Habitable zone

In astronomy and astrobiology, the habitable zone (HZ), also known as the circumstellar habitable zone (CHZ), or as the Goldilocks zone, is the range of orbits around a star within which a planetary surface could potentially support liquid water. Liquid water is considered by many scientists as necessary for a planet to be habitable. The range depends upon the brightness of the star interacting with a planet's atmosphere. At the inner edge the star's light, trapped by greenhouse gases in a planet's atmosphere, boils off the planet's water. At the outer edge light from the star is insufficient even with help from atmospheric gas and the planet's water freezes. Many other factors are added in various habitable zone models. The habitable zone has become a key tool in the search for habitable planets because discoveries of exoplanets yield approximate orbital radii.

The alternative name, Goldilocks zone, is a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".

Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sizedplanets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.

The classical habitable zone concept was defined only for planetary surfaces where habitat depends on stellar energy. Deep biospheres are known on Earth, but they would not be detectable on exoplanets. Other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.