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astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
s around a
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
within which a
planetary surface A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets (includi ...
can support
liquid water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
given sufficient
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
.J. F. Kasting, D. P. Whitmire, R. T. Reynolds, Icarus 101, 108 (1993). The bounds of the CHZ are based on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's position in the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
and the amount of
radiant energy Radiant may refer to: Computers, software, and video games * Radiant (software), a content management system * GtkRadiant, a level editor created by id Software for their games * Radiant AI, a technology developed by Bethesda Softworks for '' ...
it receives from the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. Due to the importance of liquid water to Earth's
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and
intelligence Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. More generally, it can b ...
. The habitable zone is also called the Goldilocks zone, a
metaphor A metaphor is a figure of speech that, for rhetorical effect, directly refers to one thing by mentioning another. It may provide (or obscure) clarity or identify hidden similarities between two different ideas. Metaphors are often compared wit ...
,
allusion Allusion is a figure of speech, in which an object or circumstance from unrelated context is referred to covertly or indirectly. It is left to the audience to make the direct connection. Where the connection is directly and explicitly stated (as ...
and
antonomasia In rhetoric, antonomasia is a kind of metonymy in which an epithet or phrase takes the place of a proper name, such as "the little corporal" for Napoleon I; or, conversely, the use of a proper name as an archetypal name, to express a generic idea ...
of the children's
fairy tale A fairy tale (alternative names include fairytale, fairy story, magic tale, or wonder tale) is a short story that belongs to the folklore genre. Such stories typically feature magic (paranormal), magic, incantation, enchantments, and mythical ...
of "
Goldilocks and the Three Bears "Goldilocks and the Three Bears" (originally titled "The Story of the Three Bears") is a 19th-century English fairy tale of which three versions exist. The original version of the tale tells of an obscene old woman who enters the forest home ...
", 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 either
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
s or
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, are more massive than Earth, because massive 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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
. About 11 billion of these may be orbiting Sun-like stars.
Proxima Centauri b Proxima Centauri b (or Proxima b), sometimes referred to as Alpha Centauri Cb, is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system ...
, located about 4.2 light-years (1.3
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...
s) from Earth in the constellation of
Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. ...
, 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 The habitability of natural satellites is a measure of their potential to sustain life in favorable circumstances. Habitable environments do not necessarily harbor life. Natural satellite habitability is a new area that is significant to astr ...
, because planetary-mass
moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''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 Extraterrestrial liquid water () is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential fo ...
, 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 Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on
rogue planet A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
s, or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a
solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...
, for example with sodium chlorides in
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
on Earth, chlorides and sulphates on equatorial Mars, or ammoniates, due to its different
colligative properties In chemistry, colligative properties are those properties of solutions that depend on the ratio of the number of solute particles to the number of solvent particles in a solution, and not on the nature of the chemical species present. The number ...
. In addition, other circumstellar zones, where non-water
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s favorable to hypothetical life based on
alternative biochemistries Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metab ...
could exist in liquid form at the surface, have been proposed.


History

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 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The concept was later discussed 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 Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory (1921–1952), and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid variable stars to estim ...
wrote "Liquid Water Belt", which described the same concept 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 concept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in
multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...
s, given the gravitational instabilities of those systems. The concept 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 estimating the number of habitable planets in the Milky Way to be about 600 million. At the same time, science-fiction author
Isaac Asimov yi, יצחק אזימאװ , birth_date = , birth_place = Petrovichi, Russian SFSR , spouse = , relatives = , children = 2 , death_date = , death_place = Manhattan, New York City, U.S. , nationality = Russian (1920–1922)Soviet (192 ...
introduced the concept of a circumstellar habitable zone to the general public through his various explorations of
space colonization Space colonization (also called space settlement or extraterrestrial colonization) is the use of outer space or celestial bodies other than Earth for permanent habitation or as extraterrestrial territory. The inhabitation and territor ...
. The term "
Goldilocks 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.J. F. Ka ...
" 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 James Fraser Kasting (born January 2, 1953) is an American geoscientist and Distinguished Professor of Geosciences at Penn State University. Kasting is active in NASA's search for habitable extrasolar planets. He is considered a world leader i ...
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 concept came in 2000, when astronomers Peter Ward and Donald Brownlee introduced the idea of the "
galactic habitable zone In astrobiology and planetary astrophysics, the galactic habitable zone is the region of a galaxy in which life might most likely develop. The concept of a galactic habitable zone analyzes various factors, such as metallicity (the presence of elem ...
", 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 The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact ra ...
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 Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of life, living organisms currently known on Earth all use carbon compounds for basic structural and m ...
. In 2013, further developments in habitable zone concepts were made with the proposal of a circum ''planetary'' 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. It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment. However, surface conditions are dependent on a host of different individual properties of that planet. This misunderstanding is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant CHZ worlds could host life, different terminology is needed.


