Habitable Zone

The habitable zone is the range of distances from a star where a planet's surface could hold liquid water, the key ingredient for life as we know it. Too close and water boils away; too far and it freezes. The zone's location depends on the star's brightness, so it lies much closer in around cool red dwarfs. Sitting in the habitable zone is necessary but not sufficient: a world also needs an atmosphere it can keep, which many red-dwarf planets appear to lose.

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