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Chapter 19: Future climate
evolution/Habitable planets
around other stars
Future climate evolution
• The Sun continues to get brighter at a rate
of ~ 1 percent every hundred million years
• This should increase surface
temperatures, which in turn should cause
faster silicate weathering and a
corresponding decrease in atmospheric
CO2 
Long-term implications for
habitability of Earth
Solar luminosity
C4 photosynthesis
0
0.4
1.2
1.6
• 500 m.y: CO2 falls below 150 ppmv  C3 plants
should become extinct
• 900 m.y.: CO2 falls below 10 ppmv  C4 plants
become extinct
Kump et al., The Earth System (2002), Fig. 19-1
Long-term implications
Solar luminosity
• 1.2 b.y.: The rapid rise in surface temperature
causes the stratosphere to become wet 
Earth’s oceans should be lost over the next few
hundred million years, and all life will go extinct
Is there any way to counteract these effects?
Yes, one could do this by building a solar shield!
The solar shield
• This probably isn’t a
good solution to the
problem of global
warming, as it doesn’t
solve the related
problem of ocean
acidification
• As CO2 goes down in
the more distant future,
however, this problem
goes away
uanews.org (Univ. of Arizona)
• We are also interested in the possibility
of finding habitable planets around other
stars
• As a first step, we need to figure out
where such planets might reside…
Liquid water is essential for life
(as we know it)
• Clever biochemists have
suggested that non-carbonbased, non-water-dependent
life could possibly exist
• Nonetheless, the best place
to begin the search for life is
on planets like the Earth that
have liquid water on their
surfaces
• This means that we should
look within the conventional
habitable zone around
nearby stars
Definitions
(from Michael Hart, Icarus, 1978)
• Habitable zone (HZ) -the region around a star
in which an Earth-like
planet could maintain
liquid water on its
surface at some instant
in time
• Continuously habitable
zone (CHZ) -- the
region in which a planet
could remain habitable
for some specified
period of time (e.g., 4.6
billion years)
Finding the boundaries of the
habitable zone
• Inner edge determined by loss of water
via runaway or moist greenhouse effect
• Venus is a case in point…
Venus
• 93-bar, CO2-rich atmosphere
• Practically no water (10-5
times Earth)
• D/H ratio = 150 times that on
Earth
What went wrong with it?
Positive feedback loops
(destabilizing)
Water vapor feedback
Surface
temperature
Atmospheric
H2O
(+)
Greenhouse
effect
• This feedback loop appears to have gotten out of control
on Venus because of its position closer to the Sun
Finding the boundaries of the
habitable zone
• Outer edge depends
on how large a
planet’s greenhouse
effect might be
• Mars, at 1.52 AU, is
cold and dry today
but looks as if it may
have been habitable
in the distant past…
Evidence for water on early Mars
• The ancient, heavily
cratered terrain on Mars
is cut through by fluvial
channels
• So, Mars was probably
inside the habitable
zone early in its history
• What might have kept
early Mars warm?
Warrego Vallis (image courtesy of
NASA)
The carbonate-silicate cycle
metamorphism
• CO2 builds up in a planet’s atmosphere as its climate cools
• Planets located farther from their parent star should therefore
build up dense CO2 atmospheres and large greenhouse effects
Negative feedback loops
(stabilizing)
The carbonate-silicate cycle feedback
Rainfall
Surface
temperature
(−)
Greenhouse
effect
Silicate
weathering
rate
Atmospheric
CO2
• The carbonate-silicate cycle feedback
loop ensures that the habitable zone is
relatively wide
• We can also calculate HZs and CHZs
for other types of stars…
Hertzsprung-Russell (HR) Diagram
O and B
stars
Main sequence
G stars
Sun
M-stars
See also The
Earth System,
p. 191
• Our Sun is a normal, hydrogen burning star along
the stellar main sequence
http://www.physics.howard.edu/students/Beth/bh_stellar.html
ZAMS habitable zones
• Planets orbiting late K and M stars may be tidally locked
• Early F and A stars have short lifetimes and give off lots of UV
radiation
• Habitable zones around solar-type stars appear to be relatively wide
Kasting et al., Icarus (1993)
• Intriguingly, astronomers are now
beginning to find planets around other
stars
• Most of these so far have been detected
using the radial velocity (or Doppler)
method
Radial velocity (Doppler) method
• The pull of the planet on its host star makes the star wobble
back and forth in the observer’s line of sight
http://www.eso.org/public/videos/eso1035g/
Transit Method
Kepler Mission
• This space-based telescope
points at a patch of the
Milky Way and monitors the
brightness of ~150,000 stars,
looking for transits of Earthsized (and other) planets
• 105 precision photometry
 can find Earths
• Launched: March 7, 2009
• 2,326 “planet candidates”
found so far (Dec, 2011)
http://www.nmm.ac.uk/uploads/jpg/kepler.jpg
December 2011 data release
Candidate
label
Earth-size
Super-Earths
Candidate size
(RE)
Rp < 1.25
1.25 < Rp < 2.0
Neptune-size
2.0 < Rp < 6.0
Jupiter-size
6.0 < Rp < 15
Very-large-size 15 < Rp < 22.4
TOTAL
Number of
candidates
207
680
1181
203
55
2326
Known extrasolar planets
704
• 708 extrasolar planets
identified as of Dec. 09, 2011
• Few, if any, of these planets
are very interesting, however,
from an astrobiological
standpoint
– Gliese 581g (the “Goldilocks
planet”) is probably not real
Howard et al.(2010)
~ 2300 more “candidate” planets from Kepler mission !!
Kepler-22b
• 600 l.y. distant
• 2.4 RE
• 290-day orbit, late G
star
• Not sure whether
this is a rocky planet
or a Neptune
(RNeptune = 3.9 RE)
http://www.nasa.gov/mission_pages/
kepler/news/kepscicon-briefing.html
Direct imaging
• The real payoff will come
from observing Earth-like
planets directly, i.e.,
separating their light from
that of the star, and taking
spectra of their atmospheres
• This will require large,
space-based telescopes
– Earth-sized planets could
conceivably be detected by
future 30 m-class groundbased telescopes; however,
looking for biomarker gases
through Earth’s atmosphere
is probably impossible
TPF-C
TPF-I
• What we’d really like to
do is to build a big TPF
(Terrestrial Planet Finder)
telescope and search
directly for Earth-like planets
• This can be done either in the
thermal-IR (TPF-I) or in the
visible/near-IR (TPF-C or –O)
TPF-O
Visible Spectrum of Earth
10 Å = 1 nm
O2 = life?
Integrated light of Earth, reflected from dark side of moon;
Rayleigh, chlorophyll, O2, O3, H2O
Ref.: Woolf, Smith, Traub, & Jucks, ApJ 2002; also Arnold et al. 2002
Thermal-IR
spectra
O3 = life?
Source:
R. Hanel, Goddard
Space Flight Center
Take-home lessons from this
class
• We need to preserve our environment, as
Earth is the only habitable planet that we
know of
• Global warming is a real problem with which
we will someday have to deal
• There may well be other Earth-like planets
around other stars. Looking for them, and
looking for signs of life on them, is a scientific
endeavor that is well worth undertaking