27.32. DrakeandSobel,IsAnyoneOutThere?,8–12.33. FrankDrake,“AReminiscenceofProjectOzma,”CosmicSearch1,no.1(1979):10.34. F. Ghigo, “The Tatel Telescope,” National Radio Astronomy Observatory,

http://www.gb.nrao.edu/fgdocs/tatel/tatel.html.35. Drake,“Reminiscence.”36. John R. Percy, “The Nearest Stars: A Guided Tour,” Astronomical Society of the Pacific, 1986,

https://astrosociety.org/edu/publications/tnl/05/stars2.html.37. Drake,“Reminiscence.”38. “Early SETI: Project Ozma, Arecibo Message,” SETI Institute, http://www.seti.org/seti-

institute/project/details/early-seti-project-ozma-arecibo-message.39. Drake,“Reminiscence.”40. “EarlySETI:ProjectOzma.”41. GiuseppeCocconiandPhilipMorrison,“SearchingforInterstellarCommunications,”Nature184,no.

4690(September19,1959):844–46.42. DrakeandSobel,IsAnyoneOutThere?,32.43. DrakeandSobel,IsAnyoneOutThere?,45–64.44. DrakeandSobel,IsAnyoneOutThere?,47.45. DrakeandSobel,IsAnyoneOutThere?,54.46. DrakeandSobel,IsAnyoneOutThere?,49.47. DrakeandSobel,IsAnyoneOutThere?,51.48. Maggie Masetti, “How Many Stars in the Milky Way?,” Blueshift, July 22, 2015,

https://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/.49. FredHoyle,TheBlackCloud(London:Heinemann,1957).50. Drakechosetofocusjustonourhomegalaxy,theMilkyWay,becausethedistancestoothergalaxies

aresolarge.Anysource-emittedelectromagneticradiationbecomesmoredifficulttodetectthefartherawayitis.

51. Su-ShuHuang,“TheProblemofLifeintheUniverseandtheModeofStarFormation,”PublicationsoftheAstronomicalSocietyofthePacific71,no.422(October1959):421–24.

52. StanleyL.Miller,“AProductionofAminoAcidsunderPossiblePrimitiveEarthConditions,”Science117,no.3046(May15,1953):528–29.

53. DrakeandSobel,IsAnyoneOutThere?,61.54. Ofcourse,onecanalsoaskwhetheracivilizationthatwasfarmoreadvancedthanourswouldstill

useradioatall.Butlikethepreviousissueofliferequiringplanets,onehastobeginsomewhere,anditsbettertounderestimatethelikelihoodofeachtermthangooverboard.

55. DrakeandSobel,IsAnyoneOutThere?,62.56. DrakeandSobel,IsAnyoneOutThere?,52.57. DrakeandSobel,IsAnyoneOutThere?,62.58. DrakeandSobel,IsAnyoneOutThere?,64.59. Jamieson,Reason,20.60. “The Television Infrared Observation Satellite Program (TIROS),”NASAScience, May 22, 2016,

https://science.nasa.gov/missions/tiros/.

CHAPTER2:WHATTHEROBOTAMBASSADORSSAY

1. Franklin O’Donnell, “The Venus Mission: How Mariner 2 Led the World to the Planets,” JetPropulsionLaboratorywebsite,https://www.jpl.nasa.gov/mariner2/.

2. DavidR.Williams,“ChronologyofLunarandPlanetaryExploration,”GoddardSpaceFlightCenter,lastmodifiedAugust8,2017,https://nssdc.gsfc.nasa.gov/planetary/chronology.html.

3. DavidR.Williams, “VenusFactSheet,”GoddardSpaceFlightCenter, lastmodifiedDecember23,2016,https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html.

4. O’Donnell,“TheVenusMission.”5. LarryKlaes,“RememberingtheEarlyRoboticExplorers,”CentauriDreams:ImaginingandPlanning

InterstellarExploration,August29,2012,https://www.centauri-dreams.org/?p=24285.6. O’Donnell,“TheVenusMission.”7. O’Donnell,“TheVenusMission.”8. Williams,“VenusFactSheet.”9. WilliamSheehanandJohnEdwardWestfall,TheTransitsofVenus(Amherst,NY:PrometheusBooks,

2004),213.10. SheehanandWestfall,Transits,213.11. MikhailYa.Marov,“MikhailLomonosovandtheDiscoveryoftheAtmosphereofVenusDuringthe

1761Transit,” inTransitsofVenus:NewViewsof theSolarSystemandGalaxy,Proceedingsof the196thColloquiumof theInternationalAstronomicalUnion,ed.D.W.Kurtz(Cambridge:CambridgeUniversityPress,2004).

12. F.W.TaylorandD.M.Hunten,“Venus:Atmosphere,”inEncyclopediaoftheSolarSystem,3rded.,eds.TilmanSpohn,DorisBreuer,andTorrenceV.Johnson(Amsterdam:Elsevier,2014).

13. C. H. Mayer, T. P. McCullough, and R. M. Sloanaker, “Observations of Venus at 3.15 cmWaveLength,”AstrophysicalJournal127,no.1(January1958):1–10.

14. PaoloUliviwithDavidM.Harland,RoboticExplorationoftheSolarSystem:Part1,TheGoldenAge,1957–1982(Berlin:Springer,2007),xxxi.

15. UliviandHarland,RoboticExploration,xxxii.16. “Planetary Temperatures,” Australian Space Academy,

http://www.spaceacademy.net.au/library/notes/plantemp.htm.17. KeayDavidson,CarlSagan:ALife(NewYork:Wiley,1999),39–56.18. RaySpangenburgandKitMoser,CarlSagan:ABiography(Westport,CT:Greenwood,2004),11–29.19. Kenneth R. Lang, “Global Warming: Heating by the Greenhouse Effect,”NASA’s Cosmos, 2010,

http://ase.tufts.edu/cosmos/view_chapter.asp?id=21&page=1.20. Tim Sharp, “What Is the Temperature on Earth?,” Space.com, September 28, 2012,

https://www.space.com/17816-earth-temperature.html.21. F.W.Taylor,PlanetaryAtmospheres(Oxford:OxfordUniversityPress,2010),12.

22. Svante Arrhenius, “On the Influence of Carbonic Acid in the Air upon the Temperature of theGround,”PhilosophicalMagazineandJournalofScience41,no.251(April1896):237–76.

23. SpencerWeart,“TheCarbonDioxideGreenhouseEffect,”TheDiscoveryofGlobalWarming,January2017,https://history.aip.org/climate/co2.htm.

24. SpangenburgandMoser,CarlSagan,36–38.25. Davidson,CarlSagan.26. O’Donnell,“TheVenusMission.”27. Tony Reichhardt, “The First Planetary Explorers,” Air and Space Magazine, December 14, 2012,

http://www.airspacemag.com/daily-planet/the-first-planetary-explorers-162133105/.28. O’Donnell,“TheVenusMission.”29. Asif A. Siddiqi, Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes

1958–2000(Washington,DC:NationalAeronauticsandSpaceAdministration,2002).30. Taylor,PlanetaryAtmospheres,113–15.31. Taylor,PlanetaryAtmospheres,114–24.32. Thecold trapworksbykeepingwater in the lowerpartof theatmosphere.Aswatervapor rises, it

eventually cools and condenses, falling back to Earth. This process intensifies at the tropopause(fifteen kilometers above sea level), where air temperatures drop far below freezing. Thus, allremainingwater intheatmosphereisfrozenout.MichaelDenton,“TheColdTrap:HowItWorks,”Evolution News and Science Today, May 10, 2014,https://evolutionnews.org/2014/05/the_cold_trap_h/.