Determination

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 can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
s, can have the right temperature for liquid water to exist at the surface.


Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.38 to 10.0
astronomical units The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However, their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to at which water can only exist as vapour. The entire orbits of the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
,
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. At
Hellas Basin Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the third- or fourth-largest known impact crater in the Solar System. The basin floor i ...
, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year. Despite indirect evidence in the form of
seasonal flows on warm Martian slopes Seasonal flows on warm Martian slopes (also called recurring slope lineae, recurrent slope lineae and RSL) are thought to be salty water flows occurring during the warmest months on Mars, or alternatively, dry grains that "flow" downslope of at le ...
, no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, Mars, Vesta and Ceres, suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years. According to extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
, anti-greenhouse or other possible heat sources. For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.


Extrasolar extrapolation

Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at \sqrt, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the CHZ concept is more complex.


Spectral types and star-system characteristics

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain
spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
s. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU. With regard to spectral types, Zoltán Balog proposes that
O-type stars An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, s ...
cannot form planets due to the
photoevaporation Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of mat ...
caused by their strong
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
emissions. Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones. Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of
spectral class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a
tidal locking Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
radius for red dwarfs. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment to life. Others maintain that circumstellar habitable zones are more common, and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking. Astronomy professor Eric Agol argues that even
white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...
s may support a relatively brief habitable zone through planetary migration. At the same time, others have written in similar support of semi-stable, temporary habitable zones around
brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s. Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales.


Stellar evolution

Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years, would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve. Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now, and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase. In order to deal with this increase in luminosity, the concept of a ''continuously habitable zone'' has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region. In red dwarf systems, gigantic
stellar flare The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star. Computer scientists and mathematicians often voc ...
s which could double a star's brightness in minutes and huge
starspot Starspots are stellar phenomena, so-named by analogy with sunspots. Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...
s which can cover 20% of the star's surface area, have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux, so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life. Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously habitable Earth as a red giant. However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU. At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. Given that this new equilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. However, around such a helium-burning star, important life processes like
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. Moreover, as Ramirez and Kaltenegger (2016) showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant. Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage.


Desert planets

A planet's atmospheric conditions influence its ability to retain heat, so that the location of the habitable zone is also specific to each type of planet:
desert planet A desert planet, also known as a dry planet, an arid planet, or a dune planet, is a theoretical type of terrestrial planet with a surface consistency similar to Earth's hot deserts. History A 2011 study suggested that not only are life-sustaini ...
s (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.


