In astronomy and astrobiology, the habitable zone is the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface.[1]1 Since liquid water is essential for all known forms of life, planets in this zone are considered the most promising sites to host extraterrestrial life. The terms "ecosphere" and "Liquid Water Belt" were introduced by Hubertus Strughold and Harlow Shapley respectively in 1953.[2] Contemporary alternatives include "HZ", "life zone", and "Goldilocks Zone."[3]
"Habitable zone" is sometimes used more generally to denote various regions that are considered favorable to life in some way. One prominent example is the Galactic habitable zone' (the distance from the galactic centre). Such concepts are inferred from the empirical study of conditions favorable for life on Earth. If different kinds of habitable zones are considered, their intersection is the region considered most likely to contain life.
The location of planets and natural satellites (moons) within its parent's star's habitable zone (and a near circular orbit) is but one of many criteria for planetary habitability and it is theoretically possible for habitable planets to exist outside the habitable zone. The term "Goldilocks planet" is used for any planet that is located within the CHZ[4][5] although when used in the context of planetary habitability the term implies terrestrial planets with conditions roughly comparable to those of the Earth (i.e. an Earth analog). The name originates from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one that is neither too close nor too far from a star to rule out liquid water on its surface. While only about a dozen planets have been confirmed in the habitable zone, the Kepler spacecraft has identified a further 54 candidates and current estimates indicate that there are "at least 500 million" such planets in the Milky Way.[6]
Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes.[7] Astronomical objects located in the zone are typically close in proximity to their parent star and as such more exposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, these and many other exclusionary factors reinforce a contrasting theory of interstellar "dead zones" where life cannot exist, supporting the Rare Earth Hypothesis.
Some planetary scientists have suggested that habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other solvent compounds (such as ammonia and methane) could exist in stable liquid forms. Astrobiologists theorise that these environments could be conducive to alternative biochemistry.[8] Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water. It may follow for oceans consisting of ammonia or methane.[9]
Habitable zones are used in the Search for Extra-Terrestrial Intelligence and is based on the assumption that should intelligence extraterrestrial life exists elsewhere in the universe, that it would most likely be found there.
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The concept of what is now widely known as the habitable zone originates in the 1950s. Two publications referring to the concept were written at about the same time. Hubertus Strughold wrote "The Green and the Red Planet: A Physiological Study of the possibility of Life on Mars" in which he used the term "ecosphere" and referred to "zones" in which life could exist.[2] 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.[10] In 1955 Strughold wrote a follow-up called "Ecosphere of the Sun".[11] Chinese-American astrophysicist Su-Shu Huang extended the debate in 1959 with "Life-Supporting Regions in the Vicinity of Binary Systems" proposing that life zones were rare due to the orbital instabilities of habitable zones in common multistar systems.[12]
Habitable zone theory was further developed in 1964 by Stephen H. Dole in "Habitable Planets for Man" and then popularised by science fiction writer Isaac Asimov by capturing the imagination exploring possibilities of space colonization of other planetary systems.[13]
By the 1970s, Michael H. Hart's 1979 paper "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe" outlined the first evolutionary model for a habitable zone and hist pessimistic conclusions on the distribution of extraterrestrial life fuelled the Rare Earth Hypothesis.
Beyond the outer edge of the habitable zone, a planet will be too cold to sustain liquid water on its surface. Any water present will freeze. A planet closer to its star than the inner edge of the habitable zone will be too hot. Any water present will boil away or be lost into space entirely. Liquid water is considered important because carbon compounds dissolved in water form the basis of all Earthly life, so watery planets are good candidates to support similar carbon-based biochemistries.
Theoretical determinations of the habitable zone are based on empirical observation of the habitability of the Earth and its orbit within Solar System. Various complications must be taken into account, such as the greenhouse effect and changing albedo due to clouds.
Estimates for the habitable zone within our own solar system range from 0.725 to 3.0 astronomical units based on various scientific models.
Estimation of the Solar System's habitable zone is made difficult due to a number of factors. Although the aphelion of planet Venus and the complete orbits of The Moon, the planet Mars and dwarf planet Ceres are within the habitable zone, the varying atmospheric pressures of these planets, rather than the habitable zone, determines their potential for surface water. In the case of Venus, the atmospheric pressure is far too high, and a runaway greenhouse effect raises the surface temperature massively, and in the case of Mars, the atmospheric pressure is too low.For the Moon and Ceres the atmosphere is essentially nonexistent, and therefore, surface water cannot exist on these worlds.