33. Davidson,CarlSagan.34. SpangenburgandMoser,CarlSagan,34–65.35. “Mars Exploration Rovers: Step-by-Step Guide to Entry, Descent, and Landing,” Jet Propulsion

Laboratory,https://mars.nasa.gov/mer/mission/tl_entry1.html.36. StevenW.Squyres,RovingMars:Spirit,Opportunity,and theExplorationof theRedPlanet(New

York:Hyperion,2005),292–93.37. UliviandHarland,RoboticExploration,xxxiii–xxxiv.38. Vakoch,Astrobiology,History,andSociety,108.39. WilliamSheehan,ThePlanetMars:AHistoryofObservationandDiscovery(Tucson,AZ:University

ofArizonaPress,1996).40. Sheehan,PlanetMars.41. Rod Pyle, “Alone in the Darkness:Mariner 4 to Mars, 50 Years Later,” California Institute of

Technology, July 14, 2015, https://www.caltech.edu/news/alone-darkness-mariner-4-mars-50-years-later-47324.

42. “TheDeadPlanet,”NewYorkTimes,July30,1965.43. UliviandHarland,RoboticExploration,108–12.44. UliviandHarland,RoboticExploration,114–16.45. Elizabeth Howell, “Mariner 9: First Spacecraft to Orbit Mars,” Space.com, November 12, 2012,

https://www.space.com/18439-mariner-9.html.46. “Welcome to the Planets,” Jet Propulsion Laboratory,

https://pds.jpl.nasa.gov/planets/choices/mars1.htm.47. Davidson,CarlSagan,279–80.48. DavidR.Williams,“VikingMissiontoMars,”GoddardSpaceFlightCenter,lastmodifiedSeptember

5,2017,https://nssdc.gsfc.nasa.gov/planetary/viking.html,and“AChronologyofMarsExploration,”National Aeronautics and Space Administration, last modified April 16, 2015,https://history.nasa.gov/printFriendly/marschro.htm.

49. “Overview: The Mars Exploration Program,” National Aeronautics and Space Administration,https://mars.nasa.gov/programmissions/overview/.

50. RobertHaberle,interviewwiththeauthor,March20,2017.51. “The History of Mars General Circulation Model,” Mars Climate Modeling Center,

https://spacescience.arc.nasa.gov/mars-climate-modeling-group/history.html.52. Haberle,interview.53. Williams,“VikingMissiontoMars.”54. Williams,“VikingMissiontoMars.”55. Derek Hayes,Historical Atlas of the Pacific Northwest (Vancouver, BC: Douglas and McIntyre,

2001).56. AndersPersson,“Hadley’sPrinciple:Part1—ABrainchildwithManyFathers,”Weather63,no.11

(November2008):335–38.57. DavidR.Williams, “Mars Fact Sheet,”Goddard Space Flight Center, lastmodifiedDecember 23,

2016,https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html.58. Haberle,interview.59. Rob Gutro, “Polar Vortex Enters Northern U.S.,” Goddard Space Flight Center, 2014,

https://www.nasa.gov/content/goddard/polar-vortex-enters-northern-us/#.WcAeq62UUo-.60. LauraDattaro,“ChecktheWeatheronMars,WhereNASA’sMAVENIsHeaded,”WeatherChannel,

November 19, 2013, https://weather.com/science/news/check-weather-mars-where-nasas-maven-headed-20131119.

61. AndrewP.Ingersoll,PlanetaryClimates(Princeton,NJ:PrincetonUniversityPress,2013),96–106.62. NationalAeronauticsandSpaceAdministration,“Minerals inMars ‘Berries’Adds toWaterStory,”

newsrelease,March18,2004,https://mars.nasa.gov/mer/newsroom/pressreleases/20040318a.html.63. National Aeronautics and Space Administration, “NASA Rover Finds Old Streambed on Martian

Surface,” news release, September 27, 2012,https://www.nasa.gov/mission_pages/msl/news/msl20120927.html.

64. MichaelH.CarrandJamesW.HeadIII,“GeologicHistoryofMars,”EarthandPlanetaryScienceLetters294,nos.3–4(June1,2010):185–203.

65. PaulL.Montgomery,“ThrongsFillManhattan toProtestNuclearWeapons,”NewYorkTimes,June13,1982.

66. Robert S. Norris and Hans M. Kristensen, “Global Nuclear Weapons Inventories, 1945–2010,”BulletinoftheAtomicScientists66,no.4(July/August2010):77–83.

67. R.P.Turcoetal.,“NuclearWinter:GlobalConsequencesofMultipleNuclearExplosions,”Science222,no.4630(December23,1983):1283–92.

68. Jill Lepore, “The Atomic Origins of Climate Science,” The New Yorker, January 30, 2017,http://www.newyorker.com/magazine/2017/01/30/the-atomic-origins-of-climate-science.

69. JacobDarwinHamblin,“Badash,ANuclearWinter’sTale,”Metascience21,no.3(November2012):727–31.

CHAPTER3:THEMASKSOFEARTH

1. “Earth’s Early Atmosphere,” Astrobiology Magazine, December 2, 2011,http://www.astrobio.net/geology/earths-early-atmosphere/.

2. John Reed, “Inside the Army’s Secret Cold War Ice Base,” Defense Tech, April 6, 2012,https://www.defensetech.org/2012/04/06/inside-the-armys-secret-cold-war-ice-base/, and MalcolmMellor,Oversnow Transport (Hanover, NH: U.S. Army Cold Regions Research and EngineeringLaboratory,1963),http://www.dtic.mil/dtic/tr/fulltext/u2/404778.pdf.

3. “TrailBlazedbyRenownedExplorerLeadsDanish,U.S.ScoutstoArcticAdventure,”ArmyResearchandDevelopment,December1960,14.

4. “The Ice Sheet,”Visit Greenland, http://www.greenland.com/en/about-greenland/nature-climate/the-ice-cap/.

5. Frank J. Leskovitz, “Camp Century, Greenland: Science Leads the Way,”http://gombessa.tripod.com/scienceleadstheway/id9.html.

6. Leskovitz,“CampCentury.”7. Leskovitz,“CampCentury.”8. Leon E. McKinney, “Camp Century Greenland,” West-Point.org, http://www.west-

point.org/class/usma1955/D/Hist/Century.htm.9. GordondeQ.Robin,“ProfileData,GreenlandRegion,”inTheClimateRecordinPolarIceCaps,ed.

GordondeQ.Robin(1983;repr.,Cambridge:CambridgeUniversityPress,2010),100–101.10. JosephGale,Astrobiology of Earth: The Emergence, Evolution, and Future of Life on a Planet in

Turmoil(Oxford:OxfordUniversityPress,2009),125–26.11. JohnS.Schlee,“OurChangingContinent,”UnitedStatesGeologicalSurvey,lastmodifiedFebruary

15,2000,https://pubs.usgs.gov/gip/continents/.12. Gale,AstrobiologyofEarth,125.13. Willi Dansgaard,Frozen Annals: Greenland Ice Cap Research (Copenhagen: Niels Bohr Institute,

2005),55–56.14. Dansgaard,FrozenAnnals,58.15. W. Dansgaard et al., “One Thousand Centuries of Climate Record from Camp Century on the

Greenland IceSheet,”Science 166, no. 3903 (October 17, 1969): 377–80. See also “TheYoungerDryas,”NOAANationalCenters forEnvironmental Information,https://www.ncdc.noaa.gov/abrupt-climate-change/The%20Younger%20Dryas.

16. MannedSpacecraftCenter, “Apollo8OnboardVoiceTranscription,AsRecordedon theSpacecraftOnboard Recorder (Data Storage Equipment),” January 1969, 113–14,https://www.jsc.nasa.gov/history/mission_trans/AS08_CM.PDF.

