214: planets & life phys 214

Planets & Life

Planets & Life

PHYS 214

PHYS 214

Dr Rob Thacker

Dr Rob Thacker

Dept of Physics (308A)

Dept of Physics (308A)

thacker@

astro.queensu.ca

thacker@

astro.queensu.ca

Please start all class related emails with

Please start all class related emails with ““214:

214:””

Today

Today’’ s Lecture

s Lecture

��Rare Earth H

ypothesis (REH)

Rare Earth H

ypothesis (REH)

��Peter W

ard & D

on Brownlee (Univ. of Washington)

Peter W

ard & D

on Brownlee (Univ. of Washington)

Could the Earth be

Could the Earth be ““special

special ””??

��““Who are we? W

e find that we live on an

Who are we? W

e find that we live on an

insignificant planet of a humdrum star lost in

insignificant planet of a humdrum star lost in

a galaxy tucked away in some forgotten corner

a galaxy tucked away in some forgotten corner

of a universe in which there are far more galaxies than

of a universe in which there are far more galaxies than

people

people…….. ””

Carl

Carl Sagan

Sagan

One of the un

derlying tenets of scientific investigation is that

we do not occup

y a special

place in the U

niverse. Y

et, the Earth definitely is “special”

within the solar system

as life

only app

ears to ha

ve occurred here. Could it be tha

t life is a coincidence of so m

any factors

that, in reality, the E

arth is actually special within the galaxy?

Don’t

be drawn in by an

thropocentric qu

otes:

"If we are alone in the U

niverse, it sure seems lik

e an

awful waste of space."

Underlying basis of the REH

Underlying basis of the REH

��The development of intelligent life has required

The development of intelligent life has required

many unlikely coincidences that are highly

many unlikely coincidences that are highly

improbable of occurring elsewhere

improbable of occurring elsewhere

��Note, this doesn

Note, this doesn’’ t m

ean the Earth is

t mean the Earth is uniqu

eun

ique

––just

just rare

rare

Could we actually be alone?

Could we actually be alone?

Components of the REH

Components of the REH

(1)

(1)

Galactic Habitable zone

Galactic Habitable zone

(2)

(2)

Appropriate stellar type

Appropriate stellar type

(3)

(3)

Suitable planetary system

Suitable planetary system

(4)

(4)

Suitable size of planet


(5)

(5)

Presence of a large moon


(6)

(6)

Requirem

ent of a magnetic field

Requirem

ent of a magnetic field

(7)

(7)

Plate tectonics

Plate tectonics

(8)

(8)

Appropriate atmospheric chem

istry

Appropriate atmospheric chem

istry

(9)

(9)

Evolutionary selection processes (e.g. glaciations,

Evolutionary selection processes (e.g. glaciations,

impact events)

impact events)

Galactic H

abitable Zone

Galactic H

abitable Zone

��In addition to the issues we discussed, it is also possible

In addition to the issues we discussed, it is also possible

that passage through a spiral arm

is negative

that passage through a spiral arm

is negative

precondition on life

precondition on life

��Spiral arm

s in galaxies actually represent a pattern that rotate

Spiral arm

s in galaxies actually represent a pattern that rotates s

at different speeds to the actual m

aterial

at different speeds to the actual m

aterial

��Very active star form

ation

Very active star form

ation ––

lots of material &

new

stars that could

lots of material &

new

stars that could

perturb the orbits of the planets

perturb the orbits of the planets

��It is not clear how significant an issue this is, the Sun

It is not clear how significant an issue this is, the Sun m

aymay

well

well

have passed through spiral arm

s several times

have passed through spiral arm

s several times

��Nonetheless, on the basis of the GHZ argument it is

Nonetheless, on the basis of the GHZ argument it is

expected that 5

expected that 5-- 10%

of the stars in the MW fall in the

10% of the stars in the MW fall in the

GHZ

GHZ

Appropriate stellar type for life

Appropriate stellar type for life

��Aside from the age issue we discussed,

Aside from the age issue we discussed, Wien

Wien’’ ssLaw

tells us the

Law

tells us the

peak em

ission for massive stars (such as O stars) is in the

peak em

ission for massive stars (such as O stars) is in the

ultraviolet

ultraviolet

��This would have a strong ionizing effect on any atmosphere in th

This would have a strong ionizing effect on any atmosphere in the e

habitable zone (lots of

habitable zone (lots of photodissociation

photodissociation))

