Extraterrestrial liquid water
Earth, with oceanic water covering 71% of its surface, is the only planet known to have stable bodies of liquid water on its surface and liquid water is essential to all life on Earth. Earth orbits within the Sun's habitable zone, a region in which liquid water can exist on the surface of a planet with sufficient atmospheric pressure.
There is growing evidence of liquids below the surface of several moons and dwarf planets elsewhere in the Solar System that may consist of liquid water, some are speculated to be large "oceans"; however, none have been confirmed. Oceans and water may be common in other planetary systems; for example, water vapour was found in 2007 in the proplanetary disc of a young star MWC 480, within 1 AU of the star.
Solar System 
Obsolete theories 
Lunar maria 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 expressed some doubt about the lunar 'seas' in his Dialogue Concerning the Two Chief World Systems.[a]
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 is correct, there were never real seas or oceans on the Moon, only perhaps a little moisture (liquid or ice) in some places, when the Moon had a thin atmosphere created by degassing of volcanoes or impacts of icy bodies.
Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. Given that Venus has been completely resurfaced 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; others include Hellas Planitia and Argyre Planitia.
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 mission indicate it had some long-term standing water in at least one location, but its extent is not known. The Opportunity Mars rover photographed bright veins of a mineral leading to conclusive confirmation of deposition by liquid water.
It is thought that liquid water 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, e.g. stream beds, polar caps, spectroscopic measurement, eroded craters or minerals directly connected to the existence of liquid water (such as Goethite). In an article in the Journal of Geophysical Research, scientists studied Lake Vostok 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.
Icy-moon and trans-Neptunian-object subsurface oceans 
Subsurface oceans have been postulated for most of the icy moons 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 satellites, particularly the Galilean moons of Jupiter, such as Europa (liquid water underneath its icy surface due to tidal heating), and, with less certainty, Callisto and Ganymede.
Models of heat retention and heating via radioactive decay in smaller icy bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases is 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.
Geysers have been found on Enceladus. 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 observations that Titan might have seas or oceans of liquid hydrocarbons. The Cassini–Huygens 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 
Water in Jupiter's stratosphere appears to have its origins in comet impacts. The possibility of Uranus and Neptune possessing hot, highly compressed, supercritical water[b] under their thick atmospheres has been hypothesised. While their composition is still not fully understood, a 2006 study by Wiktorowicz et al ruled out the possibility of such an water "ocean" existing on Neptune, though some studies have suggested that exotic oceans of liquid diamond are possible.
Water-ammonia layers on cool small bodies 
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 should be available in 2015, when the Dawn Mission is due to become the first space probe to explore the planet.
Liquid water inside comets 
Comets 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 
The goal of current searches is to find Earth-sized planets in the habitable zone 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 f 
55 Cancri f is a large planet orbiting in the habitable zone of the star 55 Cancri A. 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 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 and carbon dioxide. Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.
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 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’.
COROT-9b 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, 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 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 might be warm enough for oceans if a greenhouse effect was operating. Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.
Gliese 581 d 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, 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 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.
Gliese 667 Cc 
Gliese 667 Cc is one of two 'super-Earth' planets around Gliese 667 C, a dim red star that is part of a triple star system. The stars of this system have a concentration of heavy elements only 25% that of our Sun's. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for such star systems to have an abundance of low mass planets. It seems that habitable planets can form in a greater variety of environments than previously believed.
Gliese 667 Cc, in a tight 28-day orbit of a dim red star, must receive 90% of the light that Earth receives, but most of its incoming light is in the infrared, a higher percentage of this incoming energy should be absorbed by the planet. The planet is expected to absorb about the same amount of energy from its star that Earth absorbs from the Sun, which would allow surface temperatures similar to Earth and perhaps liquid water.
GJ 1214 b 
GJ 1214 b was 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 indicated that it is likely a mix of rock and water, and follow-up observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water, so it is a large waterworld. The high temperatures and pressures would form exotic materials like 'hot ice' or 'superfluid water'.
HD 28185 b 
HD 28185 b 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 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.
The planet orbits its host star or brown dwarf 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. 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 decays 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.
Kepler-22b is a planet 2.4 times the size of the Earth, with an estimated temperature of 22 °C. It was one of 54 candidates found by the Kepler telescope and reported in February as potentially habitable. It is the first of these to be formally confirmed using other telescopes. Its composition is currently unknown.
Kepler-62e and Kepler-62f 
Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth. Both are assumed to be rocky planets, but since the star is 1200 light-years away, it is hard to be sure.
