An example of a system based on stellar luminosityfor predicting the location of the habitable zonearound various types of stars. Planet sizes, starsizes, orbit lengths, and habitable zone sizes are notto scale.

Circumstellar habitable zoneFrom Wikipedia, the free encyclopedia

In astronomy and astrobiology, the circumstellar habitablezone (CHZ), or simply the habitable zone, is the range oforbits around a star within which a planetary surface cansupport liquid water given sufficient atmospheric pressure.[1][2]The bounds of the CHZ are based on Earth's position in theSolar System and the amount of radiant energy it receives fromthe Sun. Due to the importance of liquid water to Earth'sbiosphere, the nature of the CHZ and the objects within it maybe instrumental in determining the scope and distribution ofEarth­like extraterrestrial life and intelligence.

The habitable zone is also called the Goldilocks zone, ametaphor of the children's fairy tale of "Goldilocks and theThree Bears", in which a little girl chooses from sets of threeitems, ignoring the ones that are too extreme (large or small,hot or cold, etc.), and settling on the one in the middle, whichis "just right".

Since the concept was first presented in 1953,[3] many stars have been confirmed to possess a CHZ planet,including some systems that consist of multiple CHZ planets.[4] Most such planets, being super­Earths or gasgiants, are more massive than Earth, because such planets are easier to detect. On November 4, 2013, astronomersreported, based on Kepler data, that there could be as many as 40 billion Earth­sized planets orbiting in thehabitable zones of Sun­like stars and red dwarfs in the Milky Way.[5][6] 11 billion of these may be orbiting Sun­likestars.[7] Proxima Centauri b, located about 4.2 light­years (1.3 parsecs) from Earth in the constellation ofCentaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The CHZ is also ofparticular interest to the emerging field of habitability of natural satellites, because planetary­mass moons in theCHZ might outnumber planets.[8]

In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept isstill evolving.[9] Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are nowthought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that existindependently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid waterknown to exist within in lithospheres and asthenospheres of the Solar System.[10] Sustained by other energysources, such as tidal heating[11][12] or radioactive decay[13] or pressurized by non­atmospheric means, liquid watermay be found even on rogue planets, or their moons.[14] Liquid water can also exist at a wider range oftemperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides andsulphates on equatorial Mars,[15] or ammoniates,[16] due to its different colligative properties. In addition, othercircumstellar zones, where non­water solvents favorable to hypothetical life based on alternative biochemistriescould exist in liquid form at the surface, have been proposed.[17]

Contents

1 History

2 Determination of the circumstellar habitable zone

2 Determination of the circumstellar habitable zone2.1 Solar System estimates2.2 Extrasolar extrapolation

2.2.1 Spectral types and star­systemcharacteristics

2.2.2 Stellar evolution2.2.3 Desert planets2.2.4 Other considerations

3 Extrasolar discoveries3.1 Early findings3.2 Habitable super­Earths3.3 Earth­sized planets and Solar analogs

4 Habitability outside the CHZ5 Significance for complex and intelligent life6 See also7 References8 External links

History

The concept of a circumstellar habitable zone was first introduced in 1953 by Hubertus Strughold, who in histreatise 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.[3][18] In the same year, Harlow Shapleywrote "Liquid Water Belt", which described the same theory in further scientific detail. Both works stressed theimportance of liquid water to life.[19] Su­Shu 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 largebody, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.[20][21] A majorearly contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and byextension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities ofthose systems.[22]

The theory of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets forMan, in which he covered the circumstellar habitable zone itself as well as various other determinants of planetaryhabitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million.[23] Atthe same time, science­fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to thegeneral public through his various explorations of space colonization.[24] The term "Goldilocks zone" emerged inthe 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be presentin the liquid phase.[25] In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" torefer more precisely to the region then (and still) known as the habitable zone.[20]

An update to habitable­zone theory came in 2000, when astronomers Peter Ward and Donald Brownlee introducedthe idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.[26][27] The galactichabitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regionsclose enough to a galactic center that stars there are enriched with heavier elements, but not so close that starsystems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation andenormous gravitational forces commonly found at galactic centers.[26]

The range of published estimates forthe extent of the Sun's CHZ. Theconservative CHZ[23] is indicated bya dark­green band crossing the inneredge of the aphelion of Venus,whereas an extended CHZ,[29]

extending to the orbit of the dwarfplanet Ceres, is indicated by a light­green band.