17. Earthrise,Time,http://100photos.time.com/photos/nasa-earthrise-apollo-8.18. K.M.Cohen,S.Finney,andP.L.Gibbard,“InternationalChronostratigraphicChart,”International

Commission on Stratigraphy, January 2013,http://www.stratigraphy.org/icschart/chronostratchart2013-01.pdf.

19. AnnZabludoff,“Lecture13:TheNebularTheoryof theOriginof theSolarSystem,”UniversityofArizona Department of Astronomy and Steward Observatory,http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html.

20. C.Goldblattetal.,“TheEonsofChaosandHades,”SolidEarthDiscussions1,no.1(2010),1–3.21. Goldblattetal.,“ChaosandHades.”22. Thomas Holtz, “GEOL 102 Historical Geology: The Archean Eon,” University of Maryland

Department of Geology, last modified January 18, 2017,https://www.geol.umd.edu/~tholtz/G102/lectures/102archean.html.

23. Stanly M. Awramik and Kenneth J. McNamara, “The Evolution and Diversification of Life,” inPlanetsandLife:TheEmergingScienceofAstrobiology, eds.WoodruffR.Sullivan III and JohnABaross(CambridgeUniversityPress,2007),313–16.

24. AwramikandMcNamara,“EvolutionandDiversification”313–18.25. Z. X. Li et al., “Assembly, Configuration, and Break-up History of Rodinia: A Synthesis,”

PrecambrianResearch,160(2008):179–210.26. David Catling and James F. Kasting, “PlanetaryAtmospheres and Life,” inPlanets and Life: The

Emerging Science of Astrobiology, eds. Woodruff R. Sullivan III and John A Baross (CambridgeUniversityPress,2007),99.

27. AwramikandMcNamara,“EvolutionandDiversification,”321.28. DonaldE.Canfield,Oxygen:AFourBillionYearHistory(Princeton,NJ:PrincetonUniversityPress,

2014),145–46.29. “PETM: Global Warming, Natural,” Weather Underground,

https://www.wunderground.com/climate/PETM.asp?MR=1.30. Canfield,Oxygen,13.31. Canfield,Oxygen,14.32. “Opening a Tectonic Zipper,” Seismo Blog (UC Berkeley Seismology Lab), April 5, 2010,

http://seismo.berkeley.edu/blog/2010/04/05/opening-a-tectonic-zipper.html.33. Canfield,Oxygen,14.34. Canfield,Oxygen,14.35. Canfield,Oxygen,41.36. Canfield,Oxygen,41.37. Gale,AstrobiologyofEarth,110–11.38. Canfield,Oxygen,41–42.39. DavidC.Catling,Astrobiology:AVeryShortIntroduction (Oxford:OxfordUniversityPress,2013),

50–55.40. Catling,Astrobiology,52.41. AlexejM.Ghilarov,“Vernadsky’sBiosphereConcept:AnHistoricalPerspective,”QuarterlyReview

ofBiology70,no.2(June1995):193–203.42. IrinaTrubetskova, “Vladimir IvanovichVernadsky andHisRevolutionaryTheoryof theBiosphere

and the Noosphere,” University of New Hampshire, http://www-ssg.sr.unh.edu/preceptorial/Summaries_2004/Vernadsky_Pap_ITru.html.

43. Ghilarov,“Vernadsky’sBiosphereConcept.”44. VladimirVernadsky,TheBiosphere,trans.DavidB.Langmuir(NewYork:Copernicus,1998),44,56.45. Ghilarov,“Vernadsky’sBiosphereConcept.”46. James Lovelock, Homage to Gaia: The Life of an Independent Scientist (New York: Oxford

UniversityPress,2000).47. Lovelock,HomagetoGaia,242.48. Lovelock,HomagetoGaia,243.49. Lovelock,HomagetoGaia,243.50. Lovelock,HomagetoGaia,243–44.51. Lovelock,HomagetoGaia,253.52. Lovelock,HomagetoGaia,255.53. Joel Bartholomew Hagen, Douglas Allchin, and Fred Singer, Doing Biology (New York:

HarperCollins,1996).54. Lovelock,HomagetoGaia,256–57.55. MichaelRuse,“Earth’sHolyFool?,”Aeon,https://aeon.co/essays/gaia-why-some-scientists-think-it-s-

a-nonsensical-fantasy.56. JohnPostgate,“GaiaGetsTooBigforHerBoots,”NewScientist,April7,1988.57. Ruse,“Earth’sHolyFool?”58. Lovelock,HomagetoGaia,265.59. Ruse,“Earth’sHolyFool?”

CHAPTER4:WORLDSBEYONDMEASURE

1. This section on Thomas See is based on information found in Thomas J. Sherrill, “A Career ofControversy:TheAnomalyofT.J.J.See,”JournalfortheHistoryofAstronomy30,no.1 (February1999):25–50,andWilliamSheehan,“TheTragicCaseofT.J.J.See,”Mercury31,no.6(November2002):34.

2. Personalcorrespondence.3. Amy Veltman, “Dr. Jill Tarter: Looking to Make ‘Contact,’ ” Space.com, November 12, 1999,

https://web.archive.org/web/20081005020231/http://www.space.com/peopleinterviews/tarter_profile_991112.html.4. “JillTarter,”SETIInstitute,https://www.seti.org/users/jill-tarter.5. JillTarter,interviewwiththeauthor.6. JohnBillingham,“SETI:TheNASAYears,”inSearchingforExtraterrestrialIntelligence:SETIPast,

Present,andFuture,ed.H.PaulShuch(Berlin:Springer,2011),70.

7. JesseL.GreensteinandDavidC.Black,“DetectionofOtherPlanetarySystems,”inTheSearchforExtraterrestrial Intelligence: SETI, eds. Philip Morrison, John Billingham, and John Wolfe(Washington,DC:NASAScientificandTechnicalInformationOffice,1977).

8. GreensteinandBlack,“Detection.”9. Tarter,interview.10. David C. Black and William E. Brunk, eds., An Assessment of Ground-Based Techniques for

DetectingOtherPlanetarySystems,Volume1:AnOverview(MoffettField,CA:NationalAeronauticsandSpaceAdministration,1979),18.

11. MichaelD.Lemonick,MirrorEarth:TheSearch forOurPlanet’sTwin (NewYork:Walker,2012),55.

12. Lemonick,MirrorEarth.13. Lemonick,MirrorEarth,52–53.14. Lemonick,MirrorEarth,58.15. Andrew Lawler, “Bill Borucki’s Planet Search,” Air and Space, May 2003,

http://www.airspacemag.com/space/bill-boruckis-planet-search-4545405/?no-ist.16. Lawler,“Borucki’sPlanetSearch.”17. Lawler,“Borucki’sPlanetSearch.”18. WilliamJ.Boruckietal.,“KeplerPlanet-DetectionMission:IntroductionandFirstResults,”Science

327,no.5968(February19,2010):977–80.19. “LiftoffofKepler:OnaSearchforExoplanetsinSomeWayLikeOurOwn,”NationalAeronautics

and Space Administration, March 6, 2009,https://www.nasa.gov/multimedia/imagegallery/image_feature_2123.html.

20. NatalieBatalha,interviewwiththeauthor.21. MicheleJohnson,“NASA’sKeplerMissionAnnouncesaPlanetBonanza,715NewWorlds,”National

AeronauticsandSpaceAdministration,February26,2014,https://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza.

22. “ExoplanetAnniversary:FromZerotoThousandsin20Years,”JetPropulsionLaboratory,October6,2015,https://www.jpl.nasa.gov/news/news.php?feature=4733.

