Habitable worlds: How common are they?
Earth has been habitable - capable of, and in fact sustaining life - for perhaps 80-90% of its history. If Venus ever were habitable, it is not so today, and precisely when it lost its habitable environment continues to be a matter of controversy. Mars might or might not support simple life, perhaps deep in its crust, but the surface environment appears to have not been habitable for billions of years. Subsurface habitable environments might exist in the outer solar system moons Europa and Enceladus, while Titan's surface is habitable only if an exotic form of life that uses liquid hydrocarbon instead of water were possible.
In the context of this local menagerie, what are the prospects for habitable worlds around other stars? Nothing in the physics of planet formation nor of planetary evolution preclude habitability as a common cosmic phenomenon in terms of planetary size, location and composition. And planets of detectable size and mass seem almost uniformly common. Future observations of near-Earth-size planets around other stars will help test how common habitable properties might be.
Meanwhile, even the future habitability of our home planet is a matter of debate. Increasing brightness of the Sun seems inevitable, but when and how the stability of Earth's ocean and its biosphere will be compromised by this gradual change cannot be predicted with absolute certainty.
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