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 thatuse an alternative biochemistry.[17] In 2013, further developments in habitable zone theory were made with theproposal of a circumplanetary habitable zone, also known as the "habitable edge", to encompass the region arounda planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from theplanet would not cause liquid water to boil away.[28]

Determination of the circumstellar habitable zone

Whether a body is in the circumstellar habitable zone of its host star isdependent on the radius of the planet's orbit (for natural satellites, the hostplanet's orbit), the mass of the body itself, and the radiative flux of the hoststar. Given the large spread in the masses of planets within a circumstellarhabitable zone, coupled with the discovery of super­Earth planets whichcan 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 lower­mass planets like Earth orVenus can remain habitable, complemented by a larger "extended habitablezone" in which super­Earth planets, with stronger greenhouse effects, canhave the right temperature for liquid water to exist at the surface.[30]

The inner edge of the HZ is the distance where runaway greenhouse effectvaporize the whole water reservoir and,[9] as a second effect, induce thephotodissociation of water vapor and the loss of hydrogen to space. Theouter edge of the HZ is the distance from the star where adding morecarbon dioxide to the atmosphere fails to keep the surface of the planetabove the freezing point.[9]

Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.5 to 3.0 astronomical units,[31] thougharriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbitwithin, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing pointof water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches theinner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a stronggreenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapour.[32] Theentire orbits of the Moon,[33] Mars,[34] and numerous asteroids also lie within various estimates of the habitablezone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure andtemperature sufficient for water to, if present, exist in liquid form for short periods.[35] At Hellas Basin, forexample, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (around the triple pointfor water) for 70 days in the Martian year.[35] Despite indirect evidence in the form of seasonal flows on warmMartian slopes,[36][37][38][39] no confirmation has been made of the presence of liquid water there. While otherobjects orbit partly within this zone, including comets, Ceres[40] is the only one of planetary mass. A combinationof low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossiblefor these bodies to sustain liquid water on their surface. Most estimates, therefore, are inferred from the effect thata repositioned orbit would have on the habitability of Earth or Venus.

According to extended habitable zone theory, planetary mass objects with atmospheres capable of inducingsufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include thosewhose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive thanEarth (super­Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. Thereare no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheresof these kinds of extrasolar objects, and the net temperature effect of such atmospheres including induced albedo,anti­greenhouse or other possible heat sources cannot be determined by their position in the habitable zone.

Estimates of the circumstellar habitable zone boundaries of the Solar SystemInneredge(AU)

Outeredge(AU)

Year Notes

0.725 1.24 Dole1964[23]

Used optically thin atmospheres and fixed albedos. Places the aphelion of Venusjust inside the zone.

1.385–1.398

Budyko1969[41]

Based on studies of ice albedo feedback models to determine the point at whichEarth would experience global glaciation. This estimate was supported in studiesby Sellers 1969[42] and North 1975.[43]

0.88–0.912

Rasool andDe Bergh1970[44]

Based on studies of Venus's atmosphere, Rasool and De Bergh concluded thatthis is the minimum distance at which Earth would have formed stable oceans.

0.95 1.01Hart et al.1979[45]

Based on computer modelling and simulations of the evolution of Earth'satmospheric composition and surface temperature. This estimate has often beencited by subsequent publications.

3.0 Fogg1992[29]

Used the carbon cycle to estimate the outer edge of the circumstellar habitablezone.

1.37 Kasting etal. 1993[20]

Noted the cooling effect of cloud albedo.

2.0 Spiegel etal. 2010[46]

Proposed that seasonal liquid water is possible to this limit when combining highobliquity and orbital eccentricity.

