The habitable zone is the region around a star where liquid water can exist on a planet's surface. It is not too hot, not too cold. It is sometimes called the Goldilocks zone. The concept is central to the search for life beyond Earth.

The zone's distance from the star depends on the star's luminosity. A dim red dwarf has a habitable zone close in. A bright blue star has one far out. The zone is not a sharp boundary. It is a range, and its edges depend on assumptions about the planet's atmosphere. A thick atmosphere can trap heat and push the inner edge outward. A thin one can let heat escape and push the outer edge inward.

Being in the habitable zone is not enough. The planet needs the right mass to hold an atmosphere. It needs a magnetic field to deflect stellar radiation. It needs water, carbon, and other elements. It needs a stable climate over billions of years. The habitable zone is a starting point, not a guarantee.

Astronomers have found potentially habitable planets around nearby stars. Proxima Centauri b, TRAPPIST-1e, and Kepler-452b are examples. Whether any of them actually harbor life is unknown. The search continues.