Other considerations

A planet cannot have a
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The
origin of water on Earth The origin of water on Earth is the subject of a body of research in the fields of planetary science, astronomy, and astrobiology. Earth is unique among the rocky planets in the Solar System in that it is the only planet known to have oceans of ...
is still not completely understood; possible sources include the result of impacts with icy bodies,
outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...
, mineralization, leakage from
hydrous In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...
minerals from the lithosphere, and photolysis. For an extrasolar system, an icy body from beyond the
frost line The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...
could migrate into the habitable zone of its star, creating an
ocean planet An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as  subsurface oceans, or on the surf ...
with seas hundreds of kilometers deep such as
GJ 1214 b Gliese 1214 b (often shortened to GJ 1214 b) is an exoplanet that orbits the star Gliese 1214, and was discovered in December 2009. Its parent star is 48 light-years from the Sun, in the constellation Ophiuchus. As of 2017, GJ 1214 b is the most ...
or
Kepler-22b Kepler-22b (also known by its Kepler object of interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about from Earth in the constellation of Cygnus. It was discov ...
may be. Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing. Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of
atmospheric escape Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and ...
. In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as a liquid. Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth), a study led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos had eliminated the CO2-H2O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens-of-millions of years. In the case of planets orbiting in the CHZs of red dwarf stars, the extremely close distances to the stars cause
tidal locking Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
, an important factor in habitability. For a tidally locked planet, the
sidereal day Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coor ...
is as long as the
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its
bond albedo The Bond albedo (or ''spheric albedo'' or ''planetary albedo'' or ''bolometric albedo''), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic ra ...
and reducing significantly temperature differences between the two sides. Planetary-mass
natural satellites A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io, but must remain within the
Hill radius The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...
of the planet so that they are not pulled out of the orbit of their host planet. Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability. A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the CHZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation ( cryptobiosis) and/or hyperthermostability.
Tardigrades Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär ...
, for example, can survive in a dehydrated state temperatures between and . Life on a planetary object orbiting outside CHZ might hibernate on the cold side as the planet approaches the
apastron An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any el ...
where the planet is coolest and become active on approach to the
periastron An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...
when the planet is sufficiently warm.


Extrasolar discoveries

A 2015 review concluded that the
exoplanets An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
Kepler-62f Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's ''Ke ...
,
Kepler-186f Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about from Earth. It was the first planet with a radius similar to Earth's to be discovered in the habitable ...
and
Kepler-442b Kepler-442b (also known by its Kepler object of interest designation ''KOI-4742.01'') is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about from Earth in ...
were likely the best candidates for being potentially habitable. These are at a distance of 990, 490 and 1,120
light-years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates,
Tau Ceti e Tau Ceti e, also called 52 Ceti e, is an exoplanet orbiting Tau Ceti that was detected by statistical analyses of the data of the star's variations in radial velocity that were obtained using HIRES, AAPS and HARPS. Its possible properties wer ...
is 11.9 light-years away. It is in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of . Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put ''ηe'', the fraction of stars with planets in the CHZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered. Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. NASA's
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
2011 study, based on observations from the ''
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
'' mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their CHZs.


Early findings

The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant sized, and many in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water. One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the CHZ, such an orbit would causes extreme
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
al effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.
Gliese 876 b Gliese 876 b is an exoplanet orbiting the red dwarf Gliese 876. It completes one orbit in approximately 61 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf. Discovery Gliese 876 b was initiall ...
, discovered in 1998, and
Gliese 876 c Gliese 876 c is an exoplanet orbiting the red dwarf Gliese 876, taking about 30 days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star. Discovery At the time of ...
, discovered in 2001, are both gas giants discovered in the habitable zone around
Gliese 876 Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It is one of the closest known stars to the Sun confirmed to possess a planetary system with more than two planets, after Gliese 1061 ...
that may also have large moons. Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone. Announced on April 4, 2001,
HD 28185 b HD 28185 b is an extrasolar planet 128 light-years away from Earth in the constellation of Eridanus (constellation), Eridanus. The planet was discovered orbiting the Sun-like star HD 28185 in April 2001 as a part of the CORALIE survey for southern ...
is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it is unclear whether such satellites could form in the first place. HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth. The following year, 55 Cancri f was discovered within the CHZ of its host star 55 Cancri A. Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces. Though, in theory, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.


