Planets & LifePlanets & LifePHYS 214PHYS 214

Dr Rob ThackerDr Rob ThackerDept of Physics (308A)Dept of Physics (308A)

thacker@astro.queensu.cathacker@astro.queensu.caPlease start all class related emails Please start all class related emails

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Today’s LectureToday’s Lecture

Rare Earth Hypothesis (REH)Rare Earth Hypothesis (REH) Peter Ward & Don Brownlee (Univ. of Peter Ward & Don Brownlee (Univ. of

Washington)Washington)

Could the Earth be Could the Earth be “special”?“special”?

““Who are we? We find that we live on an Who are we? We find that we live on an insignificant planet of a humdrum star insignificant planet of a humdrum star lost in lost in a galaxy tucked away in a galaxy tucked away in some forgotten cornersome forgotten corner of a universe of a universe in which there are far more galaxies than in which there are far more galaxies than people….” people….” Carl SaganCarl Sagan

One of the underlying tenets of scientific investigation is that we do not occupy a specialplace in the Universe. Yet, the Earth definitely is “special” within the solar system as lifeonly appears to have occurred here. Could it be that life is a coincidence of so many factorsthat, in reality, the Earth is actually special within the galaxy?

Don’t be drawn in by anthropocentric quotes: "If we are alone in the Universe, it sure seems like an awful waste of space."

Underlying basis of the Underlying basis of the REHREH

The development of intelligent life The development of intelligent life has required many unlikely has required many unlikely coincidences that are highly coincidences that are highly improbable of occurring elsewhereimprobable of occurring elsewhere Note, this doesn’t mean the Earth is Note, this doesn’t mean the Earth is

uniqueunique – just – just rarerare

Could we actually be alone?Could we actually be alone?

Components of the REHComponents of the REH

(1)(1) Galactic Habitable zoneGalactic Habitable zone(2)(2) Appropriate stellar typeAppropriate stellar type(3)(3) Suitable planetary systemSuitable planetary system(4)(4) Suitable size of planetSuitable size of planet(5)(5) Presence of a large moonPresence of a large moon(6)(6) Requirement of a magnetic fieldRequirement of a magnetic field(7)(7) Plate tectonicsPlate tectonics(8)(8) Appropriate atmospheric chemistryAppropriate atmospheric chemistry(9)(9) Evolutionary selection processes (e.g. Evolutionary selection processes (e.g.

glaciations, impact events)glaciations, impact events)

Galactic Habitable ZoneGalactic Habitable Zone In addition to the issues we discussed, it In addition to the issues we discussed, it

is also possible that passage through a is also possible that passage through a spiral arm is negative precondition on lifespiral arm is negative precondition on life Spiral arms in galaxies actually represent a Spiral arms in galaxies actually represent a

pattern that rotates at different speeds to the pattern that rotates at different speeds to the actual materialactual material

Very active star formation – lots of material & new Very active star formation – lots of material & new stars that could perturb the orbits of the planetsstars that could perturb the orbits of the planets

It is not clear how significant an issue this is, It is not clear how significant an issue this is, the Sun the Sun maymay well have passed through spiral well have passed through spiral arms several timesarms several times

Nonetheless, on the basis of the GHZ Nonetheless, on the basis of the GHZ argument it is expected that 5-10% of the argument it is expected that 5-10% of the stars in the MW fall in the GHZ stars in the MW fall in the GHZ

Appropriate stellar type Appropriate stellar type for lifefor life

Aside from the age issue we discussed, Wien’s Law Aside from the age issue we discussed, Wien’s Law tells us the peak emission for massive stars (such as tells us the peak emission for massive stars (such as O stars) is in the ultravioletO stars) is in the ultraviolet This would have a strong ionizing effect on any atmosphere This would have a strong ionizing effect on any atmosphere

in the habitable zone (lots of photodissociation)in the habitable zone (lots of photodissociation) The most massive stars are rare, so in terms of life The most massive stars are rare, so in terms of life

around these systems we don’t care too mucharound these systems we don’t care too much Similarly we discussed briefly at the low mass end (K Similarly we discussed briefly at the low mass end (K

and M stars) the planets are probably tidally locked, and M stars) the planets are probably tidally locked, but also need to extremely close to the starbut also need to extremely close to the star If the planet is very close it is much more strongly impact by If the planet is very close it is much more strongly impact by

stellar activity (flares, winds etc) – could atmosphere stellar activity (flares, winds etc) – could atmosphere survive?survive?

