Habitable zone

The range of distances from a star where a planet with suitable atmospheric conditions could maintain liquid water on its surface. Its boundaries depend on the star and the planet’s climate.

Stellar luminosity: The total energy a star emits per unit time. A star’s energy output largely determines how far its habitable zone extends.
Exoplanet: A planet orbiting a star other than the Sun. Habitable-zone estimates are commonly used to assess exoplanets.
Circumstellar habitable zone: The region around a star where planetary conditions could permit surface liquid water. This narrower orbital definition is often mistaken for proof that a planet is habitable.
Tidal locking: A state in which an orbiting body rotates once per orbit, keeping one side facing its companion. Many planets around small stars may be tidally locked, complicating estimates of their climate.
Astrobiology: The study of life’s origin, evolution, distribution, and future in the universe. The zone is one tool astrobiology uses to select worlds for investigation.
Planetary albedo: The fraction of incoming radiation a planet reflects back into space. Reflection changes how much stellar energy remains available to warm a planet.
Kepler-186f: An Earth-sized exoplanet orbiting within the habitable zone of the red dwarf Kepler-186. It is a well-known example of a small planet found within a star’s estimated zone.
Planetary habitability: The capacity of a world to support life under its environmental conditions. Habitability depends on more than whether an orbit falls inside the zone.
M dwarf: A small, cool, long-lived star on the main sequence. Its close-in habitable zone exposes planets to stellar activity and tidal effects.
Liquid water: Water in its liquid state under a given set of temperature and pressure conditions. The zone is defined around the possibility of this substance at a planet’s surface.

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