Habitable super-Earths

The 2007 discovery of
Gliese 581 c Gliese 581c (Gl 581c or GJ 581c) is a planet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.5 times that of the Earth, it is classified as a ...
, the first
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for short time. As of 2015, the planet has no newer disconfirmations.
Gliese 581 g Gliese 581g , unofficially known as Zarmina (or Zarmina's World), was a candidate exoplanet postulated to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and was the si ...
, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014, and astronomers are divided about its existence. Discovered in August 2011,
HD 85512 b HD 85512 b is an exoplanet orbiting HD 85512, a K-type main-sequence star approximately 37 light-years from Earth in the constellation of Vela. Due to its mass of at least 3.6 times the mass of Earth, HD 85512 b is classified as a rocky Ea ...
was initially speculated to be habitable, but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.
Kepler-22 b Kepler-22b (also known by its Kepler object of interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about from Earth in the constellation of Cygnus. It was discove ...
, discovered in December 2011 by the ''Kepler'' space probe, is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet.: "If it epler-22bhad a similar composition to Earth, then we're looking at a world in excess of about 40 Earth masses".
Gliese 667 Cc Gliese 667 Cc (also known as GJ 667Cc, HR 6426Cc, or HD 156384Cc) is an exoplanet orbiting within the habitable zone of the red dwarf star Gliese 667 C, which is a member of the Gliese 667 triple star system, approximately away in the constella ...
, discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of
Gliese 667 C Gliese 667 (142 G. Scorpii) is a triple- star system in the constellation Scorpius lying at a distance of about from Earth. All three of the stars have masses smaller than the Sun. There is a 12th-magnitude star close to the other three ...
. It is one of the most Earth-like planets known.
Gliese 163 c Gliese 163 c () or Gl 163 c is a potentially habitable exoplanet, orbiting within the habitable zone of M dwarf star Gliese 163. The parent star is 15.0 parsecs (approximately 49 light-years, or 465 trillion kilometers) from the Sun, in the c ...
, discovered in September 2012 in orbit around the red dwarf
Gliese 163 Gliese 163 is a faint red dwarf star with multiple exoplanetary companions in the southern constellation of Dorado. Other stellar catalog names for it include HIP 19394 and LHS 188. It is too faint to be visible to the naked eye, havin ...
is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about ° C.
HD 40307 g HD 40307 g is an exoplanet candidate suspected to be orbiting in the habitable zone of HD 40307. It is located 42 light-years away in the direction of the southern constellation Pictor. The planet was discovered by the radial velocity method, ...
, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of
HD 40307 HD 40307 is an orange ( K-type) main-sequence star located approximately 42 light-years away in the constellation of Pictor (the Easel), taking its primary name from its Henry Draper Catalogue designation. It is calculated to be sligh ...
. In December 2012,
Tau Ceti e Tau Ceti e, also called 52 Ceti e, is an exoplanet orbiting Tau Ceti that was detected by statistical analyses of the data of the star's variations in radial velocity that were obtained using HIRES, AAPS and HARPS. Its possible properties wer ...
and Tau Ceti f were found in the circumstellar habitable zone of
Tau Ceti Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under from the Solar System, it is a rela ...
, a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone; however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable.


