In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbitsaround a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.
The bounds of the CHZ are based on Earth’s position in the Solar System and the amount of radiant energy it receives from the Sun.
Due to the importance of liquid water to Earth’s biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of Earth-like extraterrestrial life and intelligence.
Since the concept was first presented in 1953, many stars have been confirmed to possess a CHZ planet, including some systems that consist of multiple CHZ planets. Most such planets, being super-Earths or gas giants, are more massive than Earth, because such planets are easier to detect.
On November 4, 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.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 CHZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the CHZ might outnumber planets.
Stars that are smaller, cooler and lower mass than the Sun (M-dwarfs) have their habitable zone much closer to the star than the Sun (G-dwarf). Stars that are larger, hotter and more massive than the Sun (A-dwarfs) have their habitable zone much farther out from the star.