Circumstellar Habitable Zone
Beyond the outer edge of the habitable zone, a planet will be too cold to sustain liquid water on its surface. Any water present will freeze. A planet closer to its star than the inner edge of the habitable zone will be too hot. Any water present will boil away or be lost into space entirely. Liquid water is considered important because carbon compounds dissolved in water form the basis of all earthly life, so watery planets are good candidates to support similar carbon-based biochemistries.
Theoretical determinations of the habitable zone are based on empirical observation of the habitability of the Earth and its orbit within Solar System. Various complications must be taken into account, such as the greenhouse effect and changing albedo due to clouds.
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