Near Earth-sized planets and Solar analogs

Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. A genuinely Earth-like planet – an
Earth analog An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial pl ...
or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology. A
solar analog Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type ...
(or "solar twin") is a star that resembles the Sun. To date, no solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1%
solar luminosity The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal ...
variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation. Using data collected by NASA's ''Kepler'' Space observatory and the
W. M. Keck Observatory The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when comp ...
, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their
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.J. F. Kast ...
. On 7 January 2013, astronomers from the ''Kepler'' team announced the discovery of
Kepler-69c Kepler-69c (also known by its Kepler Object of Interest designation ''KOI-172.02'') is a confirmed super-Earth extrasolar planet, likely rocky, orbiting the Sun-like star Kepler-69, the outermore of two such planets discovered by NASA's ''Keple ...
(formerly ''KOI-172.02''), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting
Kepler-69 Kepler-69 (''KOI-172'', ''2MASS J19330262+4452080'', KIC 8692861') is a G-type main-sequence star similar to the Sun in the constellation Cygnus, located about from Earth. On April 18, 2013 it was announced that the star has two planet ...
, a star similar to our Sun, in the CHZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of
Kepler-62 Kepler-62 is a K-type main sequence star cooler and smaller than the Sun, located roughly 990 light years from Earth in the constellation Lyra. It resides within the field of vision of the ''Kepler'' spacecraft, the satellite that NASA ...
, by the Kepler team was announced on April 19, 2013. The planets, named
Kepler-62e Kepler-62e (also known by its Kepler Object of Interest designation KOI-701.03) is a super-Earth exoplanet (extrasolar planet) discovered orbiting within the habitable zone of Kepler-62, the second outermost of five such planets discovered by NAS ...
and
Kepler-62f Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's ''Ke ...
, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively. With a radius estimated at 1.1 Earth,
Kepler-186f Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about from Earth. It was the first planet with a radius similar to Earth's to be discovered in the habitable ...
, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method though its mass remains unknown and its parent star is not a Solar analog. Kapteyn b, discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 earth radii was found orbiting the habitable zone of the red subdwarf
Kapteyn's Star Kapteyn's Star is a class M1 red subdwarf about 12.83 light-years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With a magnitude of nearly 9 it is visible through binoculars or a te ...
, 12.8 light-years away. On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the ''Kepler'' Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s: two of the three,
Kepler-438b Kepler-438b (also known by its Kepler Object of Interest designation ''KOI-3284.01'') is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 472.9 light ...
and
Kepler-442b Kepler-442b (also known by its Kepler object of interest designation ''KOI-4742.01'') is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about from Earth in ...
, are near-Earth-size and likely
rocky ''Rocky'' is a 1976 American sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the ''Rocky'' franchise and stars Talia Shire, Burt Young, Carl Weathers, and Burge ...
; the third,
Kepler-440b Kepler-440b (also known by its Kepler Object of Interest designation KOI-4087.01) is a confirmed super-Earth exoplanet orbiting within the habitable zone of Kepler-440, about from Earth. The planet was discovered by NASA's Kepler spacecraft u ...
, is a
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
. However,
Kepler-438b Kepler-438b (also known by its Kepler Object of Interest designation ''KOI-3284.01'') is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 472.9 light ...
is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January,
K2-3d K2-3d, also known as EPIC 201367065 d, is a confirmed exoplanet of probable mini-Neptune type orbiting the red dwarf star K2-3, and the outermost of three such planets discovered in the system. It is located away from Earth in the constellat ...
a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth.
Kepler-452b Kepler-452b (sometimes quoted to be an ''Earth 2.0'' or ''Earth's Cousin'' based on its characteristics; also known by its Kepler Object of Interest designation ''KOI-7016.01'') is a super-Earth exoplanet orbiting within the inner edge of the h ...
, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star
Kepler-452 Extrasolar PlanetsEncyclopaedia Kepler-452 is a G-type main-sequence star located about 1,800 light-years away from Earth in the Cygnus constellation. Although similar in temperature to the Sun, it is 20% brighter, 3.7% more massive ...
. The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016. The discovery is considered significant because it dramatically increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets. Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf
K2-72 K2-72 (also designated EPIC 206209135) is a cool red dwarf star of spectral class M2.7V located about away from the Earth in the constellation of Aquarius. It is known to host four planets, all similar in size to Earth, with one of them re ...
around 227 light year from the Sun:
K2-72c K2-72c is a small exoplanet orbiting around the red dwarf star K2-72 approximately 227.7 light-years away. It is located in the inner edge of the habitable zone. K2-72c completes an orbit in 15.2 days, and it has a radius of only 86% of that of th ...
and
K2-72e K2-72e (also known by its EPIC designation ''EPIC 206209135.04''), is a confirmed exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star K2-72, the outermost of four such planets discovered in the system by NASA's ''Kepl ...
are both of similar size to Earth and receive similar amounts of stellar radiation. Announced on the 20 April 2017,
LHS 1140b LHS 1140 b is a massive, dense rocky planet orbiting within the conservative habitable zone of the red dwarf LHS 1140. Discovered in 2017 by the MEarth Project, LHS 1140 b is nearly 7 times the mass of Earth and over 60% larger in radius, putting ...
is a super-dense
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs. The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied. Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away. At 11 light-years away, a second closest planet,
Ross 128 b Ross 128 b is a confirmed Earth-sized exoplanet, likely rocky, orbiting within the inner habitable zone of the red dwarf Ross 128, at a distance of about 11 light-years from Earth. The exoplanet was found using a decade's worth of radial veloci ...
, was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 times Earth's mass, is it roughly Earth-sized and likely rocky in composition. Discovered in March 2018,
K2-155d K2-155d is a potentially habitable Super-Earth exoplanet in the K2-155 system. It is the outermost of three known planets orbiting around the K-type star K2-155 in the constellation Taurus. It is one of 15 new exoplanets around red dwarf stars ...
is about 1.64 time the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away. One of the earliest discoveries by the
Transiting Exoplanet Survey Satellite Transiting Exoplanet Survey Satellite (TESS, Explorer 95 or MIDEX-7) is a space telescope for NASA's Explorer program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the ''Kepler ...
(TESS) announced July 31, 2019 is a Super Earth planet
GJ 357 d Gliese 357 d is an exoplanet, considered to be a "Super-Earth" within the circumstellar habitable zone of its parent star. The planet orbits GJ 357, 31 light-years from the Solar System, The system is part of the Hydra constellation. The pla ...
orbiting the outer edge of a red dwarf 31 light years away.
K2-18b K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located away from Earth. The planet, initially discovered with the Kepler space telescope, is about eight times the mass of Earth, and is thus classified as ...
is an exoplanet 124 light-years away, orbiting in the habitable zone of the
K2-18 K2-18, also known as EPIC 201912552, is a red dwarf star located from Earth, in the constellation of Leo. Planetary system The star has an exoplanet, called K2-18b, a super-Earth located within the habitable zone In astronomy and ...
, a red dwarf. This planet is significant for water vapour found in its atmosphere; this was announced on September 17, 2019. In September 2020, astronomers identified 24
superhabitable planet A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong, who have criticized the l ...
(planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
.


