Extraterrestrial liquid water
Encyclopedia
Extraterrestrial liquid water, the presence of 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...

 in its liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

.

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...

, with oceanic water covering 71% of its surface, is the only known 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,...

 with liquid water on its surface and is certainly the only one in the 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...

. There is growing evidence of sub-surface liquid water on several moons in our solar system orbiting the gas giant planets Jupiter, Saturn, Uranus and Neptune; however, none of these sub-surface moon water bodies has received final confirmation to date. Oceans and water may be common in other star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...

s and/or on their planets and other orbiting celestial bodies; for example, water vapour
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 was found in 2007 in the proplanetary disc of a young star MWC 480, within 1AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 of the star.
Large bodies of water and extensive water systems were once thought to cover 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...

 and 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...

. With increased telescopic
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 resolution and enhanced observation
Observation
Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...

 techniques in modern times, these were ultimately disproven; however, the presence of water on Mars in the distant past remains a topic of contemporary debate.

Obsolete theories

Lunar maria
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas".
Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 expressed some doubt about the lunar 'seas' in his Dialogue Concerning the Two Chief World Systems
Dialogue Concerning the Two Chief World Systems
The Dialogue Concerning the Two Chief World Systems was a 1632 Italian language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was translated to Latin as Systema cosmicum in 1635 by Matthias Bernegger...

.

Before space probes were landed, the idea of oceans on Venus was credible science. But it was discovered to be much too hot.

Telescopic observations from the time of Galileo onward have shown that Mars had no features resembling watery oceans. Mars' dryness was long recognized, and gave credibility to the spurious Martian canals.

Past surface water

Assuming that the Giant impact hypothesis
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...

is correct, there were never real seas or oceans on the moon.
Astronomers believe that Venus
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

 had liquid water and perhaps oceans in its very early history.
Given that Venus has been completely resurfaced
Geology of Venus
Venus is a planet with striking surface characteristics. Most of what is known about its surface stems from radar observations, mainly images sent by the Magellan probe between August 16, 1990 and the end of its sixth orbital cycle in September 1994...

 by its own active geology, the idea of a primeval ocean is hard to test. Rock samples may one day give the answer.

It was once thought that Mars might have dried up from something more Earth-like. The initial discovery of a cratered surface made this seem unlikely, but further evidence has changed this view. Liquid water may have existed on the surface of Mars in the distant past, and several basins on Mars have been proposed as dry sea beds. The largest is Vastitas Borealis
Vastitas Borealis
Vastitas Borealis is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the Northern plains or Northern lowlands of Mars. The plains lie 4–5 km below the mean radius of...

; others include Hellas Planitia
Hellas Planitia
Hellas Planitia, also known as the Hellas Impact Basin, is a huge, roughly circular impact basin located in the southern hemisphere of the planet Mars. It is the second or third largest impact crater and the largest visible impact crater known in the Solar System...

 and Argyre Planitia
Argyre Planitia
Argyre Planitia is a plain located in the Argyre impact basin in the southern highlands of Mars. Its name comes from a map produced by Giovanni Schiaparelli in 1877; it refers to Argyre, a mythical island of silver in Greek mythology....

.

There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did. Recent findings by the Mars Exploration Rover
Mars Exploration Rover
NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...

 mission indicate it had some long-term standing water in at least one location, but its extent is not known.

Groundwater

It is thought that liquid water
Water on Mars
Water on Mars is a psychedelic rock and electronic music group from Quebec City, Québec, Canada. The music trio is led by Philippe Navarro, guitarist, vocalist, arranger, producer, principal lyricist, and music composer....

 may exist in the Martian subsurface. Research suggests that in the past there was liquid water flowing on the surface, creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone. There are a number of direct and indirect proofs of water's presence either on or under the surface
Geology of Mars
The geology of Mars is the scientific study of the surface, crust, and interior of the planet Mars. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is fully analogous to the field of terrestrial geology. In planetary science, the term geology is...

, e.g. stream bed
Stream bed
A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...

s, polar caps, spectroscopic measurement
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

, eroded
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 craters or mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s directly connected to the existence of liquid water (such as Goethite
Goethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...

). In an article in the Journal of Geophysical Research
Journal of Geophysical Research
The Journal of Geophysical Research is a peer-reviewed scientific journal published by the American Geophysical Union 80 times per year. It contains original research on the physical, chemical, and biological processes that contribute to the understanding of the Earth, Sun, and solar system...

, scientists studied Lake Vostok
Lake Vostok
Lake Vostok is the largest of more than 140 subglacial lakes found under the surface of Antarctica. The overlying ice provides a continuous paleoclimatic record of 400,000 years, although the lake water itself may have been isolated for 15 to 25 million years. The lake is named after the...

 in Antarctica and discovered that it may have implications for liquid water still being on Mars. Through their research, scientists came to the conclusion that if Lake Vostok existed before the perennial glaciation began, that it is likely that the lake did not freeze all the way to the bottom. Due to this hypothesis, scientists say that if water had existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps that may even contain evidence of life.

