Extra-Solar Life:
Habitable Zones
14 November 2012
AST 2037
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Life in the Solar System?
•
•
•
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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?
3
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)
8
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 stars
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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/starlight, 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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