Most estimates therefore are inferred on the effect that repositioned orbit would have on the habitability of Earth or Venus, therefore the habitable zone is based on calculations based on similar sizes and atmospheric pressures.
| Inner edge | Outer edge | References | Notes |
| 0.725 AU | 1.24 AU | Dole 1964[14] | Used optically thin atmospheres and fixed albedos. |
| 0.95 AU | 1.01 AU | Hart et al. 1978, 1979[15] | stars K0 or later cannot have HZs |
| 0.95 AU | 3.0 AU | Fogg 1992[16] | Used Carbon cycles. |
| 0.95 AU | 1.37 AU | Kasting et al. 1993[17] | |
| – | 1%–2% farther out | Budyko 1969[18] | ... and Earth would have global glaciation. |
| – | 1%–2% farther out | Sellers 1969[19] | ... and Earth would have global glaciation. |
| – | 1%–2% farther out | North 1975[20] | ... and Earth would have global glaciation. |
| 4%–7% closer | – | Rasool & DeBurgh 1970[21] | ... and oceans would never have condensed. |
| – | – | Schneider and Thompson 1980[22] | disagreed with Hart. |
| – | – | Kasting 1991[23] | |
| – | – | Kasting 1988[24] | Water clouds can shrink HZ as they counter GHG effect with higher albedos. |
| – | – | Ramanathan and Collins 1991[25] | GHG effect IR trapping is greater than water cloud albedo cooling, and Venus would have to have started "Dry." |
| – | – | Lovelock 1991[26] | |
| – | – | Whitemire et al. 1991[27] |
Astronomers use apparent magnitude, luminosity and stellar flux along with the inverse square law to calculate habitable zones for stars. The "center" of the HZ is defined as the distance that an exoplanet would have to be from its parent star in order to receive the right amount of energy from the star to maintain liquid water. For example, a star with 25% of the luminosity of the Sun will have a CHZ centered at about 0.50 AU, while a star with twice the Sun's luminosity will have a CHZ centered at about 1.4 AU.
The orbit of the Earth and other planets within our Solar System is roughly circular, however many exoplanets have been found with eccentric orbits, some of which cause them to spend some of their orbit outside of the zone. A notable example is 16 Cygni Bb. Venus is also known also to spend only some of its orbit on the inside edge of the habitable zone. Water on such planets and their moons, should it exist, may go through extreme seasonal sublimation and deposition cycles. As standing bodies or surface water may be unstable and transient rendering them not strictly habitable. It is currently unknown as to whether life is capable of adapting to such extreme cycles, should it be even capable of starting in the first place.
Small stars such as red dwarfs produce much more dangerous stellar flare activity than a star the size of the Sun. The flares would blast planets in the liquid-water-zone of red dwarfs with radiation, which would slowly erode much of the atmosphere of any planet which does not have a strong magnetic field.
Stars smaller than the Sun have liquid-water-zones much closer to the star so planets would experience larger tides which could remove axial tilt, resulting in a lack of seasons. This would lead to much colder poles and a much hotter equator, and over time the planet's water may eventually be boiled away. It is also possible that the planet's day could be synchronized with its year, causing one-half of the planet to permanently face the star and the other half to be permanently frozen.[28] Alternatively, the day could resonate with the year.
An extrasolar moon orbiting a gas giant in the habitable zone may ameliorate this somewhat. Being locked to a planet, which does not radiate substantial energy, as opposed to a star, would allow starlight to reach nearly all of the surface of the moon as it orbited its primary. However, this would eliminate seasonality, but nevertheless does offer greater prospects for complex life in red dwarf systems. With more consistent temperatures, life may not be limited to extremophilic lifeforms, assuming that other conditions for complex life are provided for on that moon.
The location of a planetary system within a galaxy must also be favorable to the development of life, and this has led to the concept of a galactic habitable zone (GHZ),[29][30] although the concept has been challenged.[31]
Planetary habitability theory suggests star systems favourable to life should be located close enough to the galactic center for sufficient levels of heavy elements to form rocky (terrestrial) planets. (This may not preclude life existing on gas giants or gaseous planets[32] which may be more common elsewhere, however life on gas giants (like Jupiter and Saturn) is currently considered less likely.)[33] On the other hand, the planetary system must be far enough from the galactic center it would not be affected by dangerous high-frequency radiation, which would damage any carbon-based life. A way for life to evolve despite these opposing requirements is that the Sun may have originated nearer the center but have migrated outwards.[34]
Also, most of the stars in the galactic center are old, unstable, dying stars, meaning few or no stars form in the galactic center.[35] Some types of spiral galaxies in later time periods have been depleted of gas in dust in regions near to the galactic center, resulting in minimal new star formation in those parts of the galaxy. Because terrestrial planets form from the same types of nebulae as stars, it can be reasoned if stars cannot form in the galactic center, terrestrial planets cannot, either.
In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years (8 kiloparsecs) from the galactic core and some 6,000 light years (2 kiloparsecs) in width, containing stars roughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smaller GHZ – or none at all (see: elliptical galaxy).
Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race.[36]
The Drake equation, which attempts to estimate the likelihood of non-terrestrial intelligent life, incorporates a factor (ne) for the average number of life-supporting planets in a star system with planets. The discovery of extrasolar Goldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earth hypothesis, which posits that a series of extremely unlikely events and conditions led to the rise of life on Earth. High estimates would reinforce the Copernican mediocrity principle, in that large numbers of Goldilocks planets would imply that Earth is not especially exceptional.
Finding Earth-sized Goldilocks planets is a key part of the Kepler Mission, which uses a space telescope (launched on 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets.[37] As of April 2011, Kepler has discovered 1,235 possible planets, with 54 of those candidates located within the Goldilocks zone.[38]
The majority of planets within our planet hunting neighbourhood are located within the GHZ, therefore the search for "habitable" planets has focused on data indicating a planet's position in the Goldilocks zone. The majority of these planets found have been gas giants, however more recently smaller Super-Earths and possible terrestrial planets have been detected in the zone.
Although the extrasolar planet 70 Virginis b (discovered in 1996) was initially nicknamed "Goldilocks" because it was thought to be within the star's CHZ, it is now believed to be closer to its sun making it far too warm to be "just right" for life, analogous to Venus thus it is not a Goldilocks planet.[39]
16 Cygni Bb (discovered in 1996) is a large gas giant with an eccentric orbit that was found to spend some of its time inside the habitable zone. However the orbit means it would experience extreme seasonal effects. Despite this, simulations suggest that an Earth-like moon would be able to support liquid water at its surface over the course of a year.[40]
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 thought not to be watery may possibility have habitable moons existing in orbit.[41]
Upsilon Andromedae d (discovered in 1999) is another gas giant discovered in the habitable zone considered large enough for the possibility of water clouds and watery moons.[42]
HD 28185 b (discovered April 4, 2001) is a gas giant found to orbit entirely within its star's habitable zone[43][44] and has a low orbital eccentricity, comparable to that of Mars in our solar system.[45] Tidal interactions suggest that HD 28185 b could harbor Earth-mass satellites in orbit around it for many billions of years.[46] Such moons, if they exist, may be able to provide a habitable environment, though it is unclear whether such satellites would form in the first place.[47]
55 Cancri f (discovered in 2005), a Jupiter like gas giant exoplanet, orbits and also resides within the yellow dwarf star companion of 55 Cancri binary star systems habitable zone.[48] While conditions upon 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 satellite moons to be orbiting the planet and thus transiting through this zone and being conducive for biological development.
The Gliese 581 system (first discovered in 2005) has a set of slightly oversized terrestrial planets mirroring our own solar system's. The third planet, planet c (discovered in 2007), is expected to be analogous to Venus's position (slightly too close), the fourth planet g (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifth planet d (discovered in 2007) to the Mars position. Planet d may be too cold, but unlike Mars, it is several times more massive than Earth and may have a dense atmosphere to retain heat. One caveat with this system is that it orbits a red dwarf, probably resulting in most of the issues regarding habitability of red dwarf systems, such as all the planets likely being tidally locked to the star.
On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "Habitable Zone."[49][50][51][52] Six candidates (KOI 326.01, KOI 701.03, KOI 268.01, KOI 1026.01, KOI 854.01, KOI 70.03) in the "Habitable Zone" are listed as smaller than twice the size of Earth,[52] although the one which got the most attention as "Earth-size" (KOI 326.01) turns out to be in fact much larger.[53] A September 2011 study by Muirhead et al. reports that a re-calibration of estimated radii and effective temperatures of several dwarf stars in the Kepler sample yields six additional Earth-sized candidates within the habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI 1361.01.[2] Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."[54] Also based on the findings, the Kepler Team estimates "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[6]
HD 85512 b (discovered in 2011) is believed to be an Earth-like planet in the HD 85512 system and declared one of the best candidates to date in terms of habitability.[55]
Kepler-22b, confirmed December 5, 2011.,[56] one of the first likely terrestrial planets detected in the habitable zone of a sun-like main sequence star (Kepler 22) using the transit method. Kepler 22-b is a super-Earth (2.4 times the size of Earth). With an atmosphere, the estimated surface temperature is around 22 degrees Celsius (-11 without an atmosphere).
Habitable zones are used in the Search for Extra-Terrestrial Intelligence and is based on the assumption that should intelligence extraterrestrial life exists elsewhere in the universe, that it would most likely be found there. Habitable zone theory has been incorporated into recent applications of the Drake equation to estimate the number of intelligent races in the Milky Way.
For Active Search for Extra-Terrestrial Intelligence, habitable zones are used as a means of selecting target stars for the transmission of interstellar radio messages (IRMs). In passive SETI, it is used to shortlist targets for sourcing non-natural radio emissions. The Allen Telescope Array is being used by the SETI Institute using a list of habitable candidate planets discovered by the Kepler space telescope.[57]
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