23. Batalha,interview.24. “Star: KOI-961—3 PLANETS,” Extrasolar Planets Encyclopaedia, http://exoplanet.eu/catalog/?

f=‘KOI-961’+in+name.25. LeeBillings,“NewfoundSuper-EarthBoostsSearch forAlienLife,”ScientificAmerican,April 19,

2017,https://www.scientificamerican.com/article/newfound-super-earth-boosts-search-for-alien-life/.26. ShannonHall,“ThisSuper-SaturnAlienPlanetMightBetheNew‘LordoftheRings,’”Space.com,

February3,2015,https://www.space.com/28435-super-saturn-alien-planet-rings.html.27. Andrew Fazekas, “Diamond Planet Found—Part of ‘Whole New Class’?,” National Geographic,

October 13, 2012, http://news.nationalgeographic.com/news/2012/10/121011-diamond-planet-space-solar-system-astronomy-science/.

28. “Hubble Finds a Star Eating a Planet,” Hubble Space Telescope, May 20, 2010,https://www.nasa.gov/mission_pages/hubble/science/planet-eater.html.

29. Amelie Saintonge, “How Many Stars Are Born and Die Each Day?,” Ask An Astronomer, lastmodified June 27, 2015, http://curious.astro.cornell.edu/about-us/83-the-universe/stars-and-star-clusters/star-formation-and-molecular-clouds/400-how-many-stars-are-born-and-die-each-day-beginner.

30. Mike Wall, “Nearly Every Star Hosts at Least One Alien Planet,” Space.com, March 4, 2014,https://www.space.com/24894-exoplanets-habitable-zone-red-dwarfs.html.

31. Robert Sanders, “Astronomers Answer Key Question: How Common Are Habitable Planets?”University of California, Berkeley, November 4, 2013,http://news.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/.

32. Inourpaper,wesetthepessimismlinebetween10–24and10–22fortechnicalreasons.Throughoutthebook,wewillusethemoreconservativevalueof10–22..

33. Ross Andersen, “Fancy Math Can’t Make Aliens Real,” Atlantic, June 17, 2016,https://www.theatlantic.com/science/archive/2016/06/fancy-math-cant-make-aliens-real/487589/, andEthan Siegel, “Humanity May Be Alone in the Universe,” Forbes, June 21, 2016,https://www.forbes.com/sites/startswithabang/2016/06/21/humanity-may-be-alone-in-the-universe/.

34. Adam Frank, “Yes, There Have Been Aliens,” New York Times, June 10, 2016,https://www.nytimes.com/2016/06/12/opinion/sunday/yes-there-have-been-aliens.html.

35. Ernst Mayr, “Can SETI Succeed? Not Likely,” The Planetary Society,http://daisy.astro.umass.edu/~mhanner/Lecture_Notes/Sagan-Mayr.pdf.

36. Brandon Carter, “The Anthropic Principle and its Implications for Biological Evolution,”PhilosophicalTransactionsoftheRoyalSocietyA310,no.1512(December1983):347–63.

37. It’sworthnotingthatauthorslikeastrophysicistMarioLiviohavepresentedargumentsthatunderminethebasisforCarter’swork.MarioLivio,“HowRareAreExtraterrestrialCivilizations,andWhenDidTheyEmerge?”TheAstrophysicalJournal511,no.1(1999):429–31.

38. Hubert P. Yockey, “A Calculation of the Probability of Spontaneous Biogenesis by InformationTheory,”JournalofTheoreticalBiology67,no.3(August7,1977):377–98.

39. WentaoMa et al., “The Emergence of Ribozymes SynthesizingMembrane Components in RNA-BasedProtocells,”Biosystems99,no.3(March2010):201–9.

CHAPTER5:THEFINALFACTOR

1. WilliamBains.“ManyChemistriesCouldBeUsed toBuildLivingSystems,”Astrobiology4,no.2(June2004):137–67.

2. J.R.Haas, “ThePotentialFeasibilityofChlorinicPhotosynthesisonExoplanets,”Astrobiology 10,no.9(November2010):953–63.

3. J.Dulcic,A.Soldo,andI.Jardas,“AdriaticFishBiodiversityandReviewofBibliographyRelatedtoCroatian Small-Scale Coastal Fisheries,”http://www.faoadriamed.org/pdf/publications/td15/wp_dulcica.pdf.

4. Sharon Kingsland, Modeling Nature: Episodes in the History of Population Ecology (Chicago:UniversityofChicagoPress,1985),106.

5. PhilipJ.Davis,“CarissimoPapà:AGreatFishStory,”SIAMNews38,no.8(October2005).6. Kingsland,ModelingNature,4.7. MarkKot,ElementsofMathematicalEcology(Cambridge:CambridgeUniversityPress,2001),11.8. Kingsland,ModelingNature,109.9. Kingsland,ModelingNature,106–15.10. Kingsland,ModelingNature,1.11. Rafael Reuveny, “Taking Stock of Malthus: Modeling the Collapse of Historical Civilizations,”

AnnualReviewofResourceEconomics4(2012):303–29.12. Reuveny,“TakingStockofMalthus,”303.13. ErichvonDäniken,ChariotsoftheGods?(1968;NewYork:Putnam,1970).14. JaredDiamond,Collapse:HowSocietiesChoosetoFailorSucceed(NewYork:Viking,2005).15. JamesA.BranderandM.ScottTaylor,“TheSimpleEconomicsofEasterIsland:ARicardo-Malthus

ModelofRenewableResourceUse,”AmericanEconomicReview88,no.1(March1998):119–38.16. Bill Basener and David S. Ross, “Booming and Crashing Populations and Easter Island,” SIAM

JournalonAppliedMathematics65,no.2(2004):684–701.17. Adam Frank and Woodruff Sullivan, “Sustainability and the astrobiological perspective,”

Anthropocene5(March2014):32.18. Adam Frank, “Could You Power Your Home With A Bike?,” NPR, December 8, 2016,

http://www.npr.org/sections/13.7/2016/12/08/504790589/could-you-power-your-home-with-a-bike.19. RudyM.BaumSr.,“FutureCalculations:TheFirstClimateChangeBeliever,”Distillations,Summer

2016,https://www.chemheritage.org/distillations/magazine/future-calculations.20. L. Miller, F. Gans, and A. Kleidon, “Estimating Maximum Global Land Surface Wind Power

ExtractabilityandAssociateClimaticConsequences,”EarthSystemDynamics2(2011):112.21. It’s worth mentioning that it is possible that the distribution of exo-civilizations might be more

complicated thanprovidingawell-defined average.Theremight, for example, be two peaks in thelifetimes of a large sample of exo-civilizations (one short and long). This kind of resultwould beinterestinginitsownright.

CHAPTER6:THEAWAKENEDWORLDS

1. MarinaAlberti,CitiesThatThinkLikePlanets(Seattle:UniversityofWashingtonPress,2016).2. DrakeandSobel,IsAnyoneOutThere?.3. DrakeandSobel,IsAnyoneOutThere?.Also,“FirstSoviet-AmericanConferenceonCommunication

withExtraterrestrialIntelligence,”Icarus16,no.2(April1972):412.4. Kenneth I. Kellermann, “Nicolay Kardashev,” National Radio Astronomy Observatory,

http://rahist.nrao.edu/kardashev_reber-medal.shtml.5. NikolaiKardashev,“TransmissionofInformationbyExtraterrestrialCivilizations,”SovietAstronomy

8,no.2(September/October1964):217,andMilanM.Cirkovic,“Kardashev’sClassificationat50+:AFineVehiclewithRoomforImprovement,”SerbianAstronomicalJournal191(2015):1–15.

6. “Energy of a Nuclear Explosion,” The Physics Factbook,https://hypertextbook.com/facts/2000/MuhammadKaleem.shtml.

7. FreemanJ.Dyson,“SearchforArtificialStellarSourcesofInfraredRadiation,”Science131,no.3414(June3,1960):1667–68.

8. J. T. Wright et al.,“The Ĝ Infrared Search for Extraterrestrial Civilizations with Large EnergySupplies,II.Framework,Strategy,andFirstResult,”AstrophysicalJournal792,no.1(2014):27.

