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Template:Life in the Universe In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.<ref>Su-Shu Huang, American Scientist 47, 3, pp. 397–402 (1959)</ref><ref name=dole-1964>Template:Cite book</ref><ref name="F. Kasting, D. P 1993">J. F. Kasting, D. P. Whitmire, R. T. Reynolds, Icarus 101, 108 (1993).</ref><ref name=kopparapu-2013>Template:Cite journal</ref><ref name="SCI-20130503">Template:Cite journal</ref> The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ 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. As such, it is considered by many to be a major factor of planetary habitability, and the most likely place to find extraterrestrial liquid water and biosignatures elsewhere in the universe.
The habitable zone is also called the Goldilocks zone, a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets.<ref name="NYT-20150106-DB">Template:Cite news</ref> Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect.<ref>Template:Cite journal</ref> On November 4, 2013, astronomers reported, based on Kepler space telescope 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.<ref name="NYT-20131104">Template:Cite news</ref><ref name="PNAS-20131031">Template:Cite journal</ref> About 11 billion of these may be orbiting Sun-like stars.<ref name="LATimes-20131104">Template:Cite news</ref> Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star.<ref name="Planet-Alpha-Centauri">Template:Cite journal</ref> The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.<ref name="Schirber 2009-10-26">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.<ref name="Review 2009">Template:Cite journal</ref> Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System.<ref name="EdwardsBecker2012">Template:Cite journal</ref> Sustained by other energy sources, such as tidal heating<ref name="Cowen2008">Template:Cite news</ref><ref name="Bryner, Jeanna">Template:Cite news</ref> or radioactive decay<ref name="AbbotSwitzer2011">Template:Cite journal</ref> or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons.<ref name=physcisarxivlab-2011>Template:Cite news</ref> Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars,<ref name="Wall-Brines 2015">Template:Cite news</ref> or ammoniates,<ref name="SunClark2015">Template:Cite journal</ref> due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.<ref name=villard-2011>Template:Cite news</ref>
HistoryEdit
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).<ref>Template:Cite book</ref> The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry, "... we may presume, that, being susceptible of vegetation, it [the Earth] has been placed, in the series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate."<ref>Template:Cite book</ref>
Possibly the earliest use of the term habitable zone was in 1913,<ref>Template:Cite book</ref> by Edward Maunder in his book "Are The Planets Inhabited?".<ref>Template:Cite journal</ref> Hubertus Strughold's 1953 treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars used the term "ecosphere" and referred to various "zones" in which life could emerge.<ref name=huggett-1995>Template:Cite book</ref><ref name=strughold-1953>Template:Cite book</ref> In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life.<ref name="Kasting2010">Template:Cite book</ref> Su-Shu Huang, an American astrophysicist argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.<ref name=kasting-1993>Template:Cite journal</ref><ref name=huang-1966>Template:Cite book</ref><ref name=huang-1960>Template:Cite journal</ref>
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 the 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.<ref name="dole-1964"/> At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization.<ref name=gilster-2004>Template:Cite book</ref> The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.<ref name=nasa-2003>Template:Cite press release</ref> In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone.<ref name=kasting-1993 /> Kasting was the first to present a detailed model for the habitable zone for exoplanets.<ref name="F. Kasting, D. P 1993"/><ref name="Seager 2013">Template:Cite journal</ref>
An update to the habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.<ref name="Rare Earth" /><ref name=gonzalez-2001>Template:Cite journal</ref> The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.<ref name="Rare Earth"/>
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.<ref name=villard-2011 /> 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.<ref name=hadhazy-2013>Template:Cite news</ref>
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.<ref name='Tasker Nature 2017'>Template:Cite journal</ref><ref name='Patel MIT 2019'/> However, surface conditions are dependent on a host of different individual properties of that planet.<ref name='Tasker Nature 2017'/><ref name='Patel MIT 2019'>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This misunderstanding is reflected in excited reports of 'habitable planets'.<ref name=":0">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name='Sci News Oct2019'>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>No, the Exoplanet K2-18b Is Not Habitable. News outlets that said otherwise are just crying wolf—but they're not the only ones at fault. Laura Kreidberg, Scientific American. 23 September 2019.</ref> Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.<ref name='Patel MIT 2019'/><ref name='Sci News Oct2019'/><ref name='Tasker Sci Am 2019'>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name='Ruhner 2019'>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
DeterminationEdit
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 that 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 effects, can have the right temperature for liquid water to exist at the surface.<ref name=kasting-1988>Template:Cite journal</ref>
Solar System estimatesEdit
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units,<ref name=zsom-2013 /><ref name= rayeric-2011 /><ref name= rk-2017 /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 Template:Convert at which water can only exist as vapor.<ref name=venus-2006>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The entire orbits of the Moon,<ref name=sharp>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Mars,<ref name="bolonkin09">Template:Cite book</ref> 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.<ref name="HaberleMcKay2001">Template:Cite journal</ref> At Hellas Basin, 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.<ref name = "HaberleMcKay2001"/> Despite indirect evidence in the form of seasonal flows on warm Martian slopes,<ref name="WRD-20140218">Template:Cite magazine</ref><ref name=voanews>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=mag>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NASA-20131210">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> no confirmation has been made of the presence of liquid water at the surface. While other objects orbit partly within this zone, including comets, Ceres<ref name=ahearn-1992>Template:Cite journal</ref> is the only one of planetary mass.
