The hypothesis argues that complex extraterrestrial life requires an Earth-like planet with similar circumstance and that few if any such planets exist.
The rare earth hypothesis is the contrary of the widely accepted principle of mediocrity (also called the Copernican principle), advocated by Carl Sagan and Frank Drake, among others. The principle of mediocrity concludes that the Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Hence it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the solar system, and our region of the Milky Way are very rare.
-A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Substantial and long-lasting oceans become impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics will either not last as long as they would on a larger planet or may not occur at all.
-The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or have tiny satellites that are probably captured asteroids (Mars).