0.75 Abe et al.2011[47]

Found that land­dominated "desert planets" with water at the poles could existcloser to the Sun than watery planets like Earth.

0.77—0.87

1.02—1.18

Vladilo etal. 2013[48]

Inner edge of circumstellar habitable zone is closer and outer edge is farther forhigher atmospheric pressures; determined minimum atmospheric pressurerequired to be 15 millibar.

0.99 1.688Kopparapuet al.2013[1]

Revised estimates using updated runaway greenhouse and water loss algorithms.According to this measure Earth is at the inner edge of the HZ and close to, butjust outside, the runaway greenhouse limit. This applies to a planet with Earth­like atmospheric composition and pressure.

0.5Zsom et al.2013[49]

Estimate based on various possible combinations of atmospheric composition,pressure and relative humidity of the planet's atmosphere.

Extrasolar extrapolation

Astronomers use stellar flux and the inverse­square law to extrapolate circumstellar habitable zone models createdfor the Solar System to other stars. For example, although the Solar System has a circumstellar habitable zonecentered at 1.34 AU from the Sun,[1] a star with 0.25 times the luminosity of the Sun would have a habitable zone

centered at , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating

A video explaining the significance of the 2011discovery of a planet in the circumbinary habitablezone of Kepler­47.

Natural defenses against spaceweather, such as the magnetospheredepicted in this artistic rendition, maybe required for planets to sustainsurface water for prolonged periods.

centered at , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicatingfactors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation ofthe CHZ concept is more complex.

Spectral types and star­system characteristics

Some scientists argue that the concept of a circumstellarhabitable zone is actually limited to stars in certain types ofsystems or of certain spectral types. Binary systems, forexample, have circumstellar habitable zones that differ fromthose of single­star planetary systems, in addition to the orbitalstability concerns inherent with a three­body configuration.[50]If the Solar System were such a binary system, the outer limitsof the resulting circumstellar habitable zone could extend as faras 2.4 AU.[51][52]

With regard to spectral types, Zoltán Balog proposes that O­type stars cannot form planets due to the photoevaporationcaused by their strong ultraviolet emissions.[53] Studyingultraviolet emissions, Andrea Buccino found that only 40% ofstars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.[54] Stars smaller thanthe Sun, on the other hand, have distinct impediments to habitability. Michael Hart, for example, proposed thatonly main­sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved inmodern times into the concept of a tidal locking radius for red dwarfs. Within this radius, which is coincidentalwith the red­dwarf 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.[55]

Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water toexist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars cansupport planets with relatively constant temperatures over their surfaces in spite of tidal locking.[56] Astronomyprofessor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetarymigration.[57] At the same time, others have written in similar support of semi­stable, temporary habitable zonesaround brown dwarfs.[55]

Stellar evolution

Circumstellar habitable zones change over time with stellar evolution. Forexample, hot O­type stars, which may remain on the main sequence forfewer than 10 million years,[58] would have rapidly changing habitablezones not conducive to the development of life. Red dwarf stars, on theother hand, which can live for hundreds of billions of years on the mainsequence, would have planets with ample time for life to develop andevolve.[59][60] Even while stars are on the main sequence, though, theirenergy output steadily increases, pushing their habitable zones farther out;our Sun, for example, was 75% as bright in the Archaean as it is now,[61]and in the future, continued increases in energy output will put Earthoutside the Sun's habitable zone, even before it reaches the red giantphase.[62] In order to deal with this increase in luminosity, the concept of a

Earth's hydrosphere. Water covers71% of Earth's surface, with theglobal ocean accounting for 97.3% ofthe water distribution on Earth.

continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is aregion around a star in which planetary­mass bodies can sustain liquid water for a given period of time. Like thegeneral circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative andextended region.[62]

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes[63] and huge starspotswhich can cover 20% of the star's surface area,[64] have the potential to strip an otherwise habitable planet of itsatmosphere and water.[65] As with more massive stars, though, stellar evolution changes their nature and energyflux,[66] so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for thedevelopment of life.[65][67]

