Extra-Solar Life:Habitable Zones

14 November 2012AST 2037

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Life in the Solar System?• Mercury/Venus too hot• Earth – just right• Mars – was OK once (?), but now cold• Jupiter, Saturn, Uranus, Neptune too cold (though some

moons may be OK)

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Life Ingredients• The ingredients of (Earth-like) life are all common:• H2O• C• Fe• etc.

• So are many possible alternatives to them (i.e. Si, NH3, etc.)• Why are these so common?

• If they are so common, why isn’t there life on ALL the planets?

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Optimal Temperature Range• It’s not unique, but H2O is our favorite solvent• It’s not unique, but we prefer it as a liquid• This means:• T > 0 C• T <100 C

• We need to find temperatures in this range (at least part of the time)

• One atmosphere of pressure• How common is that?

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What Sets Planet Temperatures?• Lots of factors (such as?) • Solar Radiation (Solar flux)• Albedo• Rotational Rate• Distance• Heat sinks on Earth• Chemical changes• Photosynthesis

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Temperature Balance• Assume that the dominant factor is stellar radiation• Total Energy (Absorbed) = Total Energy (radiated)

• Show balance; solve for T• Show range in distance

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Habitable Zone• The zone in a stellar system where water is in liquid

form (at least some of the time)

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Habitable Zones: Other Stars?• Hertzsprung-Russell

Diagram (again!)• Scaling for other stars

(same balance, now try distance versus luminosity)

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How Many Other Stars?• Stars within 4 pc:• 5 F/G stars• 5 K stars• 25 M stars

• The number of K stars is about equal to the TOTAL number of O, B, A, F, and G stars TOGETHER

• The number of M stars is greater than the TOTAL of all the other stars combined

• In other words:• Most stars are M stars• Even excluding M stars, about ½ of all the rest of

stars are K stars10

Habitable Zones: K stars

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Habitable Zones: M stars• Range in distance from star is very small there will

probably be very few planets in this range

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M Star Problems• Habitable zone is VERY close to the star• Gravitational/tidal forces are much stronger here, so

we expect tidal locking may develop this close• Why is that not good for life?

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M Star Problems• M stars have very active

magnetospheric storms and flares• Why is that not good for

life?• Why is being close a

particular problem?

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Habitable Zones: O/B stars• Range in distance from the star is huge. We expect

many planets in this range• But … lifetime issue• How long does an O stars live?• How about a B star?• An A star?

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So … What Stars?• What stars are the likeliest targets in searching for

extraterrestrial life?• Late F stars (F5)• G stars (yeah us!)• K stars

• Where around these stars do we look?• The Habitable Zone

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Is this the whole story?• What about Venus?

• Venus is MUCH hotter (by about 60K) than we expect• Due to its thick atmosphere and greenhouse gases• So … thick atmosphere can “ruin” a planet in the close

end of the HZ• Alternately, it can keep a planet just outside the

distant end of the HZ “warm”

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Is this the whole story?• What about Europa/Enceladus?

• These moons seem to have liquid water (potentially in VERY large quantities)

• Why?

• Both are WAY outside the nominal HZ• So … there are other effects (other heating sources)

which can create habitable niches as well

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What about Multiple Stars?• Most stars are in binary or

multiple systems• Can these provide HZs?

• Well … maybe• If planet is close in, orbit

will be unstable and chaotic• Moving in/out of HZ is

probably hard on life development

• If planet is far away (orbiting BOTH stars), orbit is stable, but typically outside the HZ (!)

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Galactic Habitable Zones• Stars in the inner Galaxy are very densely packed• Lots of problems with supernovae, etc.• Stars in the outer galaxy are less “chemically evolved”

– not that much C and O

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Summary• Ingredients for life are everywhere• If (a big “if”) we assume that liquid water is important

for life, then there is a limited volume of any stellar system where that might exist – the Habitable Zone

• If we assume temperature is dominated by sun/star-light, then the HZ can be calculated for any given star

• Likely star types for life are F, G, and K stars (bigger stars die fast; M stars have tiny HZs and other issues)

• Multiple stars are not likely to have good HZs• The Galaxy has its own “good neighborhood” factors• This is a VERY simplistic approximation, with lots of

exceptions (atmospheres; tidal heating of moons; etc.)

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