# Circumstellar habitable zone

(Redirected from Goldilocks zone)
An example of a system based on stellar luminosity for predicting the location of the habitable zone around various types of stars. Planet sizes, star sizes, orbit lengths, and habitable zone sizes are not to scale.

In astronomy and astrobiology, the circumstellar habitable zone (CHZ) (or simply the habitable zone) is the region around stars within which planetary-mass objects with the proper atmospheric pressure and chemistry could support liquid water at the surface.[1][2] The CHZ is inferred from known requirements of Earth's biosphere, the Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. To many, studying such objects in the CHZ appears to be the best way to estimate the scope of life in the universe and locate extraterrestrial life.

Since the concept of the CHZ was first presented in 1953,[3] numerous habitable zone planets have been discovered in the Milky Way galaxy. Most are more massive than the Earth since such planets are easier to detect. The number of planets with Earth-like composition orbiting within circumstellar habitable zones has been estimated to be anywhere from 500 million[4] to over 150 billion.[1][5] The CHZ is also of particular interest to the emerging field of natural satellite habitability, since moons are believed to greatly outnumber planets.[6]

In recent decades, the CHZ concept has been broadly challenged as a primary criterion for life. Extraterrestrial liquid water is thought to exist in substantial quantities outside of the Solar System's habitable zone, created perhaps by other energy sources, such as tidal heating[7][8] and radioactive decay.[9] The existence of these energy sources suggests that life may be possible even in interstellar space. 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.[10]

## History

The concept of a circumstellar habitable zone was first introduced in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars coined the term "ecosphere" and referred to various "zones" in which life could emerge.[3][11] In the same year, Harlow Shapley wrote the "Liquid Water Belt" which described the same theory in further scientific detail. Both stressed the importance of liquid water to life.[12] Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.[13][14] Huang, a major early contributor to habitable zone theory, 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.[15]

The theory of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he covered the circumstellar habitable zone itself 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.[16] 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,[17] and in 1993, planetary scientist James Kasting introduced the term "circumstellar habitable zone" to more precisely refer to the region previously (and still) known as the habitable zone.

The term Goldilocks zone emerged in the 1970s, referencing specifically a region around a star that is "just right" (not too hot, not too cold) for water to be present in a liquid state.[18]

In 2000, astronomers Peter Ward and Donald Brownlee introduced the theory of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.[19][20] 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.[19]

More recently, various planetary scientists have criticized the circumstellar habitable zone theory for its carbon chauvinism, proposing that the concept be extended to other solvents, such as ammonia or methane, which could be the basis of life based on an alternative biochemistry.[10] In 2013, further developments in habitable zone theory were made with the proposal of a circumplanetary habitable zone, also known as the "habitable edge," to encompass the region around a planet where the orbits of natural satellites would not be disrupted, while at the same time tidal heating from the planet would not cause a boiling away of liquid water.[21]

## Determination of the circumstellar habitable zone

Prediction of the extent of the Solar System's habitable zone indicated by two dimensional orbits of the inner Solar System bodies. The orbital region in which Earth could possess liquid water is indicated in green, with the core habitable zone indicated in dark green, along with the light-green extended habitable zone.

Whether a body is in the circumstellar habitable zone of its host star is dependent on both the radius of the planet's orbit (for natural satellites, the host planet's orbit) and the mass of the body itself. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth or Venus can remain habitable, complemented by a larger "extended habitable zone" in which super-Earth planets, which have stronger greenhouse effects, more powerful geomagnetic dynamos, and greater amounts of plate tectonics, can have the right temperature for liquid water to exist at the surface.