We aren’t sure about the effect on the low mass end We aren’t sure about the effect on the low mass end – very important, over 90% of stars are in the K & M – very important, over 90% of stars are in the K & M classesclasses

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

Suitable Planetary Suitable Planetary SystemSystem

We believe that terrestrial type planets are necessary to We believe that terrestrial type planets are necessary to form life, however, in the REH gas giants are form life, however, in the REH gas giants are hypothesized to also play an important role in the hypothesized to also play an important role in the formation of lifeformation of life Protect inner regions of the solar system by scattering or Protect inner regions of the solar system by scattering or

colliding a large fraction of incoming material from the outer colliding a large fraction of incoming material from the outer solar systemsolar system

Systems with too many gas giants tend to become Systems with too many gas giants tend to become unstable and may end up strongly disturbing the unstable and may end up strongly disturbing the dynamics (could spiral into the Sun for example)dynamics (could spiral into the Sun for example)

Similar argument for a single gas giant that is too largeSimilar argument for a single gas giant that is too large Remember we’ve found quite a few systems with gas Remember we’ve found quite a few systems with gas

giants on highly elliptical orbits inside 2 AU – just how giants on highly elliptical orbits inside 2 AU – just how common is this?common is this?

Is this too much of a stretch?Is this too much of a stretch?

Suitable size of planetSuitable size of planet On the basis of the escape velocity On the basis of the escape velocity

argument (given in lecture 22) planets argument (given in lecture 22) planets cannot be too smallcannot be too small With the loss of the atmosphere water might With the loss of the atmosphere water might

freeze, evaporate away or photodissociate due freeze, evaporate away or photodissociate due to the increased UV fieldto the increased UV field

A planet that is too large will tend to have A planet that is too large will tend to have many more impact eventsmany more impact events Will also make it harder for mountain systems Will also make it harder for mountain systems

to form and then will likely be a “water world”to form and then will likely be a “water world” In this case the carbonate-silicate cycle cannot In this case the carbonate-silicate cycle cannot

actact

Presence of a large moonPresence of a large moon The ratio of the mass of the Moon to Earth is The ratio of the mass of the Moon to Earth is

1/801/80thth – this is a surprisingly large number – this is a surprisingly large number Only Pluto and Charon come close in the rest of the Only Pluto and Charon come close in the rest of the

solar systemsolar system The pairwise nature of the Earth-Moon systems The pairwise nature of the Earth-Moon systems

means that incoming asteriods are much less means that incoming asteriods are much less likely to hit Earth than if it were one systemlikely to hit Earth than if it were one system

Also the Moon stabilizes the tilt of the Earth, if Also the Moon stabilizes the tilt of the Earth, if the tilt were to change quickly then dramatic the tilt were to change quickly then dramatic changes in climate could occur (possibly very changes in climate could occur (possibly very quickly)quickly) Tilt of 90° would lead to poles facing the Sun for ½ Tilt of 90° would lead to poles facing the Sun for ½

yearyear Would Would complexcomplex life be unable to adapt or to form life be unable to adapt or to form

in the presence of sudden changes in climate?in the presence of sudden changes in climate?

Magnetic FieldMagnetic Field Cosmic rays and the solar wind contain Cosmic rays and the solar wind contain

high energy charged particles that unless high energy charged particles that unless deflected would lead to excessive radiation deflected would lead to excessive radiation exposure for lifeexposure for life

The Earth’s magnetic field is generated by The Earth’s magnetic field is generated by the liquid Fe core by a dynamo effectthe liquid Fe core by a dynamo effect The core is kept liquid by continued decay of The core is kept liquid by continued decay of

radioactive isotopesradioactive isotopes Any planet with a long term magnetic field must Any planet with a long term magnetic field must

thus have these isotopes (uranium 238, thorium thus have these isotopes (uranium 238, thorium 232, and potassium 40 for example)232, and potassium 40 for example)

The isotopes necessary for this decay become The isotopes necessary for this decay become produced in fewer and fewer amounts with produced in fewer and fewer amounts with successive generations of supernovae!successive generations of supernovae!