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will changedramatically from its main­sequence size.[68] For example, the Sun is expected to engulf the previously­habitableEarth as a red giant.[69] However, once a red giant star reaches the horizontal branch, it achieves a new equilibriumand can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.[70]

At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense.[71] Given that this newequilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the SolarSystem at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants.[70]However, around such a helium­burning star, important life processes like photosynthesis could only happenaround planets where the atmosphere has been artificially seeded with carbon dioxide, as by the time a solar­massstar becomes a red giant, planetary­mass bodies would have already absorbed much of their free carbon dioxide.[72]

Desert planets

A planet's atmospheric conditions influence its ability to retain heat, so that the location of the habitable zone isalso specific to each type of planet: desert planets (also known as dry planets), with very little water, will have lesswater vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planetcould maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is lessice to reflect heat into space, so the outer edge of desert­planet habitable zones is further out.[73][74]

Other considerations

A planet cannot have a hydrosphere—a key ingredient for the formation ofcarbon­based life—unless there is a source for water within its stellarsystem. The origin of water on Earth is still not completely understood;possible sources include the result of impacts with icy bodies, outgassing,mineralization, leakage from hydrous minerals from the lithosphere, andphotolysis.[75][76] For an extrasolar system, an icy body from beyond thefrost line could migrate into the habitable zone of its star, creating an oceanplanet with seas hundreds of kilometers deep[77] such as GJ 1214 b[78][79] orKepler­22b may be.[80]

Maintenance of liquid surface water also requires a sufficiently thickatmosphere. Possible origins of terrestrial atmospheres are currentlytheorised to outgassing, impact degassing and ingassing.[81] Atmospheres are thought to be maintained throughsimilar processes along with biogeochemical cycles and the mitigation of atmospheric escape.[82] In a 2013 studyled by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased

Artist's concept of a planet on aneccentric orbit that passes through theCHZ for only part of its orbit

with greater atmospheric pressure.[48] Below an atmospheric pressure of about 15 millibars, it was found thathabitability could not be maintained[48] because even a small shift in pressure or temperature could render waterunable to form a liquid.[83]

In the case of planets orbiting in the CHZs of red dwarf stars, the extremely close distances to the stars cause tidallocking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbitalperiod, causing one side to permanently face the host star and the other side to face away. In the past, such tidallocking was thought to cause extreme heat on the star­facing side and bitter cold on the opposite side, makingmany red dwarf planets uninhabitable; however, three­dimensional climate models in 2013, showed that the side ofa red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducingsignificantly temperature differences between the two sides.[56]

Planetary­mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfilladditional parameters, in particular being located within the circumplanetary habitable zones of their hostplanets.[28] More specifically, moons need to be far enough from their host giant planets that they are nottransformed by tidal heating into volcanic worlds like Io,[28] but must still remain within the Hill radius of theplanet so that they are not pulled out of orbit of their host planet.[84] Red dwarfs that have masses less than 20% ofthat of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitablezone would put a habitable moon so close to the star that it would be stripped from its host planet. In such asystem, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as toeliminate any prospects of habitability.[28]

A planetary object that orbits a star with high orbital eccentricity mayspend only some of its year in the CHZ and experience a large variation intemperature and atmospheric pressure. This would result in dramaticseasonal phase shifts where liquid water may exist only intermittently. It ispossible that subsurface habitats could be insulated from such changes andthat extremophiles on or near the surface might survive through adaptionssuch as hibernation (cryptobiosis) and/or hyperthermostability. Tardigrades,for example, can survive in a dehydrated state temperatures between0.150 K (−273 °C)[85] and 424 K (151 °C).[86] Life on a planetary objectorbiting outside CHZ might hibernate on the cold side as the planetapproaches the apastron where the planet is coolest and become active onapproach to the periastron when the planet is sufficiently warm.[87]

