Habitable zone

Habitable zone

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In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and astrobiology
Astrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...

, a habitable zone (also referred to HZ, "life zone", "Comfort Zone") is an umbrella term
Umbrella term
An umbrella term is a word that provides a superset or grouping of concepts that all fall under a single common category. Umbrella term is also called a hypernym. For example, cryptology is an umbrella term that encompasses cryptography and cryptanalysis, among other fields...

 for regions (relative to star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s) that are considered favourable to life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

. The concept is inferred
Inference
Inference is the act or process of deriving logical conclusions from premises known or assumed to be true. The conclusion drawn is also called an idiomatic. The laws of valid inference are studied in the field of logic.Human inference Inference is the act or process of deriving logical conclusions...

 from the empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 study of conditions favourable for Life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

. The intersection of two kinds of habitable zones: one around a star (circumstellar habitable zone or CHZ also known as , "Green Belt", "Ecosphere" or "Goldilocks Zone"), and another within a galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 (galactic habitable zone or GHZ) is considered most favorable to life.

The CHZ is defined as the relatively narrow spherical annulus
Annulus (mathematics)
In mathematics, an annulus is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. Or, it is the area between two concentric circles...

 (spherical shell
Spherical shell
In geometry, a spherical shell is a generalization of an annulus to three dimensions. A spherical shell is the region between two concentric spheres of differing radii....

) region around a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 at which the temperature is between 273 and 373 Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, the range at which water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 can exist. Water is required to sustain life on Earth and surface water
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....

 is integral to the planet's biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

. Surface water could exist (with sufficient atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

) on an astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...

 within this zone. Therefore it follows that this region of planetary systems would be the best place to look for extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

, should it exist outside of our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

.

The location of a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s and natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s (moons) within its parent's star habitable zone (and a near circular orbit
Circular orbit
A circular orbit is the orbit at a fixed distance around any point by an object rotating around a fixed axis.Below we consider a circular orbit in astrodynamics or celestial mechanics under standard assumptions...

) is but one of many criteria for planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

. The term "Goldilocks planet" is used for any planet that is located within the CHZ although when used in the of context of planetary habitability the term implies terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s with conditions roughly comparable to those of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 (ie an Earth analog
Earth analog
An Earth analog is a theoretical other planet with conditions similar to Earth....

). 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
Goldilocks Principle
The Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. It is used, for example, in the Rare Earth hypothesis to state that a planet must neither be too far away from, nor too close to the sun to support life. Either extreme would result in...

 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

.

Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes. 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
Rare Earth hypothesis
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

.

Some planetary scientists suggests that habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other elements (such as methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

) could exist in stable liquid forms. Astrobiologists theorise that these environments could be conducive to alternative biochemistry
Alternative biochemistry
Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define...

. Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water
Extraterrestrial liquid water
Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life....

. It may follow for similar oceans consisting of ammonia or methane.

The habitable zone is used in the Active Search for Extra-Terrestrial Intelligence
Active SETI
Active SETI is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message...

 as a means of selecting of target stars for the transmission of Interstellar radio messages (IRMs). It is supposed that should intelligence extraterrestrial life exists elsewhere in the universe, that it would most likely be found in a habitable zone.

Circumstellar habitable zone


Within a planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

, a planet must lie within the habitable zone in order to sustain liquid water on its surface. Beyond the outer edge, a planet will not receive enough solar radiation to make up for radiative losses, leaving water to freeze. A planet closer than the inner edge of this zone will absorb too much radiation, boiling away surface water. The circumstellar habitable zone (CHZ) or ecosphere is the spherical shell of space surrounding a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 where such planets might exist. 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.

CHZ theory is based on empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 observation of the habitability of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and its orbit within Solar System.

Solar System Range estimation



Estimates for the CHZ within our own solar system range from 0.725 (the aphelion of planet Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 falls within this range) to 3.0 astronomical units (including the complete orbits of the planet Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and dwarf planet Ceres
CERES
CERES may refer to:* California Environmental Resources Evaluation System * Centre for European, Russian, and Eurasian Studies of the University of Toronto...

), based on various scientific models.