9. Cirkovic,“Kardashev’sClassification.”10. Carl Sagan, Carl Sagan’s Cosmic Connection: An Extraterrestrial Perspective, ed. Jerome Agel

(Cambridge:CambridgeUniversityPress,2000).11. Michio Kaku, “The Physics of Extraterrestrial Civilizations,” http://mkaku.org/home/articles/the-

physics-of-extraterrestrial-civilizations/.12. IsaacAsimov,Foundation(NewYork:GnomePress,1951).13. SecondLawofThermodynamics,http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html.14. Matt Williams, “What is the Weather Like on Mercury?,” Universe Today, July 24, 2017,

https://www.universetoday.com/85348/weather-on-mercury/.15. Volatility is a concept from physics and chemistry and is the tendency of a substance to vaporize.

Volatilesinplanetarysciencearesubstancesthatwillvaporize(orboil)at“normal”temperaturesandpressures.Iron,forexample,isnotconsideredavolatile,whilewater,CO2,andmethanearevolatiles.

16. L. Kaltenegger and D. Sasselov, “Detecting Planetary Geochemical Cycles on Exoplanets:AtmosphericSignaturesandtheCaseofSO2,”AstrophysicalJournal708,no.2(2010):1162–67,andJ.F.KastingandD.E.Canfield,“TheGlobalOxygenCycle,”inFundamentalsofGeobiology,eds.A.H.Knoll,D.E.Canfield,andK.O.Konhauser(Hoboken,NJ:Wiley-Blackwell,2012),93–104.

17. AdamFrank,AxelKleidon,andMarinaAlberti,“EarthasaHybridPlanet:TheAnthropoceneinanEvolutionaryAstrobiologicalContext,”Anthropocene(forthcoming).

18. Donald Canfield, “The Early History of Atmospheric Oxygen,” Annual Review of Earth andPlanetarySciences33(2005):1–36.

19. EleniStavrinidouetal.,“ElectronicPlants,”ScienceAdvances1,no.10(November2015).20. DavidGrinspoon,TheEarthinHumanHands(NewYork:GrandCentralPublishing,2016).

21. BasedonVernadsky’swritings,theJesuitpriestandpaleontologistPierreTeilharddeChardinworkedon his own, decidedly more mystical version of the noosphere. P. Teilhard de Chardin, ThePhenomenonofMan,trans.BernardWall(NewYork:Harper,1959),238.

22. The“BigHistoryProject”isanattempttoteachhistoryinawaythatputshumanityinitsplacealongwiththerestofthecosmos.Seehttps://www.bighistoryproject.com/home.

INDEX

Italicpagenumbersrefertoillustrations.

Pagenumberslistedcorrespondtotheprinteditionofthisbook.Youcanuseyourdevice’ssearchfunctiontolocateparticulartermsinthetext.

abiogenesis,48Ackerman,ThomasP.,93–94AdriaticSea,predatorandpreystudiesin,174–76,179agency-dominatedbiospheres,219–22aggression,ofexo-civilizations,50agriculturalrevolution,4Alberti,Marina,193,198,217aliens,13–14,171;Seealsoexo-civilizationsAlvinsubmersible,113–15AmesResearchCenter,SETImeetingat,141–42Anasazicivilization,182Anders,William,107–8,108Andersen,Ross,157anoxygenicphototrophs,116,117Anthropoceneera;SeealsoastrobiologicalperspectiveonAnthropoceneera

Earthvs.humancivilizationin,121,222–25energyconsumption,CO2concentration,andpopulationin,199energytransformationlimitsin,213–14,220frequencyofconditionsleadingto,171asGreatFilter,26–27andGreatOxidationEvent,117–18narrativesabout,55wasteproductsin,188

anthropogenicclimatechange(human-drivenclimatechange),26–27,70,100–107Apollo8mission,107–8,108Archeaneon,110–11,116–17,218Aristotle,29,158Arrhenius,Svante,69,190Asimov,Issac,212astrobiologicalperspectiveonAnthropoceneera

andclimatechange,224–25

andNoachianperiodonMars,91questionsof,12–15relationshipofcivilizationandplanetin,209roleofhumansin,15–17sociologicalquestionsaboutexo-civilizationsin,174

astrobiology,10–12,53,90astrometricskymapping,134–36,142AstronomicalJournal,135,136astronomy

historyof,30–31radio,36–37,39–42,65technologyfor,36–37

Atchley,Dana,43Atlas-Agenarocket,71atmosphere

asdetectoroflife,123–24andenergytransformationsonplanets,215–16ofMars,90–91overEarth’shistory,99–100,110andsurfacetemperatureofplanets,68–70ofVenus,65

atmosphericpressure,onMars,89–90atomicweapons,34–35atomism,28–29averagelifetimeoftechnologicalcivilization(L)

inDrakeequation,49–50andexo-civilizationmodeling,186,192,201–2andexoplanetdataonexistenceofexo-civilizations,154GreenBankconferenceestimatesof,53–54importanceofdetermining,169–71

averages,inexo-civilizationmodeling,186

Babylonians,189bacteria,inEarth’shistory,114–16Basener,Bill,183–84Batalha,Natalie,146–49,224Baum,L.Frank,38bedrock,onMars,78Berkner,Lloyd,40BigBang,9BigHistoryProject,250n.22binarystars,134,135,138,144biogeochemistry,120biosphere(s)

agency-dominated,219–22inEarthsystemscience,127–30andenergyconsumption,213,214andenergytransformationsonplanets,217–22evolutionof,221inGaiatheory,123–27influenceonEarth’shistoryof,118–22modelingexoplanet,185thick,218,219

thin,217–19bio-technicalprobability(fbt)

andconstraintsonexo-civilizationmodeling,171ofexo-civilizationsatanypointintime,153–55pessimisticestimationsof,157–64

birthrateofstars(N*),46,150BlackCloud,The(Hoyle),46BlackDeath,196“blueberries,”onMars,90Borman,Frank,107Borucki,Bill,143,145–46,224Brander,JamesA.,182–84brightness,137,139;SeealsotransitmethodBrin,David,25Bruno,Giordano,30,135,234n.16Butler,Paul,144ByurakanObservatorymeeting,206–8,207

Calvin,Melvin,43,44,54CambrianExplosion,112CampCentury,100–107,102,224Canfield,Donald,113–15,218,224carboncycle,73,74carbondioxide

inEarth’satmosphere,99andenergyconsumption/populationinAnthropoceneera,199andgreenhouseeffect,69–70inMartianatmosphere,86,124inVenusianatmosphere,65

Carboniferousera,112Carroll-Nellenback,Jonathan,194,198carryingcapacity,178,182,183Carson,Rachel,55–56Carter,Brandon,162–63,247n.37CatholicChurch,29,30CentralParknuclearfreezedemonstration(1982),92–93ChariotsoftheGods?(vonDäniken),181chemicalequilibrium,123,124Cirkovic,MilanM.,210cities,sustainable,205–6civilizations

averagelifetimeof,Seeaveragelifetimeoftechnologicalcivilization(L)coevolutionofplanetsand,14–15human,SeehumancivilizationKardashevscaleofprogressfor,208–14onotherplanets,Seeexo-civilizationsstudyingenvironment’sinteractionswith,193–98sustainable,Seesustainablecivilizations