Despite this, studies indicate the strong possibility of past liquid water on the surface of Venus,<ref name="SalvadorMassol2017">Template:Cite journal</ref> the Moon,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref> Mars,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name='Willson 2018'>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Vesta<ref name="ScullyRussell2015">Template:Cite journal</ref> and Ceres,<ref name="RaponiDe Sanctis2018">Template:Cite journal</ref><ref>NASA.gov PIA21471: Landslides on Ceres</ref> 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 the 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 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, 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.
| Inner edge (AU) | The outer edge (AU) | Year | Notes |
|---|---|---|---|
| 0.725 | 1.24 | 1964, Dole<ref name=dole-1964 /> | Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone. |
| 1.005–1.008 | 1969, Budyko<ref name=budyko-1969>Template:Cite journal</ref> | Based on studies of ice-albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969<ref>Template:Cite journal</ref> and North 1975.<ref>Template:Cite journal</ref> | |
| 0.92–0.96 | 1970, Rasool and De Bergh<ref name=rasool-1970>Template:Cite journal</ref> | Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans. | |
| 0.958 | 1.004 | 1979, Hart<ref name=hart-1979>Template:Cite journal</ref> | Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications. |
| 3.0 | 1992, Fogg<ref name=fogg-1992>Template:Cite journal</ref> | Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone. | |
| 0.95 | 1.37 | 1993, Kasting et al.<ref name=kasting-1993 /> | Founded the most common working definition of the habitable zone used today. Assumes that CO2 and H2O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate–silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU. |
| 2.0 | 2010, Spiegel et al.<ref name="speigel-2010">Template:Cite journal</ref> | Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity. | |
| 0.75 | 2011, Abe et al.<ref name="abe-2011">Template:Cite journal</ref> | Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth. | |
| 10 | 2011, Pierrehumbert and Gaidos<ref name= rayeric-2011 /> | Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in the Solar System. | |
| 0.77–0.87 | 1.02–1.18 | 2013, Vladilo et al.<ref name=vladilo-2013>Template:Cite journal</ref> | Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar. |
| 0.99 | 1.67 | 2013, Kopparapu et al.<ref name=kopparapu-2013 /><ref name="Kopparapu2013b" /> | Revised estimates of the Kasting et al. (1993) formulation using updated moist greenhouse and water loss algorithms. According to this measure, Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapor. Once the stratosphere becomes wet, water vapor photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a Template:CO2 dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in Template:CO2 will not create enough warming to prevent Template:CO2 catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.67 AU. This definition does not take into account possible radiative warming by Template:CO2 clouds. |
| 0.38 | 2013, Zsom et al. <ref name=zsom-2013>Template:Cite journal</ref> |
Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere. | |
| 0.95 | 2013, Leconte et al.<ref name= leconte-2013>Template:Cite journal</ref> | Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System. | |
| 0.95 | 2.4 | 2017, Ramirez and Kaltenegger <ref name=rk-2017>Template:Cite journal</ref> |
An expansion of the classical carbon dioxide-water vapor habitable zone<ref name=kasting-1993 /> assuming a volcanic hydrogen atmospheric concentration of 50%. |
| 0.93–0.91 | 2019, Gomez-Leal et al. <ref name=gomez-2019>Template:Cite journal</ref> |
Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of climate sensitivity in both cases. | |
| 0.99 | 1.004 | The tightest bounded estimate from above | |
| 0.38 | 10 | The most relaxed estimate from above |
Extrasolar extrapolationEdit
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,<ref name="kopparapu-2013" /> a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at <math>\sqrt{0.25}</math>, 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 HZ concept is more complex.