Estimation is made difficult due to a number of factors, notwithstanding the planetary habitability of the Earth and differences observed in other planetary bodies within the the Solar System's zone (the Earth being the only Solar System body known to have substantial surface water).
INNER edge OUTER edge References Notes
0.725 AU 1.24 AU Dole 1964 Used optically thin atmospheres and fixed albedos.
0.95 AU 1.01 AU Hart et al. 1978, 1979 stars K0 or later cannot have HZs
0.95 AU 3.0 AU Fogg 1992 Used Carbon cycles.
0.95 AU 1.37 AU Kasting et al. 1993
1%–2% farther out Budyko 1969 ... and Earth would have global glaciation.
1%–2% farther out Sellers 1969 ... and Earth would have global glaciation.
1%–2% farther out North 1975 ... and Earth would have global glaciation.
4%–7% closer Rasool & DeBurgh 1970 ... and oceans would never have condensed.
Schneider and Thompson 1980 disagreed with Hart.
Kasting 1991
Kasting 1988 Water clouds can shrink HZ as they counter GHG effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 with higher albedos.
Ramanathan and Collins 1991 GHG effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 IR trapping is greater than water cloud albedo cooling, and Venus would have to have started "Dry."
Lovelock 1991
Whitemire et al. 1991

Extrasolar extrapolation


Astronomers use apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

, luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 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
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 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.

Flare radiation


Small stars such as red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

s 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. See Habitability of red dwarf systems#Variability.

Tidal forces


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
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 resulting in a lack of seasons resulting in much colder poles and much hotter equator which over time would boil away all the water. Also 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.

Galactic habitable zone


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), although the concept has been challenged.

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
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

). This may not preclude life existing on gas giants or gaseous planets which may be more common elsewhere, however life on gas giants (like Jupiter and Saturn) is currently considered less likely.

On the other hand, the planetary system must be far enough from the galactic center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

 it would not be affected by dangerous high-frequency radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

, which would damage any carbon-based life. Also, most of the stars in the galactic center are old, unstable, dying stars, meaning few or no stars form in the galactic center. 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....

 (8 kiloparsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s) 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
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

).

Searching for planets and natural satellites in the zone


Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race
Human Race
Human Race refers to the Human species.Human race may also refer to:*The Human Race, 79th episode of YuYu Hakusho* Human Race Theatre Company of Dayton Ohio* Human Race Machine, a computer graphics device...

.

The Drake equation
Drake equation
The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...

, 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
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 Goldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earth hypothesis
Rare Earth hypothesis
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

, 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
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 mediocrity principle
Mediocrity principle
The mediocrity principle is the notion in philosophy of science that there is nothing very unusual about the evolution of our solar system, the Earth, any one nation, or humans. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the...

, 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
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

, which uses a space telescope (launched on 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets. As of April 2011, Kepler has discovered 1,235 possible planets, with 54 of those candidates located within the Goldilocks zone.

Discoveries in the zone



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.

Although the extrasolar planet 70 Virginis b
70 Virginis b
70 Virginis b is an extrasolar planet approximately 60 light-years away in the constellation of Virgo. Announced in 1996 by Geoffrey Marcy and R. Paul Butler, 70 Virginis was one of the first stars confirmed to have planets orbiting it...

 (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.

16 Cygni Bb
16 Cygni Bb
16 Cygni Bb or 16 Cyg Bb is an extrasolar planet approximately 70 light-years away in the constellation of Cygnus. The planet was discovered orbiting the sun-like star 16 Cygni B, one of two solar-mass components of the triple star system 16 Cygni...

 (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 season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

al effects. Despite this, simulations suggest that an Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

-like moon
Extrasolar moon
An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of...

 would be able to support liquid water at its surface over the course of a year.

Gliese 876 b
Gliese 876 b
Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese 876. It completes one orbit in approximately 61 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.- Discovery :...

 (discovered in 1998) and Gliese 876 c
Gliese 876 c
Gliese 876 c is an extrasolar planet orbiting the red dwarf star Gliese 876, taking about 30 days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star.- Discovery :...

 (discovered in 2001) are both gas giants discovered in the habitable zone around Gliese 876
Gliese 876
Gliese 876 is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius. As of 2011, it has been confirmed that four extrasolar planets orbit the star...

 although thought not to be watery may possibility have habitable moons existing in orbit.

Upsilon Andromedae d
Upsilon Andromedae d
Upsilon Andromedae d is an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A. Its discovery in April 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first star to be known to host a multiple-planet planetary system...

 (discovered in 1999) is another gas giant discovered in the habitable zone considered large enough for the possibility of water clouds and watery moons.

HD 28185 b
HD 28185 b
HD 28185 b is an extrasolar planet approximately 138 light-years away from Earth in the constellation of Eridanus. The planet was discovered orbiting the Sun-like star HD 28185 in April 2001 as a part of the CORALIE survey for southern extrasolar planets, and its existence was independently...