��The most m

assive stars are rare, so in terms of life around thes

The most m

assive stars are rare, so in terms of life around these e

system

s we don

system

s we don’’ t care too m

uch

t care too m

uch

��Similarly we discussed briefly at the low m

ass end (K and M

Similarly we discussed briefly at the low m

ass end (K and M

stars) the planets are probably tidally locked, but also need to

stars) the planets are probably tidally locked, but also need to

extrem

ely close to the star

extrem

ely close to the star

��If the planet is very close it is m

uch m

ore strongly impact by s

If the planet is very close it is m

uch m

ore strongly impact by s tellar

tellar

activity (flares, w

inds etc)

activity (flares, w

inds etc) ––

could atm

osphere survive?

could atm

osphere survive?

��We aren

We aren’’ t sure about the effect on the low m

ass end

t sure about the effect on the low m

ass end ––

very

very

important, over 90% of stars are in the K & M

classes

important, over 90% of stars are in the K & M

classes

��Perhaps only 5% of stars are appropriate for life?

Perhaps only 5% of stars are appropriate for life?

Suitable Planetary System

Suitable Planetary System

��We believe that terrestrial type planets are necessary to form

lWe believe that terrestrial type planets are necessary to form

life,

ife,

however, in the REH gas giants are hypothesized to also play an

however, in the REH gas giants are hypothesized to also play an

important role in the form

ation of life

important role in the form

ation of life

��Protect inner regions of the solar system

by scattering or colli

Protect inner regions of the solar system

by scattering or colliding a large

ding a large

fraction of incoming material from the outer solar system

fraction of incoming material from the outer solar system

��Systems with too m

any gas giants tend to become unstable and

Systems with too m

any gas giants tend to become unstable and

may end up strongly disturbing the dynam

ics (could spiral into

may end up strongly disturbing the dynam

ics (could spiral into

the Sun for exam

ple)

the Sun for exam

ple)

��Similar argument for a single gas giant that is too large

Similar argument for a single gas giant that is too large

��Rem

ember we

Rem

ember we ’’ve found quite a few systems with gas giants on

ve found quite a few systems with gas giants on

highly elliptical orbits inside 2 AU

highly elliptical orbits inside 2 AU ––

just how common is this?

just how common is this?

��Is this too m

uch of a stretch?

Is this too m

uch of a stretch?



��On the basis of the escape velocity argument

On the basis of the escape velocity argument

(given in lecture 22) planets cannot be too small

(given in lecture 22) planets cannot be too small

��With the loss of the atmosphere water m

ight freeze,

With the loss of the atmosphere water m

ight freeze,

evaporate away or

evaporate away or photodissociate

photodissociatedue to the

due to the

increased UV field

increased UV field

��A planet that is too large will tend to have many

A planet that is too large will tend to have many

more im

pact events

more im

pact events

��Will also m

ake it harder for mountain systems to

Will also m

ake it harder for mountain systems to

form

and then will likely be a

form

and then will likely be a ““water world

water world””

��In this case the carbonate

In this case the carbonate --silicate cycle cannot act

silicate cycle cannot act



��The ratio of the mass of the Moon to Earth is 1/80

The ratio of the mass of the Moon to Earth is 1/80

thth––

this is a surprisingly large number

this is a surprisingly large number

��Only Pluto and

Only Pluto and Charon

Charoncome close in the rest of the solar

come close in the rest of the solar

system

system

��The

The pairwise

pairwisenature of the Earth

nature of the Earth-- M

oon systems means

Moon systems means

that incoming

that incoming asteriods

asteriodsare much less likely to hit Earth

are much less likely to hit Earth

than if it were one system

than if it were one system

��Also the Moon stabilizes the tilt of the Earth, if the tilt

Also the Moon stabilizes the tilt of the Earth, if the tilt

were to change quickly then dramatic changes in

were to change quickly then dramatic changes in

climate could occur (possibly very quickly)

climate could occur (possibly very quickly)

��Tilt of 90

Tilt of 90°°would lead to poles facing the Sun for

would lead to poles facing the Sun for ½½

year

year

��Would

Would com

plex

complex

life be unable to adapt or to form

in the

life be unable to adapt or to form

in the

presence of sudden changes in clim

ate?

presence of sudden changes in clim

ate?

