1. Essentials to Life A description of factors that have allowed complex life to evolve on Earth as noted in Rare Earth: Why Complex Life is Uncommon in the Universe

2. Location, Location, Location Galactic Location The most far reaching galaxies are too old to support life Metals and other heavier elements were not created yet Type of Galaxy Most types of galaxies such as small, elliptical, and clustered ones are metal-poor and could not support the formation of Earth Solar System Location Too close to the center – too populated Too many “energetic processes” Too far away – stars are metal-poor Planet Location Too close, water would evaporate, tidal lock too far away, too cold!

3. Size DOES Matter Sun Appropriate lifetime (too short, life couldn’t develop) Energy produced / given off is not highly rich in UV rays Earth Too small – internal heat would escape faster Thin atmosphere, particles would not be gravitationally bound (ex. Mars) Oceans would also not remain if small Appropriate tilt size to prevent wild seasonal change in climate

4. Having the Right Neighbors Moon Stabilizes tilt Not too close to create chaotic tides Jupiter Not too close to disrupt orbit Close enough to attract all comets / meteors / asteroids

5. Earthly Features Oceans Not too much or too little (anyone seen Waterworld?) Plate Tectonics / Internal Heat Renews land nutrients Molten core creates magnetic field Supports biological diversity Atmosphere Enough carbon but not too much for runaway greenhouse effect (Venus) Appropriate temperatures (thanks to distance) Appropriate composition, pressure Oxygen, photosynthesis Few global catastrophes

6. What Life Needs Carbon and other life building block elements Water on / near surface Appropriate atmosphere Long period of stability Abundance of heavy elements at core, crust Fe, Mg, Si, O – earth structure U, Th, K – radioactive heat C, O, N, H, P – biogenic building blocks

7. What Allowed this to Develop? Molten surface Distance from sun Galactic location Development of plant life – photosynthesis T-tauri phase Size of planet 2 nd generation star Luck

8. Looking for Signs of Life Type of galaxy, location within the galaxy Inner – too busy, black holes Outer – metal poor Star / Sun – not too massive or give off too many damaging rays (sun only emits 10% of energy in UV), fluctuations in output are small Planet within a “habitable zone” (not too far, not too close) Correct age – too old will be metal poor, too young and perhaps not enough heat? (of galaxy, sun, planet) Metal content in the planets, presence of Carbon (other life building elements) and WATER