Essentials to LifeA 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

Location, Location, Location

Galactic LocationThe most far reaching galaxies are too old to support life

Metals and other heavier elements were not created yet

Type of GalaxyMost types of galaxies such as small , elliptical, and clustered ones are metal-poor and could not support the formation of Earth

Solar System LocationToo close to the center – too populated

Too many “energetic processes”

Too far away – stars are metal-poor

Planet LocationToo close, water would evaporate, tidal lock too far away, too cold!

Size DOES MatterSun

Appropriate lifetime (too short, life couldn’t develop)

Energy produced / given off is not highly rich in UV rays

EarthToo 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

Having the Right Neighbors

MoonStabilizes tilt

Not too close to create chaotic tides

JupiterNot too close to disrupt orbit

Close enough to attract all comets / meteors / asteroids

Earthly FeaturesOceans

Not too much or too little (anyone seen Waterworld?)

Plate Tectonics / Internal HeatRenews land nutrients

Molten core creates magnetic field

Supports biological diversity

AtmosphereEnough carbon but not too much for runaway greenhouse effect (Venus)

Appropriate temperatures (thanks to distance)

Appropriate composition, pressure

Oxygen, photosynthesis

Few global catastrophes

What Life NeedsCarbon and other life building block elements

Water on / near surface

Appropriate atmosphere

Long period of stability

Abundance of heavy elements at core, crustFe, Mg, Si, O – earth structure

U, Th, K – radioactive heat

C, O, N, H, P – biogenic building blocks

What Allowed this to Develop?

Molten surface

Distance from sun

Galactic location

Development of plant life – photosynthesis

T-tauri phase

Size of planet

2nd generation star

Luck

Looking for Signs of Life

Type of galaxy , location within the galaxyInner – 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