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 at 0.48,[1] meaning that there may be roughly 95-180 billion habitable planets in the Milky Way.[5] However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.[22]

Previous studies have been more conservative. In 2011, Seth Borenstein in 2011 concluded that there are roughly 500 million habitable planets in the Milky Way.[4] NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, concluding that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars", a figure of about 2 billion habitable planets for the entire galaxy.[23]

### Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.725 to 3.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Venus, for example, has an orbit whose aphelion touches the inner reaches of the Solar System's habitable zone, but has an extremely thick carbon dioxide atmosphere which causes the surface temperature to reach 462 °C (864 °F).[24] While the entire orbits of the Moon,[25] Mars,[26] and the dwarf planet Ceres[27] lie within various estimates of the habitable zone, and seasonal flows on warm Martian slopes have not yet been ruled out, the three bodies have atmospheric pressures that are far too low to create a strong greenhouse effect and sustain liquid water on their surfaces.

Most estimates, therefore, are inferred on the effect that repositioned orbit would have on the habitability of Earth or Venus;. According to extended habitable zone theory, however, a planet with a more dense atmosphere than Earth orbiting in the extended habitable zone, such as Gliese 667 Cd[28] or Gliese 581 d,[29][30] might theoretically possess liquid water.

Estimates of the circumstellar-habitable-zone boundaries of the Solar System
Inner Edge (AU) Outer Edge (AU) Year Notes
0.725 1.24 Dole 1964[16] Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
1.385–1.398 Budyko 1969[31] 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[32] and North 1975.[33]
0.88–0.912 Rasool and DeBurgh 1970[34] Based on studies of Venus's atmosphere, Rasool and DeBurgh concluded that this is the minimum distance at which Earth would have formed stable oceans.
0.95 1.01 Hart et al. 1979[35] Based on computer modelling and simulations of the evolution of the Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
3.0 Fogg 1992[36] Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
1.37 Kasting et al. 1993[13] Noted the cooling effect of cloud albedo.
2.0 Spiegel et al. 2010[37] Proposed that seasonal habitability is possible to this limit when combining high obliquity and orbital eccentricity.
0.75 Abe et al. 2011[38] Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
0.77—0.87 1.02—1.18 Vladilo et al. 2013[39] Inner edge of circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; no habitability below 15 millibar of pressure.
0.99 1.688 Kopparapu et 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, but just outside, of the runaway greenhouse limit.
0.5 Zsom et al. 2013
(submitted)[40]
Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.

### Extrasolar extrapolation

Artists conception of Kepler-16(AB)-b, a circumbinary planet in the circumstellar habitable zone of its two host stars.[41]

Astronomers use stellar flux and the inverse-square law to extrapolate cirumstellar-habitable-zone models created for the Solar System to other stars. For example, while the Solar System has a circumstellar habitable zone centered 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 $\sqrt{0.25}$, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Beyond this, though, there are numerous roadblocks to a perfect extrasolar extrapolation of the circumstellar habitable zone concept.

#### Spectral types and star-system characteristics

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral 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.[42] 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.[43][44]

With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to photoevaporation.[45] On the other end of the spectral scale, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could possess habitable zones, an idea which has been extended in modern times with 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 ability to support life.[46]

Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Recent climate modelling supports the idea that red dwarfs could sustain surface water suitable for supporting extremophiles and that the terminator of tidally locked planets in the habitable zone may be sufficiently "life friendly".[47] Astronomy professor Eric Agol argues that white dwarfs may support a relatively brief habitable zone through planetary migration.[48] At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.[46]

#### Stellar evolution

Natural defenses against space weather, such as the magnetosphere depicted in this artistic rendition, may be required for planets to sustain surface water for prolonged periods.

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,[49] 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 to develop complex life.[50][51] Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther and farther out; our Sun, for example, was only 75% as bright in the Archaean as it is now,[52] and in the future continued increases in energy output will put Earth outside of the Sun's habitable zone, even before it reaches the red giant phase.[53] 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 of time. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.[53]

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes[54] and huge starspots which can cover 20% of the star's surface area,[55] have the potential to strip an otherwise habitable planet of its atmosphere and water.[56] However, stellar evolution is at play with red dwarf system habitability as well, reducing the wild fluctuations in luminosity so planets are more likely to have life.[57] As red dwarf stars grow older, they have fewer and fewer violent outbursts, until by 1.2 billion years of age they become sufficiently constant to allow for the development of life.[56][58]

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously-habitable Earth as a red giant.[59] However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.[60] Given that this new equilibrium lasts for about 1 Gyr, and since life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on red-giant planets in the habitable zone.[60] However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has been artificially seeded with 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.[61]

#### Other considerations

The Earth's hydrosphere. Water covers 71% of the Earth's surface, with the global ocean accounting for 97.3% of the water distribution on Earth.