 (discovered April 4, 2001) is a gas giant was found to orbit entirely within its star's habitable zone and has a low orbital eccentricity, comparable to that of Mars in our solar system. However like earlier discoveries it is a gas giant. Tidal interactions suggest that HD 28185 b could harbor Earth-mass satellites in orbit around it for many billions of years. 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.

55 Cancri f
55 Cancri f
55 Cancri f is an extrasolar planet approximately 41 light-years away from Earth in the constellation of Cancer...

 (discovered in 2005), a Jupiter like gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

 exoplanet, orbits and also resides within the yellow dwarf star companion of 55 Cancri binary star system
55 Cancri
55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer...

s habitable zone. 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
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

 system (first discovered in 2005) has a set of slightly oversized terrestrial planets mirroring our own solar system's. The third planet, planet c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

 (discovered in 2007), is expected to be analogous to Venus's position (slightly too close), the fourth planet g
Gliese 581 g
Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth...

 (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifth planet d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....

 (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
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

, probably resulting in most of the issues regarding habitability of red dwarf systems
Habitability of red dwarf systems
Determining the habitability of red dwarf systems could help reveal the likelihood of extraterrestrial life, as red dwarfs make up most stars in the Milky Way Galaxy...

, such as all the planets likely being tidally locked to the star.

GJ 1214 b (December 16, 2009), though just outside of the habitable zone, does provide indications of being an ocean planet
Ocean planet
An ocean planet is a hypothetical type of planet whose surface is completely covered with an ocean of water.Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly half water and half rock by mass...

, meaning it is believed to be an extrasolar planet of the superearth variety, surrounded by a deep liquid ocean of water.

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." 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, although the one which got the most attention as "Earth-size" (KOI 326.01) turns out to be in fact much larger. 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.http://arxiv.org/abs/1109.1819 Based on these latest Kepler findings, astronomer Seth Shostak
Seth Shostak
Seth Shostak is an American astronomer. He grew up in Arlington, VA and earned his physics degree from Princeton University and a Ph.D...

 estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." 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.

HD 85512 b
HD 85512 b
HD 85512 b is an extrasolar planet orbiting the K-type star Gliese 370 approximately 36 light-years away in the constellation of Vela ....

 (discovered in 2011) is believed to be an Earth-like planet in the HD 85512 system.

Criticism

  • The concept of a habitable zone is criticized by Ian Stewart
    Ian Stewart (mathematician)
    Ian Nicholas Stewart FRS is a professor of mathematics at the University of Warwick, England, and a widely known popular-science and science-fiction writer. He is the first recipient of the , awarded jointly by the LMS and the IMA for his work on promoting mathematics.-Biography:Stewart was born...

     and Jack Cohen
    Jack Cohen (scientist)
    Jack Cohen, FIBiol is a British reproductive biologist also known for his popular science books and involvement with science fiction.-Life:...

     in their book Evolving the Alien
    Evolving the Alien
    Evolving the Alien: The Science of Extraterrestrial Life is a book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart.In this book Cohen and Stewart argue that any investigation of extraterrestrial life is too restrictive and unimaginative...

    , for two reasons: the first is that the hypothesis assumes alien life has the same requirements as terrestrial life; the second is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone". For instance, Jupiter
    Jupiter
    Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

    's moon Europa
    Europa (moon)
    Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

     is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophile
    Extremophile
    An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

    s (such as the tardigrades) on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan
    Carl Sagan
    Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

     believed that life was also possible on the gas giants, such as Jupiter
    Jupiter
    Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

     itself. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditions when the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".

  • Differing levels of volcanic activity, lunar effects, planetary mass, and even radioactive decay may affect the radiation and heat levels acting on a planet to modify conditions supporting life. And while it is likely that Earth life could adapt to an environment like Europa's, it is far less likely for life to develop there in the first place, or to move there and adapt without advanced technology. Therefore, a planet that has moved away from a habitable zone is more likely to have life than one that has moved into it.

  • Scientists describe extensive computer simulations in the Astrophysical Journal that show that, at least in galaxies similar to our own Milky Way, stars such as the Sun can migrate great distances, thus challenging the notion that parts of these galaxies are more conducive to supporting life than other areas.

See also


  • Alternative biochemistry
    Alternative biochemistry
    Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define...

  • Extraterrestrial liquid water
    Extraterrestrial liquid water
    Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life....

  • Rare Earth hypothesis
    Rare Earth hypothesis
    In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...


External links