Habitability outside the CHZ

Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moons Titan and
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
and
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's moons
Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliff ...
and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in
subsurface ocean Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of undergr ...
s. Outside the CHZ,
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
and radioactive decay are two possible heat sources that could contribute to the existence of liquid water. Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on
rogue planet A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
s as a result of radioactive decay-based heating and insulation by a thick surface layer of ice. With some theorising that life on Earth may have actually originated in stable, subsurface habitats, it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'. Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface. Another possibility is that outside the CHZ organisms may use
alternative biochemistries Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metab ...
that do not require water at all. Astrobiologist Christopher McKay, has suggested that
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
() may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on from the star. This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife. In addition, testing of a number of organisms has found some are capable of surviving in extra-CHZ conditions.


Significance for complex and intelligent life

The
Rare Earth hypothesis In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligenc ...
argues that complex and intelligent life is uncommon and that the CHZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a CHZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics) and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation). But others, such as Ian Stewart and Jack Cohen in their 2002 book ''Evolving the Alien'' argue that complex intelligent life may arise outside the CHZ. Intelligent life outside the CHZ may have evolved in subsurface environments, from alternative biochemistries or even from nuclear reactions. On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvental ecosystems, supporting large complex life forms such as ''Riftia pachyptila''. Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone. List of microorganisms tested in outer space, Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the ''Milnesium tardigradum'', which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. In addition, the lichens ''Rhizocarpon geographicum'' and ''Xanthoria elegans'' have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. The fungi ''Cryomyces antarcticus'' and ''Cryomyces minteri'' are also able to survive and reproduce in Mars-like conditions. Species, including humans, known to possess animal cognition require large amounts of energy, and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true
Earth analog An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial pl ...
s in the CHZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits or space habitat analogues on the surface may be eliminated, and complex Earth life can thrive. Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe. The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter , which is the average number of planetary-mass objects orbiting within the CHZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican principle, Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe. A 1971 NASA report by Drake and Bernard M. Oliver, Bernard Oliver proposed the "Water hole (radio), water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterhole searches and the Allen Telescope Array now extends Project Phoenix (SETI), Project Phoenix to such candidates. Because the CHZ is considered the most likely habitat for intelligent life, active SETI, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and the 2003 Cosmic Call, Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the CHZ. The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its CHZ. A Message from Earth in 2008, and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.


References


External links

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Interstellar Real Estate: Location, Location, Location – Defining the Habitable Zone
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