Rare Earth Hypothesis That occurrence of Earth-like planets with complex life forms in the universe is rare. So many geologic and astronomical conditions worked together to create Earth-like environments. Complex life is not common.

Competing Theory: Principle of Mediocrity States that Earth-like planets are common in the universe; so is complex life. Earth is “mediocre” … not unique. Carl Sagan & Francis Drake

Which Theory is Accurate? … More data is needed.

Planet Size & Mass Iron Core Atmosphere Jupiter-like Neighbor Large Moon Occasional Catastrophes Right Distance from Sun Average Mass Star Rare Earth Hypothesis: Conditions needed to support complex life

Planet Size & Mass Iron Core Atmosphere Jupiter-like Neighbor Large Moon Occasional Catastrophes Right Distance from Sun Average Mass Star Rare Earth Hypothesis: Conditions needed to support complex life Large size = Large gravity. Jupiter attracts meteorites that would otherwise collide w/ inner planets. -Burns up meteorites so they don’t impact. -Regulates temps. -Filters out dangerous wavelengths of energy (UV) -Oxygen for life! Moon’s gravity pulls on Earth’s axis. This lessens the amount that Earth WOBBLES on its axis. WOBBLING results in CLIMATE CHANGE (Ice age  > warming). Cause mass extinctions that stimulate EVOLUTION. New species can be successful and diversify. Temperatures for LIQUID WATER Last long enough for evolution. Massive stars last only a few million yrs. Large/Massive enough to have enough gravity to hold on to an atmosphere; Large enough to maintain internal heat (needed for volcanism & plate tectonics) Creates a magnetic field that protects organisms from dangerous solar wind.