Extrasolar discoveries

Among exoplanets, a review in 2015 came to the conclusion that Kepler­62f, Kepler­186f and Kepler­442b werelikely the best candidates for being potentially habitable.[88] These are at a distance of 1200, 490 and 1,120 light­years away, respectively. Of these, Kepler­186f is similar in size to Earth with a 1.2­Earth­radius measure, and it islocated towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanetcandidates, Tau Ceti e is 11.9 light­years away. It is in the inner edge of its solar system's habitable zone, giving itan estimated average surface temperature of 68 °C (154 °F).[89]

Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zonetend 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,[1] meaning that there may be roughly 95–180 billion habitable planets in the

The habitable zone of Gliese 581compared with our Solar System'shabitable zone.

Milky Way.[90] However, this is merely a statistical prediction; only a small fraction of these possible planets haveyet been discovered.[91]

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500million habitable planets in the Milky Way.[92] NASA's Jet Propulsion Laboratory 2011 study, based onobservations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" ofall stars of spectral class F, G, and K are expected to have planets in their CHZs.[93][94]

Early findings

The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planetswere discovered. One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due toit being neither "too hot" nor "too cold." Later study revealed temperatures analogous to Venus, ruling out anypotential for liquid water.[95] 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that causesextreme seasonal effects on the planet's surface. In spite of this, simulations have suggested that it is possible for aterrestrial natural satellite to support water at its surface year­round.[96]

Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in thehabitable zone around Gliese 876. Although they are not thought to possess significant water at their surfaces, bothmay have habitable moons.[97] Upsilon Andromedae d, discovered in 1999, is a gas giant in its star's circumstellarhabitable zone considered to be large enough to favor the formation of large, Earth­like moons.[98]

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellarhabitable zone[99] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.[100] Tidalinteractions suggest that HD 28185 b could harbor habitable Earth­mass satellites in orbit around it for manybillions of years,[101] though it is unclear whether such satellites could form in the first place.[102]

HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellarhabitable zone of HD 69830, 41 light years away from Earth.[103] The following year, 55 Cancri f was discoveredwithin the CHZ of its host star 55 Cancri A.[104][105] Although conditions on this massive and dense planet are notconducive to the formation of water or life as we know it, a hypothetical moon of this planet with the proper massand composition could be able to support liquid water at its surface.[106]

Habitable super­Earths

The 2007 discovery of Gliese 581 c, the first super­Earth in thecircumstellar habitable zone, created significant interest in the system bythe scientific community, although the planet was later found to haveextreme surface conditions that may resemble Venus.[107] Gliese 581 d,another planet in the same system and thought to be a better candidate forhabitability, was also announced in 2007. Its existence was laterdisconfirmed in 2014. Gliese 581 g, yet another planet thought to have beendiscovered in the circumstellar habitable zone of the system, wasconsidered to be more habitable than both Gliese 581 c and d. However, itsexistence was also disconfirmed in 2014.[108]

A diagram comparing size (artist'simpression) and orbital position ofplanet Kepler­22b within Sun­likestar Kepler 22's habitable zone andthat of Earth in the Solar System

Comparison of the CHZ position ofEarth­radius planet Kepler­186f andthe Solar System (17 April 2014)

Discovered in August 2011, HD 85512 b was initially speculated to behabitable,[109] but the new circumstellar­habitable­zone criteria devised byKopparapu et al. in 2013 place the planet outside the circumstellarhabitable zone.[91] With an increase in the frequency of exoplanetdiscovery, the Earth Similarity Index was devised in October 2011 as a wayof comparing planetary properties, such as surface temperature and density,to those of Earth in order to better gauge the habitability of extrasolarbodies.[110]

Kepler­22 b, discovered in December 2011 by the Kepler space probe,[111]is the first transiting exoplanet discovered around a Sun­like star. With aradius 2.4 times that of Earth, Kepler­22b has been predicted by some to bean ocean planet.[112] Gliese 667 Cc, discovered in 2011 but announced in2012,[113] is a super­Earth orbiting in the circumstellar habitable zone ofGliese 667 C. Subsequently in June 2013, two other habitable super­Earthsorbiting the same star, Gliese 667 Cf and Gliese 667 Ce, were discovered in

the CHZ.[114]

