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Extra-Solar Life: Habitable Zones

Extra-Solar Life: Habitable Zones. 14 November 2012 AST 2037. 1. 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). 2. Life Ingredients.

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Extra-Solar Life: Habitable Zones

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  1. Extra-Solar Life:Habitable Zones 14 November 2012 AST 2037 1

  2. 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) 2

  3. 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

  4. 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? 4

  5. What Sets Planet Temperatures? • Lots of factors (such as?) • Solar Radiation (Solar flux) • Albedo • Rotational Rate • Distance • Heat sinks on Earth • Chemical changes • Photosynthesis 5

  6. 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 6

  7. Habitable Zone • The zone in a stellar system where water is in liquid form (at least some of the time) 8

  8. Habitable Zones: Other Stars? • Hertzsprung-Russell Diagram (again!) • Scaling for other stars (same balance, now try distance versus luminosity) 9

  9. 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 10

  10. Habitable Zones: K stars 11

  11. Habitable Zones: M stars • Range in distance from star is very small  there will probably be very few planets in this range 12

  12. 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? 13

  13. 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? 14

  14. 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? 15

  15. 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 16

  16. 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” 17

  17. 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 18

  18. 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 (!) 19

  19. 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 20

  20. 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.) 21

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