List of extrasolar candidates for liquid water

Artist's illustration of the signatures of water in exoplanet atmospheres by Hubble.^[1] Water vapor and ice have been found to be common elements of extraterrestrial atmospheres, however water in liquid form has not been confirmed beyond the Earth.

Extraterrestrial liquid water in the Solar System is likely uncommon, although it has been hypothesized to exist in some of its moons, and to have formerly existed on Mars and Venus.^[2][3] Extrasolar liquid water has not yet been found to exist. The following list contains candidates from the list of confirmed exoplanets.

Most known extrasolar planetary systems appear to have very different compositions from the Solar System, though there is probably sample bias arising from the detection methods.

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).^[4] 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.^[5] But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.^[6]

Summary

Image	Planet	Star	Mass (x M⊕)	Temp. (surf. mean, K)	Distance (ly)	Age (gy)	Orbit (days)	Discovery (year)	Notes
	Earth	Sun	1	288	0	4.54	365		For comparison.
	55 Cancri e	55 Cancri	8.63	<2000	41	7.4–8.7	0.73	2004
	55 Cancri f	55 Cancri	~47.5	200–300	41	7.4–8.7	260	2005
		AA Tauri			456.4
		Alpha Centauri B			4.37
	COROT-7b	COROT-7	<9	1300–1800	489	1.2–2.3	0.85	2009
	COROT-9b	COROT-9	266.9	250–430	1500		95	2010
		GD 61			150
	Gliese 581 c	Gliese 581	5.5	700–1000	20	7–11	12.9	2007
	Gliese 581 d	Gliese 581	6.98	~181	20	7–11	66.8	2007	Disputed.
	Gliese 581 g	Gliese 581	2.2	242–261	20	7–11	32	2010	Disputed.
	Gliese 667 Cc	Gliese 667 C	3.7	277.4	23	>2	28.1	2011
	GJ 1214 b	GJ 1214	7	393–555	42	6	1.58	2009
	HD 28185 b	HD 28185	1811.46		138	7.5	379.6	2001
	HD 85512 b	HD 85512	3.6	298	36	5.6	54.43	2011
	MOA-2007-BLG-192Lb	MOA-2007-BLG-192L	3.3		3000			2008
	Kapteyn b	Kapteyn's Star	4.8	205	12.8	~11	48.61	2014
	Kepler-22b	Kepler-22	<52.8	295	600	~4	289.8	2011
	Kepler-62e	Kepler-62	5	~270	1200	7	122.3	2013
	Kepler-62f	Kepler-62	1.413–2.80	~208	1200	208	267.2	2013
	Kepler-69c	Kepler 69	~6	548	2700	0.4	242.4	2013
	Proxima b	Proxima Centauri	1.27 (min.)	234	4.2	<4.85	11.1	2016
		TW Hydrae			176

Description (alphabetical order)

55 Cancri e

55 Cancri e orbits very close to 55 Cancri, but observations suggest that it is 30% supercritical water, a global ocean.^[7] Follow-up analysis of its transit failed to detect traces of either water or hydrogen^[8] making it highly unlikely that liquid water is present at the surface.

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.^[9]

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.^[10] Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.

Alpha Centauri B

The nearby star Alpha Centauri B may have a planet with oceans.^[11] This is speculative: the only confirmed planet in the system is Alpha Centauri Bb, which would be much too hot.

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 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’.^[12]

COROT-9b

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 planet 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.^[13]

GD 61

GD 61 is a white dwarf star, with an asteroid which has given the first direct evidence of a water-rich rocky planetary body outside the Solar System. The asteroid may be part of the debris from what might once have been a rocky planet with either ice or oceans.^[14]

Previous detections of water vapor have been in giant planets. This find confirms that rocky planets with water exist outside of the Solar System.

Artist's impression of Upsilon Andromedae d, portrayed as a class II planet with water vapor clouds, as seen from a hypothetical large moon with surface liquid water

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.^[15] 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.^[16] 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.^[17] 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.^[18]

The unconfirmed planet Gliese 581 g is another good candidate. This planet is estimated to be between three and 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.^[19]

Gliese 667 C - three planets

Artist's impression of Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system.^[20]

Gliese 667 Cc was originally described as 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.^[21] 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, so 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.^[22]

Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone.^[23]

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,^[24] 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'.^[25][26]

HD 28185 b

HD 28185 b was the first exoplanet to be detected in the habitable zone.^[27] 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.^[28]

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, and is a potential candidate for a life-supporting world.^[29][30]

MOA-2007-BLG-192Lb

MOA-2007-BLG-192Lb is a small planet orbiting a small star. It is about 3 Earth masses, currently the second smallest detected extrasolar planet orbiting a normal star, after Gliese 581 e.

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 decay 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.^[31]

Kapteyn b

Kapteyn b is a super-Earth orbiting within the habitable zone of Kapteyn's Star, which is 13 light-years away and is 11 billion years old.^[32]

Kepler-22b

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.^[33]

Kepler-62e and Kepler-62f

The star Kepler-62 has five planets, two of which are the right distance from the star to have liquid water and potentially sustain life.^[34]

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.^[35] Both are assumed to be rocky planets, but since the star is 1200 light-years away, it is hard to be sure.

Kepler-69c

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.

NASA announced its discovery on 18 April 2013, along with the two Earth-like planets of Kepler 62.^[35]

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.^[36] 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.^[37]

Proxima b

Proxima Centauri b, the nearest known exoplanet, is in the habitable zone of its host star, and might contain liquid water.^[38][39] More details about the planet's physical conditions are needed for a proper evaluation of its habitability.^[40][41][42]

TW Hydrae

TW Hydrae is a very young star is in the process of forming a planetary 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.^[43]

See also

• Lists of exoplanets
• List of potentially habitable exoplanets

References