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163[115] is located 49 light yearsfrom Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percentmore stellar radiation than Earth, leading to surface temperatures of about 60° C.[116][117][118] HD 40307 g, acandidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.[119]In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun­likestar 12 light years away.[120] Although more massive than Earth, they are among the least massive planets found todate orbiting in the habitable zone;[121] however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellarhabitable zone criteria established by the 2013 Kopparapu study.[122]

Earth­sized planets and Solar analogs

Recent discoveries have uncovered planets that are thought to be similar insize or mass to Earth. While there is no universal definition of "Earth­sized", ranges are typically defined by mass. The lower range used in manydefinitions of the super­Earth class is 1.9 Earth masses, likewise, sub­Earths range up to the size of Venus (~0.815 Earth masses). An upper limitof 1.5 Earth radii is also considered, given that above 1.5 R⊕ the averageplanet density rapidly decreases with increasing radius, indicating that theseplanets have a large fraction of volatiles by volume overlying a rockycore.[123] Solar analogs and solar twins are stars that resemble the Sun. Todate no solar twin with an exact match as that of the Sun has been found,however, there are some stars that are nearly identical to the Sun, and aresuch considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billionyears old, with the correct metallicity and a 0.1% solar luminosity variation.[124] Stars with an age of 4.6 billionyears are at the most stable state. Proper metallicity and size are also very important to low luminosityvariation.[125][126][127]

While larger than Kepler 186f,Kepler­452b's orbit and star are moresimilar to Earth's.

On 7 January 2013, astronomers from the Kepler team announced thediscovery of Kepler­69c (formerly KOI­172.02), an Earth­size exoplanetcandidate (1.7 times the radius of Earth) orbiting Kepler­69, a star similarto our Sun, in the CHZ and expected to offer habitableconditions.[128][129][130][131] The discovery of two planets orbiting in thehabitable zone of Kepler­62, by the Kepler team was announced on April19, 2013. The planets, named Kepler­62e and Kepler­62f, are likely solidplanets with sizes 1.6 and 1.4 times the radius of Earth,respectively.[130][131][132]

With a radius estimated at 1.1 Earth, Kepler­186f, discovery announced inApril 2014, is the closest yet size to Earth of an exoplanet confirmed by thetransit method[133][134][135] though its mass remains unknown and its parentstar is not a Solar analog.

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 stars: two ofthe three, Kepler­438b and Kepler­442b, are near­Earth­size and likely rocky; the third, Kepler­440b, is a super­Earth.[136] Announced 16 January, EPIC 201367065 d is a planet of 1.5 Earth radii found to orbit within thehabitable zone of EPIC 201367065, receiving 1.4 times the intensity of visible light as Earth.[137]

Kepler­452b, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385Earth days to orbit the habitable zone of its G­class (solar analog) star Kepler­452.[138][139]

The discovery of a system of three tidally­locked planets orbiting the habitable zone of an ultracool dwarf star,TRAPPIST­1, was announced in May 2016.[140] The discovery is considered significant because it greatlyincreases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.

Notable exoplanets – Kepler Space Telescope

Confirmed small exoplanets in habitable zones. (Kepler­62e, Kepler­62f, Kepler­186f, Kepler­296e, Kepler­296f, Kepler­438b, Kepler­440b, Kepler­

442b) (Kepler Space Telescope; January 6, 2015).[136]

Habitability outside the CHZ

Liquid­water environments have been found to exist in the absence of atmospheric pressure, and at temperaturesoutside the CHZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europaand Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurfaceoceans.[141]

Outside the CHZ, tidal heating and radioactive decay are two possible heat sources that could contribute to theexistence of liquid water.[11][12] Abbot and Switzer (2011) put forward the possibility that subsurface water couldexist on rogue planets as a result of radioactive decay­based heating and insulation by a thick surface layer ofice.[14]

