Pop Quiz!
1. What possible feature of Europa and Enceladus makes
them interesting as possible homes for life?
2. Name two things that make Venus seem to be an unlikely
home for life.
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Extra-Solar Life:
Habitable Zones
Stephen Eikenberry
30 October 2012
AST 2037
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Life in the Solar System?
•
•
•
•
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
• 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?
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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>273 K)
• T <100 C (T<373K)
• We need to find temperatures in this range (at least
part of the time)
• How common is that?
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What Sets Planet Temperatures?
• Lots of factors (such as?)
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Temperature Balance
• Assume that the dominant factor is stellar radiation
• Flux(emitted) = Flux(radiated)
• Show balance; solve for T
• Show range in distance
• Sun  0.84 – 1.7 AU “Habitable Zone”
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Habitable Zone
• The zone in a stellar system where water is in liquid
form (at least some of the time)
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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 of Earth:
• 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
• Calculate on board
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Habitable Zones: M stars
• Show on board
• 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
• Show on board
• Range in distance from the star is huge
• We expect many planets in this range
• But … lifetime issue
• How long does an O star 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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