This large rocky planets is one of two known to be orbiting the star Kepler 69, which is similar to our sun. It is believed to be in the star's habitable zone.
It is 70% more massive than the Earth and has a 242-day orbit, similar to that of Venus in our own solar system.
Kepler (other results) 
Among the 1,235 possible extrasolar 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.
TW Hydrae 
TW Hydrae is a 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.
See also 
- Extraterrestrial water vapor
- Gliese 163 c
- Gliese 581 d
- Gliese 581 g
- Gliese 667 Cc
- Habitable zone
- HD 85512 b
- Kepler 22 b
- List of nearest terrestrial exoplanet candidates
- Planetary habitability
- 'Salviati', who normally gives Galileo's own opinions, says:
I say then that if there were in nature only one way for two surfaces to be illuminated by the sun so that one appears lighter than the other, and that this were by having one made of land and the other of water, it would be necessary to say that the moon's surface was partly terrene and partly aqueous. But because there are more ways known to us that could produce the same effect, and perhaps others that we do not know of, I shall not make bold to affirm one rather than another to exist on the moon...
What is clearly seen in the moon is that the darker parts are all plains, with few rocks and ridges in them, though there are some. The brighter remainder is all fill of rocks, mountains, round ridges, and other shapes, and in particular there are great ranges of mountains around the spots...
I think that the material of the lunar globe is not land and water, and this alone is enough to prevent generations and alterations similar to ours.
- At pressures above one million atmospheres (for example, centre of Uranus is about 8 million atmospheres and 5000 Kelvin) water exists in a supercritical state with properties very different from water at one atmosphere. (Water Phase Diagram)
- "Earth". Nineplanets.org.
- Josh A. Eisner (2007). "Water vapour and hydrogen in the terrestrial-planet-forming region of a protoplanetary disk". Nature 447 (447): 562–564. arXiv:0706.1239. Bibcode:2007Natur.447..562E. doi:10.1038/nature05867.
- Did oceans on Venus harbour life?, issue 2626 of New Scientist magazine.
- "Mars Probably Once Had A Huge Ocean". Sciencedaily.com. 2007-06-13. Retrieved 2012-01-22.
- Jpl.Nasa.Gov. "NASA Mars Rover Finds Mineral Vein Deposited by Water — NASA Jet Propulsion Laboratory". Jpl.nasa.gov. Retrieved 2012-01-22.
- "Science@NASA, The Case of the Missing Mars Water". Retrieved 2009-03-07.
- "Water on Mars: Where is it All?". Retrieved 2009-03-07.
- "Water at Martian south pole". 17 March 2004. Retrieved 29 September 2009.
- "A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars". Retrieved 2009-04-08.
- Hussmann, H.; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
- "Cassini Images of Enceladus Suggest Geysers Erupt Liquid Water at the Moon's South Pole". Ciclops.org. 2006-03-09. Retrieved 2012-01-22.
- "Saturn's Moon Enceladus Is Unlikely To Harbor Life". Sciencedaily.com. 2007-08-14. Retrieved 2012-01-22.
- "Possible salty ocean hidden in depths of Saturn moon". Astronomynow.com. 2009-06-25. Retrieved 2012-01-22.
- "Saturn's Moon Titan – Land Of Lakes And Seas". Sciencedaily.com. 2007-10-11. Retrieved 2012-01-22.
- "Mysterious signal hints at subsurface ocean on Titan". Space.newscientist.com. Retrieved 2012-01-22.
- Briggs, Helen (2008-03-20). "Saturn moon may have hidden ocean". BBC News. Retrieved 2012-01-22.
- Cavalié, T.; Feuchtgruber, H.; Lellouch, E.; de Val-Borro, M.; Jarchow, C.; Moreno, R.; Hartogh, P.; Orton, G.; Greathouse, T. K.; Billebaud, F.; Dobrijevic, M.; Lara, L. M.; González, A.; Sagawa, H. (2013). "Spatial distribution of water in the stratosphere of Jupiter fromHerschelHIFI and PACS observations". Astronomy & Astrophysics 553: A21. doi:10.1051/0004-6361/201220797. ISSN 0004-6361.
- Wiktorowicz, Sloane J.; Ingersoll, Andrew P. (2007). "Liquid water oceans in ice giants". Icarus 186 (2): 436–447. doi:10.1016/j.icarus.2006.09.003. ISSN 00191035.
- Silvera, Isaac (2010). "Diamond: Molten under pressure". Nature Physics 6 (1): 9–10. doi:10.1038/nphys1491. ISSN 1745-2473.