With some theorising that life on Earth may have actually originated in stable, subsurface habitats,[142][143] it hasbeen suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem withlife'.[144] Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface.[145]

Another possibility is that outside the CHZ organisms may use alternative biochemistries that do not require waterat all. Astrobiologist Christopher McKay, has suggested that methane (CH

4) may be a solvent conducive to thedevelopment of "cryolife", with the Sun's "methane habitable zone" being centered on 1,610,000,000 km(1.0 ×109 mi; 11 AU) from the star.[17] This distance is coincidental with the location of Titan, whose lakes and

The discovery ofhydrocarbon lakes onSaturn's moon Titan hasbegun to call into questionthe carbon chauvinism thatunderpins CHZ theory.

rain of methane make it an ideal location to find McKay's proposed cryolife.[17] Inaddition, testing of a number of organisms has found some are capable of survivingin extra­CHZ conditions.[146]

Significance for complex and intelligent life

The Rare Earth hypothesis argues that complex and intelligent life is uncommonand 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 requirementto sustain life but a requirement to support the secondary conditions required formulticellular life to emerge and evolve. The secondary habitability factors are bothgeological (the role of surface water in sustaining necessary plate tectonics)[26] andbiochemical (the role of radiant energy in supporting photosynthesis for necessaryatmospheric oxygenation).[147] But others, such as Ian Stewart and Jack Cohen intheir 2002 book Evolving the Alien argue that complex intelligent life may ariseoutside the CHZ.[148] Intelligent life outside the CHZ may have evolved insubsurface environments, from alternative biochemistries[148] or even from nuclearreactions.[149]

On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential tosurvive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancientcircumvental ecosystems, supporting large complex life forms such as Riftia pachyptila.[150] Similar environmentsmay be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of thehabitable zone.[151] 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 extremetemperatures well above the boiling point of water and the cold vacuum of outer space.[152] In addition, the plantsRhizocarpon geographicum and Xanthoria elegans have been found to survive in an environment where theatmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower thanthat which most plants require to photosynthesize.[153][154][155] The fungi Cryomyces antarcticus and Cryomycesminteri are also able to survive and reproduce in Mars­like conditions.[155]

Species, including humans, known to possess animal cognition require large amounts of energy,[156] and haveadapted to specific conditions, including an abundance of atmospheric oxygen and the availability of largequantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earthanalogs 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, thenecessity of spacesuits or space habitat analogues on the surface may be eliminated and complex Earth life canthrive.[23]

Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in theuniverse.[157] The 1961 Drake equation, sometimes used to estimate the number of intelligent civilizations in ourgalaxy, contains the factor or parameter ηe, which is the fraction of stars that have planetary­mass objects orbitingwithin the CHZ. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarityin the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view thathabitability—and therefore life—is common throughout the Universe.[26] A 1971 NASA report by Drake andBernard Oliver proposed the "waterhole", based on the spectral absorption lines of the hydrogen and hydroxylcomponents of water, as a good, obvious band for communication with extraterrestrial intelligence[158][159] that has

since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to JillTarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterholesearches[160][161] and the Allen Telescope Array now extends Project Phoenix to such candidates.[162]

Because the CHZ is considered the most likely habitat for intelligent life, METI efforts have also been focused onsystems likely to have planets there. The 2001 Teen Age Message and the 2003 Cosmic Call 2, for example, weresent to the 47 Ursae Majoris system, known to contain three Jupiter­mass planets and possibly with a terrestrialplanet in the CHZ.[163][164][165][166] The Teen Age Message was also directed to the 55 Cancri system, which has agas giant in its CHZ.[104] A Message from Earth in 2008,[167] and Hello From Earth in 2009, were directed to theGliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.

See also

References

Hypothetical types of biochemistryEarth analogEarth Similarity IndexExtraterrestrial liquid waterExtraterrestrial lifeGalactic habitable zoneStar Trek's Class M planet classification

Star Trek's Class M planet classificationNatural satellite habitabilityPlanetary habitabilityRare Earth hypothesisVenus zone

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