Magnetic Field

Magnetic Field

��Cosm

ic rays and the solar wind contain high energy

Cosm

ic rays and the solar wind contain high energy

charged particles that unless deflected would lead to

charged particles that unless deflected would lead to

excessive radiation exposure for life

excessive radiation exposure for life

��The Earth

The Earth’’ s m

agnetic field is generated by the liquid Fe

s magnetic field is generated by the liquid Fe

core by a dynam

o effect

core by a dynam

o effect

��The core is kept liquid by continued decay of radioactive

The core is kept liquid by continued decay of radioactive

isotopes

isotopes

��Any planet with a long term

magnetic field m

ust thus have

Any planet with a long term

magnetic field m

ust thus have

these isotopes (

these isotopes (uranium

uranium

238,

238, thorium

thorium

232, and

232, and potassium

potassium

40

40

for exam

ple)

for exam

ple)

��The isotopes necessary for this decay become produced in

The isotopes necessary for this decay become produced in

fewer and few

er amounts with successive generations of

fewer and few

er amounts with successive generations of

supernovae!

supernovae!

��Intriguingly this m

ight put an upper limit on the age of the Uni

Intriguingly this m

ight put an upper limit on the age of the Uni verse

verse

capable of supporting LAWKI

capable of supporting LAWKI

Plate tectonics

Plate tectonics

��We have already mentioned the importance of the carbonate

We have already mentioned the importance of the carbonate --

silicate cycle

silicate cycle ––plate tectonics is certainly necessary on Earth to

plate tectonics is certainly necessary on Earth to

create land m

asses capable of weathering

create land m

asses capable of weathering

��Plate tectonics also leads to the development of different

Plate tectonics also leads to the development of different

environments which m

ay promote biodiversity

environments which m

ay promote biodiversity

��Note a large satellite increases the probability of plate tecton

Note a large satellite increases the probability of plate tectonics

ics

due to the tidal forces on the planet

due to the tidal forces on the planet

��In the Earth

In the Earth-- M

oon system it is also possible that the initial collision m

ay

Moon system it is also possible that the initial collision m

ay

have initiated plate tectonics

have initiated plate tectonics

��Around 2.5

Around 2.5 G

yrGyr

ago there is evidence of tectonic activity

ago there is evidence of tectonic activity

form

ing major land m

asses, perhaps creating

form

ing major land m

asses, perhaps creating favourable

favourable

environments for photosynthesizing bacteria (recall the oxygen

environments for photosynthesizing bacteria (recall the oxygen

bloom between 2.7

bloom between 2.7 G

yrGyr

and 1.6

and 1.6 G

yrGyr

ago)

ago)

��This is then given as a precursor to the development of the euka

This is then given as a precursor to the development of the eukaryotic cell

ryotic cell

Atm

ospheric Chemistry

Atm

ospheric Chemistry

��We

We ’’ve seen the effect of a run

ve seen the effect of a run-- away greenhouse

away greenhouse

on Venus

on Venus ––yet it is only 0.3 AU closer to the

yet it is only 0.3 AU closer to the

Sun

Sun

��To m

uch CO

To m

uch CO

22is clearly a problem if photosynthesis

is clearly a problem if photosynthesis

becomes unable to start

becomes unable to start

��Is this too Earth

Is this too Earth-- centric?

centric?

��Also perhaps need some O

Also perhaps need some O

33to shield against UV

to shield against UV

radiation that is harmful to complex life evolved

radiation that is harmful to complex life evolved

on land?

on land?

Evolutionary selection processes

Evolutionary selection processes

��Mass extinction events have played an enorm

ous

Mass extinction events have played an enorm

ous

role in the evolution of life on Earth

role in the evolution of life on Earth

��Each event can serve as an

Each event can serve as an ““evolutionary pump

evolutionary pump””

by creating many em

pty ecological niches

by creating many em

pty ecological niches

��Systems in which all the niches are filled will see

Systems in which all the niches are filled will see

evolution occur more slowly

evolution occur more slowly

��The time to fill empty niches seems to be short

The time to fill empty niches seems to be short

however (geologically speaking)

however (geologically speaking)

��Of course, such events can select against complex

Of course, such events can select against complex

life

life

This grap

h should be view

ed as incomplete though

As we only have records for biota that are easily fossiliz

ed.