- "Largest Asteroid Might Contain More Fresh Water than Earth". Space.com. 2005-09-07. Retrieved 2012-01-22.
- "Asteroid Ceres". Solarviews.com. Retrieved 2012-01-22.
- Dawn's destinations, Astronomy Now June 2007.
- "Frozen comet's watery past: Discovery challenges paradigm of comets as 'dirty snowballs' frozen in time". Sciencedaily.com. 2011-04-05. doi:10.1016/j.gca.2011.03.026. Retrieved 2012-01-22.
- "Habitable planets may be common". Newscientist.com. Retrieved 2012-01-22.
- "The hunt for habitable exomoons". Astronomynow.com. 2009-09-04. Retrieved 2012-01-22.
- "Water, water everywhere". Astronomynow.com. Retrieved 2012-01-22.
- "Astrophile: Supercritical water world does somersaults". Newscientist.com. Retrieved 2012-01-22.
- Bowdler, Neil (2007-11-07). "Astronomers discover new planet". BBC News. Retrieved 2012-01-22.
- "Organics and water found where new planets may grow". Spaceflightnow.com. Retrieved 2012-01-22.
- "CoRoT discovers most Earth-like exoplanet yet". Astronomynow.com. 2009-02-03. Retrieved 2012-01-22.
- "Sizing up a temperate exoplanet". Astronomynow.com. 2010-03-17. Retrieved 2012-01-22.
- "New Planet Could Harbor Water and Life". Space.com. 2007-04-24. Retrieved 2012-01-22.
- "Scientists might have picked right star, wrong world for hosting life". MSNBC. 2007-06-18. Retrieved 2012-01-22.
- "Sibling worlds may be wettest and lightest known". Newscientist.com. Retrieved 2012-01-22.
- "Exoplanet near Gliese 581 star 'could host life'". BBC News. 2011-05-17. Retrieved 2012-01-22.
- "NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet". Release 10-237. NASA. 2010-09-29.
- "Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way". ESO Press Release. Retrieved 29 March 2012.
- New Super-Earth Detected Within the Habitable Zone of a Nearby Cool Star
- Super-Earth orbits in habitable zone of cool star
- "The small planet with a thick coat". Astronomynow.com. 2009-12-17. Retrieved 2012-01-22.
- Distant 'waterworld' is confirmed
- Hubble Reveals a New Class of Extrasolar Planet
- Jones, B. W.; Sleep, P. N.; Underwood, D. R. (2006). "Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties". The Astrophysical Journal 649 (2): 1010. arXiv:astro-ph/0603200. Bibcode:2006ApJ...649.1010J. doi:10.1086/506557.
- Barnes, J. W.; O’Brien, D. P. (2002). "Stability of Satellites around Close‐in Extrasolar Giant Planets". The Astrophysical Journal 575: 1087. arXiv:astro-ph/0205035. Bibcode:2002ApJ...575.1087B. doi:10.1086/341477.
- "Exoplanet Looks Potentially Lively". Scientificamerican.com. Retrieved 2012-01-22.
- "'Super-Earth,' 1 of 50 Newfound Alien Planets, Could Potentially Support Life". News.yahoo.com. 2011-09-12. Retrieved 2012-01-22.
- "Small Planet Discovered Orbiting Small Star". Sciencedaily.com. 2008-06-02. Retrieved 2012-01-22.
- "Kepler 22-b: Earth-like planet confirmed". BBC News. 2011-12-05. Retrieved 2012-01-22.
- Kepler telescope spies 'most Earth-like' worlds to date
- NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date
- "Kepler detects more than 1,200 possible planets". Spaceflightnow.com. Retrieved 2012-01-22.
- "NASA Finds Earth-Size Planet Candidates in Habitable Zone, Six Planet System". Sciencedaily.com. 2011-02-02. doi:10.1038/nature09760. Retrieved 2012-01-22.
- "Nearby Planet-Forming Disk Holds Water for Thousands of Oceans". Sciencedaily.com. 2011-10-20. doi:10.1126/science.1208931. Retrieved 2012-01-22.
- The Extrasolar Planets Encyclopaedia
- Gliese 581: Extrasolar Planet Might Indeed Be Habitable
- Jupiter's Moon Europa: What Could Be Under The Ice?
- To Curious Aliens, Earth Would Stand Out As Living Planet
- Ocean-bearing Planets: Looking For Extraterrestrial Life In All The Right Places