Extra-Solar Life: Habitable Zones 14 November 2012 AST

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 The ingredients of (Earth-like) life are all common: H 2 O C Fe etc. So are many possible alternatives to them (i.e. Si, NH 3, 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 H 2 O 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

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

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

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) 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

Habitable Zones: K stars 11

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

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

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

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

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

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

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

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

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