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 unknown, possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis.[62][63] 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[64] such as GJ 1214 b[65][66] or Kepler-22b may be.[67]

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing.[68] Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.[69] 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.[39] Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained[39] because even a small shift in pressure or temperature could render water unable to form a liquid.[70]

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.[21] More specifically, planets need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io,[21] but within the Hill radius of the planet so that they are not pulled out of orbit of their host planet.[71] 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 astar that it would be stripped from its host planet. Moving a moon closer to a host planet to maintain its orbit would create tidal heating so intense as to eliminate any prospects of habitability.[21]

In order for planets and natural satellites to remain habitable, they (or their host planets) need to have low orbital eccentricity so that they can orbit within the circumstellar habitable zone throughout the year. In the case of planets moving into the circumstellar habitable zone outward, though, extremophiles may be able to increase their metabolism rates as the planet approaches its periastron and becomes warmer, while going into a state of hibernation near apastron, when the planet would be at its coldest.[72]

## Discoveries in the circumstellar habitable zone

Planets in the circumstellar habitable zone are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race.[73] The Drake equation, used in order to calculate the number of intelligent civilizations in our galaxy, contains a parameter ηewhich estimates the fraction of stars that have terrestrial planets or exomoons in a circumstellar habitable zone. A low value lends support to the rare Earth hypothesis, while a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe.[19] CHZ candidates have been proposed as a focus for the search for extraterrestrial intelligence.[74]

### A decade of gas giants

Artist's impression of Upsilon Andromedae d, as seen from a hypothetical large moon with surface liquid water.

From 1996 to 2006, many extrasolar planets were found to be located in the circumstellar habitable zones of their parent stars. One of the first to be discovered was 70 Virginis b, a gas giant which was initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold." Later study, though, revealed 70 Virginis b to be an inhospitable planet whose surface temperatures would be more analogous to Venus than to Earth.[75] 16 Cygni Bb, also discovered in 1996, is another gas giant orbiting in the habitable zone; however, it has an extremely eccentric orbit that causes extreme seasonal effects on the planet's surface. In spite of this, simulations suggest that a terrestrial natural satellite would be able to support water at its surface year-round.[76]

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. although they are not thought to themselves possess significant water at their surfaces, both may have habitable moons.[77] Upsilon Andromedae d, discovered in 1999, is a gas giant in its star's circumstellar habitable zone considered to be large enough to favor the formation of large, Earth-like moons.[78]

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone[79] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.[80] Tidal interactions suggest that HD 28185 b could harbor habitable Earth-mass satellites in orbit around it for many billions of years,[81] though it is unclear whether such satellites could form in the first place.[82]

HD 69830 d, a gas giant with 17 times the mass of the Earth, was in 2006 found orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth.[83] The following year, 55 Cancri f was discovered within 55 Cancri A's circumstellar habitable zone.[84] [85] While conditions on this massive and dense planet are not conducive to the formation of water or for that matter biological life as we know it, the potential exists for a system of moons to be orbiting the planet and thus transiting through this zone and being conducive for biological development.

### Habitable super-Earths

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

The 2007 discovery of Gliese 581 c, 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 surface conditions that likely resemble Venus more than Earth.[86] Gliese 581 d, another planet in the same system and a better candidate for habitability, was also discovered in 2007. Though it receives only 30% of the stellar flux that the Earth gets from the Sun, a strong greenhouse effect may allow it to retain liquid water at its surface, thereby creating a water cycle and perhaps life.[29][87][88][89] Gliese 581 g, yet another planet discovered in the circumstellar habitable zone of the system, has the best prospects for habitability of all three planets, but its existence has recently been put into doubt in a study by Mikko Tuomi,[90] an analysis that was subsequently disputed.[91][92] The planet is currently listed as unconfirmed by the Extrasolar Planets Encyclopedia.[93]