Class1planets,217Class2planets,217,219Class3planets,217–19Class4planets,218–19

Class5planets,218–22climate,11,84

onEarthvs.Mars,86–89onMars,79,83–84,86–89asmilitaryconcern,103–4negativefeedbackcycleon,74innuclearwintermodeling,94positivefeedbackloopon,73–74onVenus,63–67

climatechange,12andAnthropoceneeraasGreatFilter,26–27anthropogenic(human-driven),26–27,70,100–107andastrobiologicalperspectiveonAnthropoceneera,224–25inEarth’shistory,223exo-civilizationsinstudyof,54–58,164–66andhabitabilityofMars,89–91icecoredataon,106–7andinfluenceofhumancivilizationonEarth,6,7Marsmodel,84–86sciencevs.storytellingon,8–10wasteproductsofcivilizationbuildingascauseof,188

climatescience,94–95,102–4Cocconi,Giuseppe,41–44coevolution,14–15,130ColdRegionsResearchandEngineeringLaboratory,105coldtrap,74,238n.32ColdWar,21,35,39,56,58,93,101,103,122Collapse(Diamond),182collapse,inexo-civilizationmodeling,196,197collisiontheory,33,38combustion,189,190–91communications,withexo-civilizations,43–50compactmultisolarsystems,148conservation,55–56continentmaking,110–11,113convectivecirculation,215ConversationsonthePluralityofWorlds(deFontenelle),30–31,31Copernicus,Nicolaus,29–30Coriolisforce,88CornellUniversity,38Coruscant,212craters,onMars,80–81cratons,110Crick,Francis,207Curiosityrover,78,89,90cyanobacteria,116

Daisyworldmodel,129D’Ancona,Umberto,175–77,176,179Dansgaard,Willi,105,224Darwin,Charles,9,31,32,118,172Darwinianevolution,172–73“dead”worlds,216,217

deFontenelle,Bernard,30–31,31,34delayedcollapse,196,197–98desertgreening,219–20Diamond,Jared,182“die-off”trajectory,195–96,196,233n.11Drake,Frank,40,57,104,166,223

atByurakanObservatorymeeting,206andDrakeequation,47,51–52andexoplanetdiscovery,140,150andGreenBankconference,43–45,51–52MilkyWayasfocusof,236n.50andProjectOzma,37–42radioastronomytechniquesof,65

Drakeequation;Seealsoaveragelifetimeoftechnologicalcivilization(L)componentsof,45–50constraintsonexo-civilizationmodelingfrom,171–74effectsofexoplanetdiscoveryon,149–51,153–55focusofpessimismlinevs.,165atGreenBankconference,50–54optimisticestimationsforcomponentsof,158–59pessimisticestimationsforcomponentsof,159–64

Dunn,Alan,22duststorms,Martian,81–82,94Dutch,onEasterIsland,180–81,184Dyson,Freeman,209–11Dysonspheres,210,211

EarthinAnthropoceneera,121atmosphericcarbondioxideconcentration,65brightnessofSunvs.,137climatechangeon,incontextofexo-civilizations,54–58coevolutionoflifeand,10–11effectsofAnthropoceneeraforhumancivilizationvs.,222–25energyfromSunon,209energytransformationson,218,220environmentalimpactofcombustionon,190–91equilibriumtemperatureof,68–69formationof,109greenhouseeffecton,69–70influenceofhumancivilizationon,4–8onKardashevscale,211mechanicsofclimateonMarsvs.,86–89proximitytoVenus,148system-basedunderstandingof,56tradewindson,87–88usesofexo-civilizationmodelingon,198–202waterlosson,74

Earthrisephotograph,107–8,108,121Earth’shistory,6–7,99–130

anthropogenicclimatechangein,100–107atmosphericchangesin,99–100andEarthriseimage,107–8

Earthsystemscienceperspectiveon,127–30eonsof,109–13Gaiatheoryof,122–27GreatOxidationEventin,113–18influenceofbiosphereon,118–22

Earth-sizedexoplanets,144–45Earthsystemscience,127–30,222EasterIsland,180–84,181ecology,176EgyptianChurchoftheEternalSource,127Einstein,Albert,52,118,137electromagneticspectrum,36endosymbiosis,125energyconsumption

andCO2concentration/populationinAnthropoceneera,199environmentalimpactof,190–91impactoftechnologyon,187inKardashevscale,209–13

energysourceseffectsofswitching,196,197–98inexo-civilizationmodeling,186–90planetarynumbersof,188–90

energytransformationsconstraintson,213–14planetaryclassificationbasedon,214–22inthermodynamics,214–15

environmentcivilizations’interactionswith,181–84,193–98onEasterIsland,181–84impactofenergysourceuseon,190–91

environmentalcollapse,182–84environmentalmovement,128eons,109–13Epicurus,28,158EpsilonEridani,41equilibrium,chemical,123,124equilibriumtemperature,68,126evolutionarybiology,171–73evolutiontheory,31,32,172–73exo-civilizationmodeling,169–202

averagelifetimeofcivilizationsfrom,169–71,201–2averagesin,186constraintson,171–74energysourcesin,186–90environmentalimpactofenergyconsumptionin,190–91historiesofexo-civilizationsin,185–86andhistoryofEasterIsland,180–84forstudyingcivilizationandenvironmentalinteractions,193–98theoreticalarchaeologyofexo-civilizations,184–202theoreticalbiologyasbasisfor,174–80usesof,onEarth,198–202

exo-civilizations,21–58

inastrobiologyofAnthropocene,13–14Byurakanmeetingon,206–8climatechangeonEarthincontextof,54–58distributionof,248–49n.21Drake’sequationfor,50–54FrankDrake’ssearchfor,37–42Fermi’sParadoxon,21–28formationof,171,173–74GreenBankconferenceon,43–50historicalviewson,28–34historiesof,185–86probabilityofexistenceof,151–64instudyofclimatechange,164–66andtechnologicaladvancesduringatomicage,34–37theoreticalarchaeologyof,184–202

exoplanetdiscovery,133–66effectsof,onDrake’sequation,150–51andexo-civilizationsinstudyofclimatechange,164–6651Pegasib,143–44inKeplermission,144–47knowledgeaboutplanetarysystemarchitecturefrom,147–48precision-relatedproblemswith,137–40andprobabilityofexo-civilizationexistence,151–64byThomasSee,133–37ofsuper-planets,148–50withtransitmethod,140–43

exoplanetsbeliefsaboutexistenceof,33–34climatemechanicsof,89modelingbiosphereson,185

“ExplanationfortheAbsenceofExtraterrestrialsonEarth,An”(Hart),24extinctionevents,115extraterrestrialintelligentspecies,23;Seealsoexo-civilizations

fbt,Seebio-technicalprobabilityfc(fractionofplanetswithtechnologicalcivilizations),49feedback

onclimate,73–74inDaisyworldmodel,129inGaiatheory,125–29

FellowshipofIsis,127Fermi,Enrico,21,23,25–28,57,166Fermi’sParadox,21–28,170fi(fractionofplanetswhereintelligenceevolves),4951Pegasib,143–44,14755Cancrie,149fl(fractionofplanetswherelifeforms),48,163–64Flammarion,Camille,32,34,158Flammarion,Claude,64,64–65,79fossilfuels,5,54,186,187fp(fractionofstarswithplanets),47,150fractionofplanetswhereintelligenceevolves(fi),49

fractionofplanetswherelifeforms(fl),48,163–64fractionofplanetswithtechnologicalcivilizations(fc),49fractionofstarswithplanets(fp),47,150Frank,Adam,156fusion,nuclear,34–35

Gaiatheory,122–29,220–22galaxy(-ies)

exo-civilizationscolonizationof,24numberofdetectabletechnologicallyadvancedcivilizationsin,SeeDrakeequationsizeof,45–46

geochemistry,120geothermalenergy,189glaciation,112GOE(GreatOxidationEvent),113–18,218Goering,Kent,103GoldilocksZone,47–48,150,151,155Golding,William,124–25GreatFilter,25–28GreatOxidationEvent(GOE),113–18,218GreatSilence,25Greeks,ancient,28–29,135GreenBankconference,43–54,207greenhouseeffect,190

runaway,72–75onVenus,67–72,184,198

Greenland,9,56,101,113,182;SeealsoCampCenturyGregersen,Soren,100–101,103,104Grinspoon,David,220GuaymasBasin,114–15GulfStream,27