Spectral types and star-system characteristicsEdit
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 types. 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.<ref>Template:Cite journal</ref> 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.<ref name="forget-1997">Template:Cite journal</ref><ref name="mischna-2000">Template:Cite journal</ref>
With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions.<ref>Template:Cite press release</ref> Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.<ref name="BuccinoLemarchand2006">Template:Cite journal</ref> 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 K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking 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 for life.<ref name=barnes-2013>Template:Cite journal</ref>
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 modeling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces despite tidal locking.<ref name=yang-2013 /> Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.<ref>Template:Cite journal</ref> At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.<ref name=barnes-2013 /> 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.<ref name=rk-2014>Template:Cite journal</ref>
Stellar evolutionEdit
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,<ref name="carroll">Template:Cite book</ref> 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.<ref name="richmond">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="guo-2009">Template:Cite journal</ref> 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,<ref name="Kasting-1968">Template:Cite journal</ref> 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.<ref name=franck-2002>Template:Cite conference</ref> 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.<ref name=franck-2002 />
In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Full reprint</ref> and huge starspots which can cover 20% of the star's surface area,<ref name=alekseev-2002>Template:Cite journal</ref> have the potential to strip an otherwise habitable planet of its atmosphere and water.<ref name=alpert-2005 /> As with more massive stars, though, stellar evolution changes their nature and energy flux,<ref name=west-2006>Template:Cite journal</ref> so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.<ref name=alpert-2005>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> For example, the Sun is expected to engulf the previously habitable Earth as a red giant.<ref name=christensen-2005>Template:Cite news</ref><ref name=rk-2016 /> 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.<ref name=lopez-2005>Template:Cite journal</ref> At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense.<ref name="LorenzLunine1997">Template:Cite journal</ref> 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.<ref name=lopez-2005 /> However, around such a helium-burning star, important life processes like photosynthesis 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.<ref name=voisey-2011>Template:Cite news</ref> Moreover, as Ramirez and Kaltenegger (2016)<ref name=rk-2016>Template:Cite journal</ref> 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.<ref name=rk-2016/> 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.<ref name=rk-2016/>
Desert planetsEdit
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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 planets (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, 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.<ref>Alien Life More Likely on 'Dune' Planets Template:Webarchive, 09/01/11, Charles Q. Choi, Astrobiology Magazine</ref><ref>Template:Cite journal</ref>
Other considerationsEdit
Template:See also A planet cannot have a hydrosphere—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 is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis.<ref name="source_mrk1">Template:Cite journal</ref><ref name="source_mrk2">Template:Cite conference</ref> For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep<ref name=kuchner-2003>Template:Cite journal</ref> such as GJ 1214 b<ref name="disco-charbonneau">Template:Cite journal</ref><ref name="planetmodels">Template:Cite journal</ref> or Kepler-22b may be.<ref name=vastag-2011>Template:Cite news</ref>
Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorized to outgassing, impact degassing, and ingassing.<ref name="RobinsonCatling2012">Template:Cite journal</ref> Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.<ref name="Shizgal, 1996">Template:Cite journal</ref> 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.<ref name=vladilo-2013 /> Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained<ref name=vladilo-2013 /> because even a small shift in pressure or temperature could render water unable to form as a liquid.<ref name=chaplin-2013>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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),<ref name=kasting-1993 /> a study<ref name="rk-2017"/> 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<ref name=rayeric-2011>Template:Cite journal</ref> 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 HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, 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 and reducing significantly temperature differences between the two sides.<ref name=yang-2013>Template:Cite journal</ref>
Planetary mass natural satellites 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.<ref name=hadhazy-2013 /> 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,<ref name=hadhazy-2013 /> but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet.<ref name="HamiltonBurns92">Template:Cite journal</ref> 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.<ref name=hadhazy-2013 />
A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the HZ 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, for example, can survive in a dehydrated state temperature between Template:Convert<ref>Template:Cite journal</ref> and Template:Convert.<ref name="survival">Template:Cite book</ref> Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm.<ref name=kane-2012>Template:Cite journal</ref>
Extrasolar discoveriesEdit
Template:See also A 2015 review concluded that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable.<ref name=centauridreams>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These are at a distance of 990, 490 and 1,120 light-years 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 is 11.9 light-years away. It is in the inner edge of its planetary system's habitable zone, giving it an estimated average surface temperature of Template:Convert.<ref>Template:Cite book</ref>
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 HZ, at 0.48,<ref name="kopparapu-2013" /> meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.<ref name=wethington-2008>Template:Cite news</ref> However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.<ref name=torres-2013-2>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.<ref name="BorensteinS">Template:Cite news</ref> NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler 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 HZs.<ref name="ChoiCQ">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=shao-2011>Template:Cite journal</ref>
Early findingsEdit
Template:Category see also The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.<ref>Template:Cite journal</ref> 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.<ref name="Extrasolar.net">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the HZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.<ref>Template:Cite journal</ref>
Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons.<ref name="Sudarsky2003">Template:Cite journal</ref> Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.
Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone<ref>Template:Cite journal</ref> and has a low orbital eccentricity, comparable to that of Mars in the Solar System.<ref>Template:Cite journal</ref> Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,<ref>Template:Cite journal</ref> though it is unclear whether such satellites could form in the first place.<ref>Template:Cite journal</ref>
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.<ref name=lovis-2006>Template:Cite journal</ref> The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A.<ref name="ScienceDaily">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Fischer2008">Template:Cite journal</ref> Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.<ref name="guardian">Template:Cite news</ref>
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-EarthsEdit
The 2007 discovery of Gliese 581c, the first super-Earth 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.<ref>Template:Cite news</ref> 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 a short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g, 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,<ref name="SCI-20140703">Template:Cite journal</ref> and astronomers are divided about its existence.
Discovered in August 2011, HD 85512 b was initially speculated to be habitable,<ref name=maxisciences>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.<ref name=torres-2013-2 />
Kepler-22 b, discovered in December 2011 by the Kepler space probe,<ref name="bbc20111205">Template:Cite news</ref> 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.<ref name="Caleb Scharf Blog">{{#invoke:citation/CS1|citation |CitationClass=web }}: "If it [Kepler-22b] had a similar composition to Earth, then we're looking at a world in excess of about 40 Earth masses".</ref> Gliese 667 Cc, discovered in 2011 but announced in 2012,<ref name=arxiv12020446>Template:Cite journal</ref> is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planets known.
Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163<ref name="Simbad-20120920">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 60° C.<ref name="PHL-20120829">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Space-20120920">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.<ref name="hd40307g_tuomi12">Template:Cite journal</ref> In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away.<ref name=aron-2012>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;<ref name="tuomi-2013">Template:Cite journal</ref> however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study.<ref name=mendes-2013-3>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is now considered as uninhabitable.
Near Earth-sized planets and Solar analogsEdit
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 Template:Earth radius the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core.<ref>Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"</ref> A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology.
A solar analog (or "solar twin") is a star that resembles the Sun. 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 variation.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Using data collected by NASA's Kepler space telescope and the W. M. Keck Observatory, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone.<ref name="NOAA 2017">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to the Sun, in the HZ and expected to offer habitable conditions.<ref name="Space-20130109">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="arXiv-20130417">Template:Cite journal</ref><ref name="NASA-20130418" /><ref name="NYT-20130418">Template:Cite news</ref> The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.<ref name="NASA-20130418">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NYT-20130418"/><ref name="Borucki-2013">Template:Cite journal</ref>
With a radius estimated at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method<ref name="NYT-20140417">Template:Cite news</ref><ref name="AP-20140417">Template:Cite news</ref><ref name="BBC-20140417">Template:Cite news</ref> though its mass remains unknown and its parent star is not a Solar analog.
Kapteyn b, discovered in June 2014, was thought to is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii orbiting the habitable zone of the red subdwarf Kapteyn's Star, 12.8 light-years away.<ref name="SP-20140603">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> However, further analysis concluded that this claim was an artefact of stellar rotation and activity.<ref name="BortleFausey2021">Template:Citation</ref>
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 of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.<ref name="NASA-20150106" /> However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d 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.<ref>Template:Cite news</ref>
Kepler-452b, 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.<ref name=Jenkins2015>Template:Cite journal</ref><ref name="bno">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 around 227 light years from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.<ref name="DressingVanderburg2017">Template:Cite journal</ref>
Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite magazine</ref>
At 11 light-years away, the second closest planet, Ross 128 b, 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Discovered in March 2018, K2-155d is about 1.64 times the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.<ref name="Exoplanet Exploration">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=CNET>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=ExtremeTech>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
One of the earliest discoveries by the Transiting Exoplanet Survey Satellite (TESS) announced on July 31, 2019, is a Super-Earth planet GJ 357 d orbiting the outer edge of a red dwarf 31 light years away.<ref name="LuquePallé2019">Template:Cite journal</ref>
K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.