Thomas Gold
Thomas Gold
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...

 has posited that many Solar System bodies could potentially hold groundwater farther down. http://www.pnas.org/cgi/reprint/89/13/6045.pdf

Icy moon and trans-Neptunian object subsurface oceans

Subsurface oceans have been postulated for most of the icy moon
Icy moon
Icy moons are believed to be a common class of natural satellites or planetoids with surfaces composed mostly of ice. An icy moon may harbor an ocean underneath the surface, and possibly include a rocky core of silicate or metallic rocks. It is thought that they may be composed of ice II...

s of the outer planets, which are covered by a thick layer of water ice. In some cases it is thought that an ocean layer may have been present in the past, but has since cooled into solid ice.

Liquid water is thought to be present under the surface of several 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, particularly the Galilean moons of Jupiter, such as 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...

 (liquid water underneath its icy surface due to tidal heating
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

), and, with less certainty, Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

 and Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

.

Models of heat retention and heating via radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 in smaller icy bodies suggest that Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

, Titania
Titania (moon)
Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...

, Oberon
Oberon (moon)
Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies...

, Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, Pluto, Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

, Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

, and Orcus
90482 Orcus
90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified...

 may have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases are the fact that the models predict that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.

Geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...

s have been found on Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

. These contain water vapour and may mean liquid water deeper down. The water is either heated tidally or geothermally. It is known that Enceladus has liquid water, as there are active cryovolcanic mountains around its southern pole. It could also be just ice. In June 2009, evidence was put forward for salty subterranean oceans.

It was believed after the Voyager
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 observations that Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 might have seas or oceans of liquid hydrocarbons. The Cassini-Huygens
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

 space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. A more recent fly-by of Titan made by Cassini has produced radar images that strongly suggest hydrocarbon lakes near the polar regions where it is colder. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.

Gas giants

Jupiter possesses a gaseous layer where, because of the Earthlike temperature and pressure, droplets may condense
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 from the water vapor.

Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 may possess large oceans of hot, highly compressed, supercritical water under their thick atmospheres, though their internal structure is not well understood at this time. It is agreed that they are different from the 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...

s Jupiter and Saturn—some astronomers would class them separately as 'ice giants'.

Water-ammonia layers on cool small bodies

A solution of water with 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...

 would have an antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....

 effect, and enable liquid water to exist in small bodies that would otherwise be too cold.

The dwarf planet Ceres is believed to contain large amounts of water-ice, and might possess a tenuous atmosphere. It is too cold for liquid water, but an ocean of water plus ammonia has been suggested. More information will be available in 2015, when the Dawn Mission
Dawn Mission
Dawn is a NASA spacecraft tasked with the exploration and study of the two largest members of the asteroid belt – Vesta and the dwarf planet Ceres. The spacecraft was constructed with some European cooperation, with partners in Germany, Italy, and the Netherlands providing Dawns framing...

 visits it.

Liquid water inside comets

Comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s contain large proportions of water ice, but are generally thought to be completely frozen due to their small size and large distance from the Sun.
However, studies on dust collected from comet Wild-2 show evidence for liquid water inside the comet at some point in the past. It is yet unclear what source of heat may have caused melting of some of the comet's water ice.

Extrasolar Habitable zone Candidates for water

Most of the 450+ detected extrasolar planetary systems
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...

 look very different from ours, making 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...

 appear atypical, though there is probably a bias arising from the detection methods
Methods of detecting extrasolar planets
Any planet is an extremely faint light source compared to its parent star. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out...

.
The goal of current searches is to find Earth-sized planets in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of their planetary systems (also sometimes called the Goldilocks zone). Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist. The Kepler telescope might be sensitive enough to detect them. But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.

55 Cancri e

55 Cancri e
55 Cancri e
55 Cancri e is an extrasolar planet with half the mass of Neptune orbiting the Sun-like star 55 Cancri A. Its mass is about 8.3 Earth masses, thus classifying it as the first Super-Earth discovered around a main sequence star, predating Gliese 876 d by a year...

 orbits very close to its sun, but observations suggest that it is 30% supercritical water, a global ocean.

55 Cancri f

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...

 is a large planet orbiting in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of the star 55 Cancri A
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...

. Its composition is unknown but it is believed to be a gas giant. If it has rocky moons, these could have liquid water.

There is also a gap in the orbits of that system's five (known) planets which might contain something more Earth-like. If it exists, it cannot be detected by present methods, though these are constantly being improved.

AA Tauri

AA Tauri
AA Tauri
AA Tauri is a young star in the constellation of Taurus, located in the young Taurus-Auriga Star Forming Region, roughly at 460 light years away from Sun.-A planetary system?:...

 is a young star, less than a million years old and a typical example of a young star with a protoplanetary disk. Astronomers have recently found the spectral signatures of water vapor, plus three simple organic molecules - hydrogen cyanide, acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.