Haberle,Robert,84,86,88,224habitability

andclimatechangeforMars,89–91zonesof,SeeGoldilocksZone

Hadeaneon,109–10Hadley,George,87Hadleycells,88hardsteps,ofevolution,162–63Hart,Michael,24,28Hawking,Stephen,118heatengines,214–15Hiroshima,bombingof,34Holoceneepoch,12,16,91,105–6,109Homosapiens,ageof,24hotEarths,148hotJupiters,147,149hotNeptunes,148“hot”worlds,13,147–49Hoyle,Frank,46Huang,Su-Shu,43,47–48

Hubel,David,207humancivilization

inAnthropoceneera,121inastrobiologicalperspectiveonAnthropoceneera,15–17as“cosmicteenagers,”3–8,224onEasterIsland,181–84effectsofAnthropoceneeraforEarthvs,222–25influenceof,onEarth,4–8onKardashevscale,211instorytellingaboutclimatechange,9–10

hydroenergysources,189hydrogenbomb,22,34–35

ICBMs(intercontinentalballisticmissiles),35,56iceages,104–5icecoredrilling,atCampCentury,104–7industrialrevolution,5–6,55infraredradiation,69intelligence(intelligentlife);Seealsosearchforextra-terrestrialintelligence(SETI)

evolutionof,160–61,171–73fractionofplanetswith,49

intercontinentalballisticmissiles(ICBMs),35,56

J1407B(exoplanet),149James,Jack,62,75,104,174,223

andJamesLovelock,122inMariner1mission,61–63inMariner2mission,71,72

Jeans,James,33JetPropulsionLaboratory(JPL),71,72,77,122–24jetstreams,Martian,88Johnson,Lyndon,54–58JPL,SeeJetPropulsionLaboratoryJupiter,95Jurassicera,112

Kardashev,NikolaiSemenovich,208–9,211–12,223Kardashevscale,208–14Keeling,Charles,54Kepler42system,148Keplermission,144–47,150–51,154,155Kleidon,Axel,193–94,198,217Konopinski,EmilJan,22,23

L,SeeaveragelifetimeoftechnologicalcivilizationLangway,Chester,105LateHeavyBombardmentperiod,109–10Leovy,Conway,84life

attemptstodetect,onMars,122–23basicsof,forexo-civilizationmodeling,171,172beliefsabout,onMars,78–84coevolutionofplanetand,130

inEarth’shistory,110–12effectofGreatOxidationEventon,117evolutionofEarthand,10–11fractionofplanetswherelifeforms,48,163–64onotherplanets,7–8;Seealsoexo-civilizations

Lifemagazine,108lifetimeoftechnologicalcivilization,Seeaveragelifetimeoftechnologicalcivilization(L)light,visible,36,41,69Lilly,JohnC.,44liquids,energytransformationswith,216lithosphere,121Livio,Mario,247n.37LosAlamosNationalLaboratory,21LoveCanaldisaster,128Lovelock,James,122–29,126,174,213,220,224Lowell,James,107Lowell,Percival,32,34,79–80,134–35

Man’sPlaceintheUniverse(Wallace),33mantle,110Marcy,Geoff,143–44Margulis,Lynn,126,174,213,224

Gaiatheoryof,125–29,220andCarlSagan,75–76ontoughnessofEarth,9

Mariner1probe,61–63Mariner2probe,63,71–76Mariner4probe,79–81Mariner9probe,81–83,82,94Mars,76–95

inastrobiologyofAnthropocene,13atmosphereon,216attemptstodetectlifeon,122–23beliefsaboutlifeon,32,34,78–84climatechangemodelfor,84–86climateof,11,86–89energytransformationson,217,218formationof,109habitabilityandclimatechangesfor,89–91nuclearwintermodelingwith,92–95,184OpportunityandSpiritroverson,76–78spaceprobesto,63

MarsExplorationRoverprogram,76–79MarsGlobalClimateModel,84–86,89–90,94mathematicalmodeling,intheoreticalbiology,177–80MaxPlanckInstitute,191Mayacivilization,182Mayor,Michel,143,144,224Mayr,Ernst,159–62Mercury,109,215,217MeridianiPlanum,76MilkyWay,24,236n.50Miller,Harold,48

Minsky,Marvin,207Moon,probesto,61,63–64Morrison,Philip,41–43Moulton,ForestRay,135,136MuseumofScienceandIndustry(Chicago,Illinois),37myths,8–10

N*(birthrateofstars),46,150Nagasaki,bombingof,34NationalAcademyofSciences,43NationalAeronauticsandSpaceAdministration(NASA),57,61,62,71,81,141,142,145;SeealsoJet

PropulsionLaboratory(JPL)NationalRadioAstronomicalObservatory,39–42,40NavalObservatory(MareIsland,California),136–37NavalResearchLaboratory(NRL),65–66,70,72negativefeedbackcycle,74Neptune,30,148netproductivity,ofplanet,218NewAgeism,127–28Newton,Isaac,30,177NewYorker,The22,22NewYorkTimes,81,92NirgalVallis,82Noachianperiod,91,218noosphere,221Norsecolony,onGreenland,182np,SeenumberofplanetsinGoldilocksZoneNRL,SeeNavalResearchLaboratorynuclearenergy,190nuclearfreezemovement,92–93nuclearfusion,34–35nuclearwar,26,27,92–93nuclearweapons,56nuclearwinter,92–95,184“NuclearWinter”(TTAPSstudy),93–94numberofplanetsinGoldilocksZone(np),48,150,151

OfthePluralityofWorlds(Whewell),32–33Oliver,Barney,43OlympusMons,83OntheRevolutionofHeavenlySpheres(Copernicus),29Opportunityrover,76–78,90orbitalmotion,exoplanetdetectionbasedon,138,142organelles,125oxygen;SeealsoGreatOxidationEvent

inEarth’satmosphere,99–100andGaiatheory,126–27

Oxygen(Canfield),114ozonelayer,117

Paleocene-EoceneThermalMaximum,113Pangaea,113

Parademagazine,93Pearman,J.Peter,43,44pessimismline

andaveragelifetimeofcivilizations,170andbio-technicalprobability,186defined,155–57andhistoryofexo-civilizations,185–86limitationsof,157–59inunderstandingofclimatechange,164–66

Phanerozoiceon,112–13photosynthesis,111,115–16phototrophs,anoxygenic,116,117planetaryscience,66PlanetarySociety,160planetarysystems,architectureof,147–48planetesimals,109planet(s);Seealsoexoplanetdiscovery

atmosphereandsurfacetemperatureof,68–70coevolutionof,14–15,130energytransformation-basedclassificationof,214–22asenginesofinnovation,221–22environmentsforsustainablecivilizationson,205–6fractionof,wherelifeforms,48,163–64fractionof,withintelligentlife,49fractionof,withtechnologicalcivilization,49fractionofstarswith,47,150implicationsofenergyconsumptionfor,212–13lawsof,75lifeonother,7–8;Seealsoexo-civilizationsnumberof,inGoldilocksZone,47–48,150,151numberofenergysourceson,188–90similaritiesofEarthtoother,7universalityofforces/processeson,70–71

Pleistoceneepoch,106,109“Pluralityofworlds”question,28–34Pollack,James,84,93–94,184populations,lawof,177–78positivefeedbackloop,73–74Postgate,John,128power,211precision,exoplanetdiscoveryand,137–40predator-preymodel,174–80,183,185ProjectOzma,40–43prokaryotes,111Proterozoiceon,111–12

