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Habitable zone

The habitable zone around a star is the region where the temperature range allows for liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star; this means the habitable zone will also vary accordingly. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet.

Habitable zones have usually been defined in terms of surface temperature; however, over half of Earth's biomass is from subsurface microbes, and temperature increases as depth underground increases, so the subsurface can be conducive for life when the surface is frozen; if this is considered, the habitable zone extends much further from the star.

Studies in 2013 indicate that an estimated 22±8% of Sun-like stars have an Earth-sized planet in the habitable zone.