COROT-7b

COROT-7b is an exoplanet less than twice the size of Earth orbiting a Sun-like star very closely. It was discovered by the CoRoT
Corot
Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...

 space telescope and was announced to the public on 3 February 2009. The surface is predicted to be in the range of 1,000–1,500 degrees Celsius, but since its composition is not known it could be covered in molten lava or enshrouded in a thick water vapour cloud layer. The planet could also be made up of water and rock in almost equal amounts. If COROT-7b is water-rich it could be 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...

’.

COROT-9b

COROT-9b
COROT-9b
COROT-9b is an exoplanet orbiting the star COROT-9, approximately 1500 light years away in the constellation Serpens. COROT-9b's distance of nearest approach to its parent star of approximately 0.36 AU is the largest of all known transiting planets, with an orbital period of 95 days. The transit of...

 has been called a temperate exoplanet as its cloudtop temperature ranges from -20 degrees to 160 degrees Celsius. It is the size of Jupiter but a similar distance as Mercury is from our Sun. There are other similar planets cases known, but this planet can be studied in detail because it transits its star. Although it is mostly made of hydrogen and helium it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures.

Gliese 581 c, d and g

Gliese 581 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...

, a world five times the size of the Earth, was originally reported to be the right distance from its sun for liquid water to exist on the planet's surface. Since it does not transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

 its sun, there is no way to know if there is any water there.

Later work suggests that Gliese 581 c would probably be too hot for liquid water. It was then suggested that Gliese 581 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....

 might be warm enough for oceans if a greenhouse 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...

 was operating. Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.

Gliese 581 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....

 looks an even better candidate. The orbital period was originally estimated at 83 days and has now been revised to 66 days. This was announced along with another new world, Gliese 581 e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...

, which is next to twice the mass of Earth but too close to its sun for liquid water. In May 2011, a new study suggested that the planet might have a thick atmosphere, oceans and even life.

The unconfirmed planet Gliese 581 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...

 is another good candidate . This planet is estimated to be between three to four times as massive as the earth, and as such it is too small to be a gas giant. The orbital period is estimated at 37 days, which places its orbit right in the middle of the habitable zone of the star Gliese 581.

GJ 1214 b

GJ 1214 b is only the second exoplanet (after CoRoT-7b) to have an established mass and radius less than those of the giant Solar System planets. It is three times the size of Earth and about 6.5 times as massive. Its low density indicates that it is likely a mix of rock and water. However, the phase state of the water is unknown. Studying its atmosphere should be possible and will tell us more.

HD 28185 b

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...

 was the first exoplanet to be detected in the habitable zone. The planet has only been detected indirectly, but is believed to be a gas giant, with no solid surface. Some scientists have argued that it could have moons large and stable enough to have oceans.

HD 85512 b

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 ....

 was discovered in August 2011. It is larger than Earth, but small enough to be probably a rocky world. It is on the borders of its star's habitable zone and might have liquid water, maybe even life.

MOA-2007-BLG-192Lb

MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of...

 is a small planet orbiting a small star. It is about three Earth masses, currently the second smallest detected extrasolar planet orbiting a normal star, after Gliese 581 e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...



The planet orbits its host star or brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

 with an orbital radius similar to that of Venus. But the host is likely to be between 3,000 and 1 million times fainter than the sun, so the top of the planet's atmosphere is likely to be colder than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

. However, the planet is likely to maintain a massive atmosphere that would allow warmer temperatures at lower altitudes. It is even possible that interior heating by radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

s would be sufficient to make the surface as warm as the Earth, but theory suggests that the surface may be completely covered by a very deep ocean.

TW Hydrae
TW Hydrae
TW Hydrae is an orange dwarf star approximately 176 light-years away in the constellation of Hydra . The star is the closest T Tauri star to the Solar System. TW Hydrae is similar in mass to the Sun, but is only about 5-10 million years old...

This very young star is in the process of forming a solar system. Scientists have now detected clouds of water vapour cold enough to form comets. This could eventually deliver oceans to dry planets, which is believed by most scientists to have happened on the early Earth and other rocky planets.

Water vapour has previously been detected in planet-forming disks, but too warm to form comets. This cloud is cool enough and is estimated to contain thousands of Earth-oceans' worth of water.

Kepler First Results

Among the 1,235 possible extra-solar planet candidates detected by NASA's planet-hunting Kepler space telescope during its first four months of operation, 54 are orbiting in the parent star's habitable 'Goldilocks' zone where liquid water could exist. Five of these are near Earth-size, and the remaining 49 habitable zone candidates range from twice the size of Earth to larger than Jupiter.

See also

  • Extraterrestrial water vapor

External links

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