Discovered in August 2011, HD 85512 b was initially believed to be habitable,[94] but the new circumstellar-habitable-zone criteria devised by Kopparapu et al. in 2013 preclude the planet from being habitable.[22] With an increase in the intensity of exoplanet discovery, the Earth Similarity Index was devised in October 2011 as a way of comparing planetary properties, such as surface temperature and density, to those of the Earth in order to better gauge the habitability of extrasolar bodies.[95]

A diagram comparing size (artists impression) and orbital position of planet Kepler-22b within Sun-like star Kepler 22's habitable zone and that of the Earth in the Solar System

Kepler-22 b, discovered in December 2011 by the Kepler space probe,[96] is the first transiting exoplanet discovered around a sunlike star. With a radius 2.4 times that of the Earth, Kepler-22b has been predicted by some to be an ocean planet.[97] Gliese 667 Cc, discovered in 2011 but announced in 2012,[98] is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. Described as one of the best candidates to support liquid water at its surface, Gliese 667 Cc has an Earth Similarity Index of 0.79.

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163[99] is located 49 ly from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40% more stellar radiation than the Earth, leading to surface temperatures of about 60° C.[100][101][102] HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.[103] In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a sunlike star just 12 light years away.[104] While more massive than Earth, they are among the least massive planets found to date orbiting in the zone;[105] however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar-habitable-zone criteria established by the 2013 Kopparapu study.[106]

On January 7, 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-like exoplanet candidate (1.7 times the radius of Earth) orbiting a star, Kepler-69, similar to our Sun in the "habitable zone" and possibly a "prime candidate to host alien life".[107][108][109][110] 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 diameter of earth, respectively.[109][110][111]

## SETI/METI

Many argue that an orbit in the CHZ is a requirement for a planet or exomoon to host complex multicellular life, and by extension intelligent life. Proponents of the rare Earth hypothesis posit that Earth's location in the CHZ (in addition to the fulfillment by Earth of numerous other requirements) had a pivotal role in the development of photosynthesis and the subsequent Great Oxygenation Event that led to the emergence of complex life on Earth.[112][113]

To that end, many SETI and METI efforts have been focused on systems likely to have planets in the CHZ. The 2001 Teen Age Message and the 2003 Cosmic Call 2 for example, was sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the CHZ.[114][115][116][117] The Teen Age Message, and the later Wow! reply, were also directed to the 55 Cancri system, with a habitable gas giant.[84] A Message to Earth in 2008, and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three potentially habitable planets—Gliese 581 c, d, and the unconfirmed g.[118]

## Criticism

The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the carbon chauvinism that underpins habitable zone theory.

The concept of a habitable zone is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien based on the fact that habitable environments can be found outside of the circumstellar habitable zone. For example, Jupiter's moon Europa is believed to have a subsurface ocean with an environment similar to Earth's deep ocean, which may be conducive to extremophilic forms of life akin to the tardigrades found on Earth.[119]

Stewart and Cohen also criticize the circumstellar habitable zone for applying the restrictions of terrestrial biochemistry to life on other worlds, despite the fact that such life could indeed be based on a form of alternative biochemistry.[119] In a similar vein, some astrobiologists, including NASA's Christopher McKay, have suggested that methane may be a solvent conducive to the development of "cryolife", with the Sun's methane habitable zone being centered around 1.6×109 km (1,000,000,000 mi) from the star.[10] This distance is coincidental with the location of Saturn's moon Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife.[10]

The habitable zone has also been criticized[according to whom?] because it does not take into account the fact that solutions, with water as the solvent, may retain a liquid state at temperatures or pressures that are not traditionally considered habitable. For example, seasonal flows on warm Martian slopes may be caused by briny water, despite the fact that Mars is at the very edge of the traditional circumstellar habitable zone[1][120] and has an atmospheric pressure that can barely turn pure water to liquid.[70][121]

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