5 m

ain extinctions

are circled:

Ordovician

Late Devonian

End

Permian

End

Triassic

End

Cretaceous

(dinosau

r extinction)

Snowball Earth events

Snowball Earth events

��Ward & Brownlee

Ward & Brownlee suggest

suggestthat two key events were caused, or

that two key events were caused, or

strongly tied to, snowball Earth events

strongly tied to, snowball Earth events

��the sudden increase in oxygen in the atmosphere around 2.5

the sudden increase in oxygen in the atmosphere around 2.5 G

yrGyr

ago and

ago and

the appearance of the eukaryotic cell

the appearance of the eukaryotic cell

��Perhaps the first oxygen leaking into the atmosphere reacted wit

Perhaps the first oxygen leaking into the atmosphere reacted with m

ethane

h m

ethane

(CH

(CH

44+2O

+2O

22→→2H2H

22O+CO

O+CO

22) triggering a reduction in the greenhouse effect and

) triggering a reduction in the greenhouse effect and

the onset of a snowball which selected the photosynthesizing bac

the onset of a snowball which selected the photosynthesizing bacteria

teria

��The Cam

brian explosion seems to coincide with the end of evidenc

The Cam

brian explosion seems to coincide with the end of evidenc e for

e for

global

global glaciation

glaciation

��The second period of

The second period of glaciation

glaciationwas perhaps driven by plate tectonics and the

was perhaps driven by plate tectonics and the

break up of

break up of Rodinia

Rodiniaincreasing the am

ount of material that can weather, thus

increasing the am

ount of material that can weather, thus

sequestering more CO

sequestering more CO

22

��Both these events actually predate the

Both these events actually predate the ““big five

big five ””

extinctions,

extinctions,

although

although K

asting

Kastingbelieves the second Snowball Earth events m

ay

believes the second Snowball Earth events m

ay

have precipitated the biggest relative extinction ever

have precipitated the biggest relative extinction ever

Impact events

Impact events

�The most fam

ous impact

event is the Cretacious-

Tertiary event that ended the

era of dinosaurs

�50% of all species becam

e extinct

�Gave mam

mals new

ecological

niches that they evolved to fill

�However, only the

Cretacious-Tertiary event is

clearlyrelated to impacts

�Others may possibly be related,

but the evidence is m

uch

weaker

�Note that the Manicouagan

crater in Q

uebec was form

ed

214 Myrago, very close to the

end-Triassic extinction

Bacterial life might well be common

Bacterial life might well be common

��The geological record shows a large difference in time

The geological record shows a large difference in time

between life appearing and complex life evolving

between life appearing and complex life evolving

��Microbial life form

s appear a mere 700 million years after the

Microbial life form

s appear a mere 700 million years after the

form

ation of the Earth

form

ation of the Earth

��Complex life seem

s to take 3

Complex life seem

s to take 3 Gyr

Gyr

��Perhaps then single

Perhaps then single --cell life is quite common given the

cell life is quite common given the

appropriate conditions

appropriate conditions

��Indeed, given the presence of life in extreme environments

Indeed, given the presence of life in extreme environments

this almost seems probable

this almost seems probable

��Although again, w

e do not understand enough about the

Although again, w

e do not understand enough about the

evolution/adaptation issue

evolution/adaptation issue

3333

33Human intelligence

Human intelligence

55150

150

250

250

Animal intelligence

Animal intelligence

55100

100

400

400

Land ecosystem

sLand ecosystem

s

5555

510

510

Development of

Development of

animals

animals

??2000

2000

550

550

Tissue

Tissue multicellularity

multicellularity

100

100

1000

1000

2500

2500

Oxygen in the

Oxygen in the

environment

environment

Very sm

all?

Very sm

all?

