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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
Suitable size of planet
(5)
(5)
Presence of a large moon
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?
Suitable size of planet
Suitable size of planet
��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
Presence of a large moon
Presence of a large moon
��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