Circumstellar habitable zone

In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around 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. In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around 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. The habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale of 'Goldilocks and the Three Bears', in which a little girl chooses from sets of three items, ignoring 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 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. In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist within in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars, or ammoniates, due to its different colligative properties. In addition, 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. An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4). The concept of a circumstellar habitable zone was first introduced in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars, coined the term 'ecosphere' and referred to various 'zones' in which life could emerge. In the same year, Harlow Shapley wrote 'Liquid Water Belt', which described the same theory in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist, first introduced the term 'habitable zone' in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. A major early contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems. The theory of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of circumstellar habitable zone as well as various other determinants of planetary habitability, eventually guestimating the number of habitable planets in the Milky Way to be about 600 million. At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization. The term 'Goldilocks zone' emerged in the 1970s, referencing specifically a region around a star whose temperature is 'just right' for water to be present in the liquid phase. In 1993, astronomer James Kasting introduced the term 'circumstellar habitable zone' to refer more precisely to the region then (and still) known as the habitable zone. Kasting was the first to present a detailed model for the habitable zone for exoplanets. An update to habitable zone theory came in 2000, when astronomers Peter Ward and Donald Brownlee introduced the idea of the 'galactic habitable zone', which they later developed with Guillermo Gonzalez. The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers. Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry. In 2013, further developments in habitable zone theory were made with the proposal of a circumplanetary habitable zone, also known as the 'habitable edge', to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away. Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a 'conservative habitable zone' in which lower-mass planets like Earth or Venus can remain habitable, complemented by a larger 'extended habitable zone' in which super-Earth planets, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.