In September 2020, astronomers identified 24 superhabitable planet (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.<ref name="AB-20200918">Template:Cite journal</ref>
| Notable exoplanets – Kepler space telescope | |
|---|---|
| File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg
Confirmed small exoplanets in habitable zones. (Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).<ref name="NASA-20150106">{{#invoke:citation/CS1|citation |
CitationClass=web }}</ref>{{safesubst:#invoke:Check for unknown parameters|check|unknown=|preview=Page using Template:Center with unknown parameter "_VALUE_"|ignoreblank=y| 1 | style }} |
Habitability outside the HZEdit
Liquid-water environments have been found to exist in the absence of atmospheric pressure and at temperatures outside the HZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Outside the HZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.<ref name="Cowen2008"/><ref name="Bryner, Jeanna"/> Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.<ref name="physcisarxivlab-2011"/>
With some theorising that life on Earth may have actually originated in stable, subsurface habitats,<ref name=Munro2013>Template:Citation</ref><ref name=Davies2013>Template:Cite journal</ref> it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.<ref name=Taylor1996>Template:Citation</ref> On Earth itself, living organisms may be found more than Template:Convert below the surface.<ref name=Doyle2013>Template:Citation</ref>
Another possibility is that outside the HZ organisms may use alternative biochemistries that do not require water at all. Astrobiologist Christopher McKay, has suggested that methane (Template:Chem) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on Template:Convert from the star.<ref name=villard-2011 /> 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.<ref name=villard-2011 /> In addition, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.<ref> Template:Cite journal</ref>
Significance for complex and intelligent lifeEdit
The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the HZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a HZ 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)<ref name="Rare Earth">Template:Cite book</ref> and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation).<ref name="DeckerHolde2011">Template:Cite book</ref> 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 HZ.<ref name=cohen-2002>Template:Cite book</ref> Intelligent life outside the HZ may have evolved in subsurface environments, from alternative biochemistries<ref name=cohen-2002 /> or even from nuclear reactions.<ref name="GO247"> Template:Cite book</ref>
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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 circumvent ecosystems, supporting large complex life forms such as Riftia pachyptila.<ref name="Smil2003">Template:Cite book</ref> Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> A desert moss, Syntrichia caninervis is one of few plants believed capable of surviving on Mars.<ref name=":1">Template:Cite journal
- Template:Cite news</ref> In addition, the lichens Rhizocarpon geographicum and Rusavskia 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.<ref name="Skymania-20120426">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref><ref name="EGU-20120426">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Onofride Vera2015">Template:Cite journal</ref> The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.<ref name="Onofride Vera2015"/>
Species, including humans, known to possess animal cognition require large amounts of energy,<ref name="Islervan Schaik2006">Template:Cite journal</ref> 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 analogs in the HZ 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 analogs on the surface may be eliminated, and complex Earth life can thrive.<ref name=dole-1964 />
Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.<ref>Template:Cite news</ref> The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter Template:Mvar, which is the average number of planetary-mass objects orbiting within the HZ 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 mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe.<ref name="Rare Earth" /> A 1971 NASA report by Drake and Bernard Oliver proposed the "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<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Angelo2007">Template:Cite book</ref> that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, HZ candidates are the priority targets to narrow waterhole searches<ref name="TurnbullTarter2003">Template:Cite journal</ref><ref name="SiemionDemorest2013">Template:Cite journal</ref> and the Allen Telescope Array now extends Project Phoenix to such candidates.<ref name=Wall2011>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Because the HZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and 2003 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 HZ.<ref name="cplire.ru">Template:Cite conference</ref><ref>Template:Cite magazine</ref><ref name="bayesian"> Template:Cite journal</ref><ref> Template:Cite journal</ref> The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its HZ.<ref name="ScienceDaily" /> A Message from Earth in 2008,<ref name="moore">Template:Cite news</ref> and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the HZ—Gliese 581 c, d, and the unconfirmed g.
See alsoEdit
- Habitability of binary star systems
- Habitability of F-type main-sequence star systems
- Habitability of K-type main-sequence star systems
- Habitability of neutron star systems
- Habitability of red dwarf systems
- Habitability of yellow dwarf systems
- Habitable zone for complex life
ReferencesEdit
External linksEdit
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