Intriguingly this might put an upper limit on the age of Intriguingly this might put an upper limit on the age of the Universe capable of supporting LAWKI the Universe capable of supporting LAWKI

Plate tectonicsPlate tectonics We have already mentioned the importance of the We have already mentioned the importance of the

carbonate-silicate cycle – plate tectonics is certainly carbonate-silicate cycle – plate tectonics is certainly necessary on Earth to create land masses capable of necessary on Earth to create land masses capable of weatheringweathering

Plate tectonics also leads to the development of Plate tectonics also leads to the development of different environments which may promote biodiversitydifferent environments which may promote biodiversity

Note a large satellite increases the probability of plate Note a large satellite increases the probability of plate tectonics due to the tidal forces on the planettectonics due to the tidal forces on the planet In the Earth-Moon system it is also possible that the initial In the Earth-Moon system it is also possible that the initial

collision may have initiated plate tectonicscollision may have initiated plate tectonics Around 2.5 Gyr ago there is evidence of tectonic Around 2.5 Gyr ago there is evidence of tectonic

activity forming major land masses, perhaps creating activity forming major land masses, perhaps creating favourable environments for photosynthesizing favourable environments for photosynthesizing bacteria (recall the oxygen bloom between 2.7 Gyr and bacteria (recall the oxygen bloom between 2.7 Gyr and 1.6 Gyr ago)1.6 Gyr ago) This is then given as a precursor to the development of the This is then given as a precursor to the development of the

eukaryotic celleukaryotic cell

Atmospheric ChemistryAtmospheric Chemistry

We’ve seen the effect of a run-away We’ve seen the effect of a run-away greenhouse on Venus – yet it is only greenhouse on Venus – yet it is only 0.3 AU closer to the Sun0.3 AU closer to the Sun To much COTo much CO22 is clearly a problem if is clearly a problem if

photosynthesis becomes unable to startphotosynthesis becomes unable to start Is this too Earth-centric? Is this too Earth-centric?

Also perhaps need some OAlso perhaps need some O33 to shield to shield against UV radiation that is harmful against UV radiation that is harmful to complex life evolved on land?to complex life evolved on land?

Evolutionary selection Evolutionary selection processesprocesses

Mass extinction events have played an Mass extinction events have played an enormous role in the evolution of life on enormous role in the evolution of life on EarthEarth

Each event can serve as an “evolutionary Each event can serve as an “evolutionary pump” by creating many empty ecological pump” by creating many empty ecological nichesniches Systems in which all the niches are filled will Systems in which all the niches are filled will

see evolution occur more slowlysee 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 Of course, such events can select against

complex lifecomplex life

This graph should be viewed as incomplete though As we only have records for biota that are easily fossilized.

5 main extinctionsare circled:OrdovicianLate DevonianEnd PermianEnd TriassicEnd Cretaceous (dinosaur extinction)

Snowball Earth eventsSnowball Earth events Ward & Brownlee Ward & Brownlee suggestsuggest that two key events were that two key events were

caused, or strongly tied to, snowball Earth eventscaused, or strongly tied to, snowball Earth events the sudden increase in oxygen in the atmosphere around the sudden increase in oxygen in the atmosphere around

2.5 Gyr ago and the appearance of the eukaryotic cell2.5 Gyr ago and the appearance of the eukaryotic cell Perhaps the first oxygen leaking into the atmosphere reacted Perhaps the first oxygen leaking into the atmosphere reacted

with methane (CHwith methane (CH44+2O+2O22→2H→2H22O+COO+CO22) triggering a reduction in ) triggering a reduction in the greenhouse effect and the onset of a snowball which the greenhouse effect and the onset of a snowball which selected the photosynthesizing bacteria selected the photosynthesizing bacteria

The Cambrian explosion seems to coincide with the end of The Cambrian explosion seems to coincide with the end of evidence for global glaciationevidence for global glaciation

The second period of glaciation was perhaps driven by plate The second period of glaciation was perhaps driven by plate tectonics and the break up of Rodinia increasing the amount tectonics and the break up of Rodinia increasing the amount of material that can weather, thus sequestering more COof material that can weather, thus sequestering more CO22

Both these events actually predate the “big five” Both these events actually predate the “big five” extinctions, although Kasting believes the second extinctions, although Kasting believes the second Snowball Earth events may have precipitated the Snowball Earth events may have precipitated the biggest relative extinction everbiggest relative extinction ever

Impact eventsImpact events The most famous impact

event is the Cretacious-Tertiary event that ended the era of dinosaurs 50% of all species became

extinct Gave mammals new

ecological niches that they evolved to fill

However, only the Cretacious-Tertiary event is clearly related to impacts Others may possibly be

related, but the evidence is much weaker

Note that the Manicouagan crater in Quebec was formed 214 Myr ago, very close to the end-Triassic extinction

Bacterial life might well be Bacterial life might well be commoncommon

The geological record shows a large The geological record shows a large difference in time between life appearing difference in time between life appearing and complex life evolvingand complex life evolving Microbial life forms appear a mere 700 million Microbial life forms appear a mere 700 million

years after the formation of the Earthyears after the formation of the Earth Complex life seems to take 3 Gyr Complex life seems to take 3 Gyr

