1. 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.

2. 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

3. Which Theory is Accurate? … More data is needed.

4. Rare Earth Hypothesis: Conditions needed to support complex life

5. Large/Massive enough to have enough gravity to hold on to an atmosphere; Large enough to maintain internal heat (needed for volcanism & plate tectonics) Last long enough for evolution. Massive stars last only a few million yrs. Creates a magnetic field that protects organisms from dangerous solar wind. Rare Earth Hypothesis: Conditions needed to support complex life • Burns up meteorites so they don’t impact. • Regulates temps. • Filters out dangerous wavelengths of energy (UV) • Oxygen for life! Temperatures for LIQUID WATER Cause mass extinctions that stimulate EVOLUTION. New species can be successful and diversify. Large size = Large gravity. Jupiter attracts meteorites that would otherwise collide w/ inner planets. 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).