<500

<500

<3500

<3500

Oxygen

Oxygen

photosynthesis

photosynthesis

1010<500

<500

3800

3800-- 3500

3500

Origin of Life

Origin of Life

Estimated

Estimated

minimum

minimum

possible time /

possible time /

Myr

Myr

Time taken /

Time taken /

Myr

Myr

Time in the past

Time in the past

event occurred /

event occurred /

Myr

Myr

Event

Event

The W

ard & Brownlee riposte to the

The W

ard & Brownlee riposte to the

Drake Equation

Drake Equation

��N=N*

N=N* ff ggff ppnnEEff pm

pm ffiiff ccff l l ff m

m ffjjff m

eme

N*=

number of stars in M

ilky Way

N*=

number of stars in M

ilky Way

ff gg= fraction of stars in G

HZ

= fraction of stars in G

HZ

ff pp= fraction of stars with planets

= fraction of stars with planets

ff pm

pm= fraction of planets that are rocky

= fraction of planets that are rocky

nnEE= number of planets in H

Z= number of planets in H

Z

ff ii= fraction of habitable planets where microbial life arises

= fraction of habitable planets where microbial life arises

ff ll= fraction of planet

= fraction of planet’’ s life span in which complex life is present

s life span in which complex life is present

ff mm= fraction of habitable planets with a large m

oon

= fraction of habitable planets with a large m

oon

ff jj= fraction of planets with appropriate

= fraction of planets with appropriate Jovian

Jovian

planets

planets

ff me

me= fraction of planets with a small enough number of

= fraction of planets with a small enough number of

extinction events

extinction events

If any one of the f factors is very close to zero, then so will N

!

Counter arguments

Counter arguments

��Rare Earth hypothesis assumes that animal life will be somehow

Rare Earth hypothesis assumes that animal life will be somehow

Earth

Earth-- like in that it has some form

of DNA

like in that it has some form

of DNA

��How representative is Earth

How representative is Earth-- life of all life? D

oes the hypothesis

life of all life? D

oes the hypothesis

ultimately lack imagination?

ultimately lack imagination?

��We

We ’’ve seen how

ve seen how extremophiles

extrem

ophilescan adapt on Earth, perhaps there are

can adapt on Earth, perhaps there are

instances where more complex life evolved out of apparently blea

instances where more complex life evolved out of apparently blea k

k environments

environments

��We can also question assumptions about the availability of free

We can also question assumptions about the availability of free

oxygen being the impetus in the development of the eukaryote

oxygen being the impetus in the development of the eukaryote

cell

cell

��Unfortunately evolutionary biologists have stayed away from addr

Unfortunately evolutionary biologists have stayed away from addr essing

essing

this question

this question

��Most importantly: one could expect unusual things about every

Most importantly: one could expect unusual things about every

planet where intelligent life form

s planet where intelligent life form

s ––have Ward and Brownlee

have Ward and Brownlee

dem

onstrated that any of the factors they m

ention are ultimately

dem

onstrated that any of the factors they m

ention are ultimately

necessary?

necessary?

Summary of lecture 23

Summary of lecture 23

��The Rare Earth H

ypothesis states that the challenging part of

The Rare Earth H

ypothesis states that the challenging part of

the creation of intelligent life is the evolution from simple to

the creation of intelligent life is the evolution from simple to

multicelled

multicelledanimals

animals

��Microbial life on the other hand m

ay well be common throughout t

Microbial life on the other hand m

ay well be common throughout t he

he

galaxy

galaxy

��This evolution has been influenced by many environmental factors

This evolution has been influenced by many environmental factorsand

and

we could be one really lucky event

we could be one really lucky event

��Rem

ember though the argument does not suggest that Earth

Rem

ember though the argument does not suggest that Earth--

type evolution is unique

type evolution is unique ––just

just rare

rare

��There are at least 100 billion galaxies out there

There are at least 100 billion galaxies out there ……

��The hypothesis is extrem

ely interesting and challenges

The hypothesis is extrem

ely interesting and challenges

(optimistic) m

ainstream

SETI thinking

(optimistic) m

ainstream

SETI thinking……

Next lecture

Next lecture

��Looking at M

ars in m

ore detail

Looking at M

ars in m

ore detail ––our best hope for

our best hope for

finding traces of life

finding traces of life

214: planets & life phys 214

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