Perhaps then single-cell life is quite Perhaps then single-cell life is quite common given the appropriate conditionscommon given the appropriate conditions Indeed, given the presence of life in extreme Indeed, given the presence of life in extreme

environments this almost seems probableenvironments this almost seems probable Although again, we do not understand enough about Although again, we do not understand enough about

the evolution/adaptation issuethe evolution/adaptation issue

EventEvent Time in the Time in the past event past event

occurred / Myroccurred / Myr

Time taken /Time taken /

MyrMyrEstimated Estimated minimum minimum possible possible

time / Myrtime / Myr

Origin of LifeOrigin of Life 3800-35003800-3500 <500<500 1010

Oxygen Oxygen photosynthesisphotosynthesis

<3500<3500 <500<500 Very small?Very small?

Oxygen in the Oxygen in the environmentenvironment

25002500 10001000 100100

Tissue Tissue multicellularitymulticellularity

550550 20002000 ??

Development of Development of animalsanimals

510510 55 55

Land ecosystemsLand ecosystems 400400 100100 55

Animal Animal intelligenceintelligence

250250 150150 55

Human Human intelligenceintelligence

33 33 33

The Ward & Brownlee The Ward & Brownlee riposte to the Drake riposte to the Drake

EquationEquation N=N* fN=N* fg g ffp p nnE E ffpm pm ffi i ffc c ffl l ffm m ffj j ffmeme

N*=number of stars in Milky WayN*=number of stars in Milky Wayffgg = fraction of stars in GHZ = fraction of stars in GHZffpp= fraction of stars with planets= fraction of stars with planetsffpmpm = fraction of planets that are rocky = fraction of planets that are rockynnEE= number of planets in HZ= number of planets in HZffii = fraction of habitable planets where microbial life = fraction of habitable planets where microbial life arisesarisesffll = fraction of planet’s life span in which complex life is = fraction of planet’s life span in which complex life is presentpresentffmm = fraction of habitable planets with a large moon = fraction of habitable planets with a large moonffjj = fraction of planets with appropriate Jovian planets = fraction of planets with appropriate Jovian planetsffmeme = 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 argumentsCounter arguments Rare Earth hypothesis assumes that animal life will Rare Earth hypothesis assumes that animal life will

be somehow Earth-like in that it has some form of be somehow Earth-like in that it has some form of DNA DNA

How representative is Earth-life of all life? Does the How representative is Earth-life of all life? Does the hypothesis ultimately lack imagination?hypothesis ultimately lack imagination? We’ve seen how extremophiles can adapt on Earth, perhaps We’ve seen how extremophiles can adapt on Earth, perhaps

there are instances where more complex life evolved out of there are instances where more complex life evolved out of apparently bleak environments apparently bleak environments

We can also question assumptions about the We can also question assumptions about the availability of free oxygen being the impetus in the availability of free oxygen being the impetus in the development of the eukaryote celldevelopment of the eukaryote cell Unfortunately evolutionary biologists have stayed away Unfortunately evolutionary biologists have stayed away

from addressing this questionfrom addressing this question Most importantly: one could expect unusual things Most importantly: one could expect unusual things

about every planet where intelligent life forms – about every planet where intelligent life forms – have Ward and Brownlee demonstrated that any of have Ward and Brownlee demonstrated that any of the factors they mention are ultimately the factors they mention are ultimately necessary?necessary?

Summary of lecture 23Summary of lecture 23 The Rare Earth Hypothesis states that the The Rare Earth Hypothesis states that the

challenging part of the creation of intelligent life is challenging part of the creation of intelligent life is the evolution from simple to multicelled animalsthe evolution from simple to multicelled animals Microbial life on the other hand may well be common Microbial life on the other hand may well be common

throughout the galaxythroughout the galaxy This evolution has been influenced by many environmental This evolution has been influenced by many environmental

factors and we could be one really lucky eventfactors and we could be one really lucky event Remember though the argument does not suggest Remember though the argument does not suggest

that Earth-type evolution is unique – just that Earth-type evolution is unique – just rarerare There are at least 100 billion galaxies out there…There are at least 100 billion galaxies out there…

The hypothesis is extremely interesting and The hypothesis is extremely interesting and challenges (optimistic) mainstream SETI thinking…challenges (optimistic) mainstream SETI thinking…

Next lectureNext lecture

Looking at Mars in more detail – our best Looking at Mars in more detail – our best hope for finding traces of lifehope for finding traces of life