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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 Planet with Stable Environment Right Elements: ¡SPONCH! Carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus Trace elements, like iron (Fe) & magnesium (Mg) Liquid Solvent (Water) Right Temperature (Energy Source Light Chemicals ) 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. Element Distribution (¡SPONCH!) in the Solar System Distributed by volatility! O N C H are very volatile (gasses / 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 ¡SPONCH! mix in Venus, Earth, Mars; plus Titan (Saturn); Europa, Callisto, Ganymede (Jupiter) A b u n d a n c e ¡SP M V ONCH! E M J S Why Liquid? Why Water? Why Liquid? Allows easy movement of molecules to/from reactions Allows creation of complex molecules Why Water? H2O is abundant in solar system, and is versatile. Liquid ammonia (NH3) and hydrocarbons (methane, ethane) are possible. 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) Planets around Other Stars? Do they have Stable Environments? Planets are common Hundreds now known! Known ones not good. Hot gas giants, or Hghly elliptical orbits (unstable environments). Very tough to detect planet like Earth - small and far from star. 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? ¡SPONCH ! 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/r2 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."