Queloz,Didier,143,144

R-7missiles,35radioastronomy,36–37,39–42,65rangesafetyofficer,63Reagan,Ronald,92,93reflexmotion,exoplanetdetectionbasedon,139,142–44

resources,overharvestingof,183–84Revelle,Roger,54ribonucleicacid(RNA),164Riftiatubeworms,114RNA(ribonucleicacid),164Robinson,KimStanley,10rockets,35–36,71Rodinia,111Romans,ancient,49Ross,DavidS.,183–84Rossbywaves,88runawaygreenhouseeffect,72–75Ruse,Michael,128Russia

atomicweaponsof,34–35Martianprobes,81inspacerace,35–36,61Venusianprobes,72

Sagan,Carl,104,118,166,174,224atByurakanmeeting,206–8,207andGaiatheory,124,125atGreenBankconference,44onhumancivilizationas“cosmicteenagers,”6KardashevvalueforEarthfrom,211andLynnMargulis,75–76inMariner4mission,79,80as“NuclearWinter”author,93–94andJimPollack,84andSETI,140,160Venusiangreenhouseeffectstudiesof,66–68,70–72,184inVikingmission,83

SanJoseStateUniversity,84satellites,35–37,57Saturn,95Schiaparelli,Giovanni,79Schneiderman,Dan,62science

ofclimatechange,9contradictorydatain,136developmentof,inhumancivilization,5exo-civilizationmodelingusingexisting,171–74importanceofwell-posedquestionsin,27–28insearchforexo-civilizations,42,52–53transformativeinfluenceofnewdatain,104

searchforextra-terrestrialintelligence(SETI),53,141–43,152,160,208;SeealsoByurakanObservatorymeeting

“SearchingforInterstellarSignals”(Cocconi&Morrison),41–42SecondLawofThermodynamics,187–88See,Thomas,133–38,134,142“Self-regulatingEarthSystemTheory,”124SETI,Seesearchforextra-terrestrialintelligenceSETIInstitute,141

70Ophiuchisystem,135Siegel,Ethan,157SilentSpring(Carson),55–56Snow,C.P.,21sociology,50,171,173–74“softlanding”trajectory,196,197sol(Martianday),83solarcells,220solarenergy,190solarsystem(s)

architecturesofother,144,147–48explorationof,95

SovietUnion,92,93,208Spiritrover,77,78“spots,”onstars,140spreadingzones,114Sputniksatellite,35–36Squyers,Steven,76–78,90,223–24stars

birthrateof,46,50fractionof,withplanets,47,150planetsorbitingother,135,136

StarWars(film),212steadystate,126,127storytelling

aboutclimatechange,8–10,55abouthumancivilizationincontextofplanet,57–58

Struve,Otto,38,42,43,47,54Suess,Eduard,120Sullivan,Woody,152–57,159,164–65Sun,109,137,209super-Earths,13,148–49super-planets,148–50super-Saturn,149surfacetemperature

atmosphereand,68–69icecorestudyofEarth’s,105–7,106ofMars,86,89duringPhanerozoiceon,113ofVenus,65–68

sustainability,56,91,205–6sustainablecivilizations,205–25

andanthropogeniceffectsforEarthvs.civilization,222–25inastrobiologicalperspectiveonAnthropocene,12,16andaveragelifetimeofcivilizations,169–70Byurakanmeetingoninterplanetarycivilizations,206–8inexo-civilizationmodeling,196,196,198,202andKardashevscaleofcivilizationprogress,208–14andplanetaryclassificationsbasedonenergytransformations,214–22planetaryenvironmentsfor,205–6

Tarter,Jill,141–43,142,223TauCeti,41

Taylor,M.Scott,182–84technologicalcivilization

averagelifetimeof,Seeaveragelifetimeoftechnologicalcivilization(L)fractionofplanetswith,49

technologyatomicageadvancesin,34–37ofClass5planets,221andenergyconsumption,187inexo-civilizationmodeling,188–89forexoplanetdiscovery,141andKardashevscale,211,212

TeilharddeChardin,Pierre,250n.21teleology,128–29TelevisionInfraredObservationSatellite(TIROS),57Teller,Edward,21–22,23temperature

equilibrium,68,126atsurfaceofplanet,Seesurfacetemperature

TerresCiel,Les(Flammarion),64theoreticalarchaeologyofexo-civilization,184–202

averagesin,186energysourcesin,186–90environmentalimpactofenergyconsumptionin,190–91historiesofexo-civilizationsin,185–86studyingcivilizationandenvironmentalinteractionswith,193–98usesof,onEarth,198–202

theoreticalbiology,174–80theory,defined,179thermodynamics,187–88,214–15thickbiospheres,218,219thinbiospheres,217–19ThomasAquinas,29ThreeMileIslandmeltdown,128ThuleAirBase,101,103tides,asenergysource,189TIROS(TelevisionInfraredObservationSatellite),57Titan,11,232n.11Toon,Owen,93–94Tovmassian,Hrant,207Townes,Charles,207tradewinds,87–88transitmethod,139–43,145–47Trantor,212Trinityatomicbombtest,23Turco,RichardP.,93–94Type1civilizations,209,211Type2civilizations,209–11Type3civilizations,210

UFOs,23ultravioletradiation,73,117UnitedStates

atomicweaponsof,34–35

inspacerace,35–36,61universe,beliefsaboutotherworldsin,28–34UniversityofChicago,67UniversityofMaryland,142,143UniversityofWashington,152UnparalleledDiscoveriesofT.J.J.See,The(Webb),137Uranus,30

V-2missiles,35VallesMarineris,83Venus,61–76

beliefsaboutclimateof,63–67climateof,11,86,89energytransformationson,217formationof,109greenhouseeffecton,67–71,184,198Mariner1missionto,61–63Mariner2missionto,71–76proximityofEarthto,148speculationaboutlifeon,32

Verhulst,Pierre,177Vernadsky,VladimirIvanovich,118–22,119,129,221,224Vikinglanders,83visiblelight,36,41,69volatility,250n.15volcaniceruptions,72–73Volterra,Luisa,176,176Volterra,Vito,176,176–79,182,183,185vonDäniken,Erich,181

Wallace,AlfredRussel,33,34WaroftheWorlds(Wells),80WASP-12b(exoplanet),149wasteproducts,ofcivilizationbuilding,187–88water

onMars,82–84,87,90–91andrunawaygreenhouseeffect,73–74

Watson,James,129weather,83,84,89weathering,73,74,216weathersatellites,57,57Wells,H.G.,80WentaoMa,164Whewell,William,32–33,158windpower,189,191WonderfulWorldofOz,The(Baum),38WorldWarI,122,175,179WorldWarII,35,36,104,122Wright,Jason,210,233n.6

YerkesObservatory,67,70“Yes,ThereHaveBeenAliens”(Frank),156,157Yockey,Hubert,163–64

York,Herbert,22,23YoungerDryasperiod,106youngtechnologicalcivilizations,188–90

Zell-Ravenheart,Oberon(TimothyZell),127

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Names:Frank,Adam,1962–author.Title:Lightofthestars:alienworldsandthefateoftheEarth/AdamFrank.Othertitles:AlienworldsandthefateoftheEarthDescription:NewYork:W.W.Norton&Company,Inc.,[2018]|Includesbibliographicalreferencesandindex.

Identifiers:LCCN2017061640|ISBN9780393609011(hardcover)Subjects:LCSH:Cosmology—Popularworks.|Earth(Planet)—Popularworks.|Humanecology—Popularworks.|Exobiology—Popularworks.

Classification:LCCQB982.F732018|DDC523.1—dc23LCrecordavailableathttps://lccn.loc.gov/2017061640

ISBN978-0-393-60902-8(e-book)

W.W.Norton&Company,Inc.,500FifthAvenue,NewYork,N.Y.10110www.wwnorton.com

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