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/star-light, 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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