The Rare Earth hypothesis claims that the Earth is rare, and that few planets can support complex life. The Rare Earth hypothesis stands in contrast to the principle of mediocrity, which assumes that the Earth must be a typical planet, as there should be nothing special about humanity or the Earth. Numerous arguments for the Rare Earth hypothesis have been put forward, to explain why the Earth is unique; some analyses suggest that the Earth is the only planet capable of supporting complex life in the Milky Way, or even the entire visible universe.
Supporters of the Rare Earth hypothesis point out the long list of conditions needed to evolve complex life on Earth. Abiogenesis, the creation of life, requires a broth for organic chemicals to react in, which in turn requires large bodies of water. No planet other than Earth is known to have liquid water in significant quantities, and few planets have the right temperature for water to stay a liquid; if the planet is too hot, the water will boil off into space, and cold planets will keep all their water frozen.
The development of complex life also requires the planet to be protected from hazards such as cosmic radiation, extreme weather conditions, and asteroid impacts. According to the Rare Earth hypothesis, the Earth may be unusually lucky in this regard. The outer planet Jupiter has a strong enough gravitational field to bend the paths of comets headed towards the inner solar system and Earth. The Moon's unusually large size allows it to stabilize the Earth's axis, keeping climate patterns stable, and the Earth's geology gives it a strong magnetic field, helping to deflect high-energy protons and other charged cosmic radiation. It is unlikely that any random planet, even if it has liquid water, would have all of these shields in place to prevent extinction events.
The Rare Earth hypothesis remains controversial, and many scientists have pointed out that an Earth-type environment may not be the only one which can support life. Even if the evolution of complex life is rare, the Milky Way galaxy alone contains more than a hundred billion stars, providing many chances for life to evolve. Future space missions, designed to directly observe extrasolar planets, could help to resolve some of these questions by measuring the habitability of large numbers of planets orbiting stars close to the Sun.