Habitability: Earth to Universe But could we recognize life?

Plan of Presentation What ‘Our Kind’ of life needs to form and thrive Types of Planets?

Our Solar System Other Planetary Systems Other Stars Galaxies The Universe

Requirements for ‘Our Kind’ of Life Right Elements: ¡SPONCH! Carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus Trace elements, like iron (Fe) & magnesium (Mg) Stable Environment Right Temperature Liquid Solvent (Water) Energy Source Light Chemicals

Element Distribution (¡SPONCH!) in the Solar System Distributed by volatility! O N C H are very volatile (gases / ices) • Burned mostly out of inner solar system by the Sun • Some carried back in by comets and asteroids S P are not volatile - stay with rocky solid Good ¡SP ONCH!

mix in Venus, Earth, Mars; plus Titan (Saturn); Europa, Callisto, Ganymede (Jupiter); Ceres?

A b u n d a n c e

¡SP

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ONCH!

M V E M J S

Stable Planetary Environment Planets have near-circular orbits No huge ellipses with hot and cold times Sun is stable Sunspot cycle is really minor Not many asteroids or comets Not very many Dinosaur-killers!

Few nearby stars Near pass would disrupt planet orbits.

Explosion would fill solar system with lethal radiation.

Temperature in a solar system Heat mostly from star, decreases away from it. Yellow zone = liquid water at planet’s surface. Stars get hotter as they age, so yellow zone moves out.

Planet rotation smooths out temperature.

Tidal lock = same face to sun (sort-of)

Why Liquid? Why Water?

Why Liquid? Allows easy movement of molecules to/from reactions Allows creation of complex molecules Why Water? H 2 O is abundant in solar system, and is versatile. Liquid ammonia (NH 3 ) and hydrocarbons (methane, ethane) are possible.

Where in the Solar System could we find liquid water?

Earth: Certainly.

Mars: Certainly in the past, maybe now Europa: Liquid water ocean (blue) under ice; rocky mantle, metal core.

Venus: ? Possibly, in its distant past. Europa

Ceres: Dwarf Planet, Ex-asteroid Avg. diameter 942 km Equatorial - 975 km Polar - 909 km Density = 2.07 gm/cm 3 mix rock + water Polar flattening suggests rotating ‘fluid’ glob (ice or water shell over rock.

Maybe liquid water, depending on how much internal heat from radioactivity.

Enceladus: Moon of Saturn, with water vapor plumes Plumes erupting from Enceladus.

Known to be water vapor by its temperature, just about 0°C

Titan - Another solvent!

Dense atmosphere, thick cloud cover. Orange color is hydrocarbon smog.

RADAR imager on Cassini spacecraft shows river beds and lakes on surface.

Lakes probably made of liquid hydrocarbons, like methane (CH 4 ) and ethane (C 2 H 6 ).

lake land

Planets around Other Stars?

Do they have Stable Environments?

Planets are common Hundreds now known!

Most known are not good.

Hot gas giants, or Highly elliptical orbits (unstable environments).

Very tough to detect planet like Earth - small and far from star.

New Extra-solar Planets Water vapor! On Hot Jupiter HD 189733b 4.5 million km from star (Mercury is 70 mil km) 2.2 day orbit, surface T 700°C (1000K) Habitable? Gliese 581 C - Super-Earth Red dwarf star (M-type), 50 times dimmer than sun • C is 15 times closer to G581 than Earth to Sun. Half again as big as Earth, 5 times more massive Surface T at 0 - 40°C. !!! Surface G is 2.2 x Earth.

What sort of Star? Only some main sequence cool stars (types F, G, K, M ) are suitable Hot, large stars (O, B, some A) explode too soon. Hot stars (O, B) make too much deadly ultraviolet radiation. Variable stars, flare stars don’t provide stable environments Giants, supergiants White dwarfs are remnants after star explosions.

Multiple Stars?

Most stars are double, triple or more. Some have planets! (HD88753) Planet orbits are stable only near a star or far from them all.

A multiple star system is as bad for life as its worst star. And … multiple stars have more restricted habitable zones, and more variable planetary environments. Imagine our solar system with a small star in place of Jupiter!

Young or Old Stars? ¡SPONC ! form in stars during normal burning Most important trace elements (like Fe) also Heavier elements form in star explosions Star explosions release these atoms as dust More star generations, more ¡SPONCH!

Younger, multi-generation stars more likely to have solid planets!

Where in the galaxy can life survive?

Far from the core Intense radiation from its huge black hole Too many stars, will disrupt planets’ orbits In from the rim Rim stars tend to be older, poor in SPONCH Outside galaxy arms Too many stars, will disrupt planets’ orbits

Why is our Universe Habitable? The Anthropic Principle The Universe seems right for life Space is ‘flat’ Atom stability is right Gravity works as 1/r 2 How to explain these coincidences? If intelligent life weren’t possible, we wouldn't be here to think about it. Our the universe is designed for life.

"If we weren't here, the universe couldn't exist."