List of extrasolar candidates for liquid water

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Artist's illustration of the signatures of water in exoplanet atmospheres by Hubble. 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. Atmosphere of exoplanet.jpg
Artist's illustration of the signatures of water in exoplanet atmospheres by Hubble. 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 confirmed to exist. The following list contains candidates that meet the following criteria:

Contents

Most known extrasolar planetary systems appear to have very different compositions from the Solar System, though there is sampling 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 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]

In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies. [7]

Planets

ImagePlanetStarMass
(× M⊕)		Temp.
(surf. mean, K)		Distance
(ly)		Age
(gy)		Orbit
(days)		Discovery
(year)		Notes
The Blue Marble (remastered).jpg Earth Sun 128804.54365For comparison.
Artist's concept shows four of the five planets that orbit 55 Cancri, a star much like our own.jpg Harriot [ citation needed ] Copernicus ~47.5200–300417.4–8.72602005* Gas giant likely has no surface, liquid water if present could only be on a large satellite (none known). [8]
* Orbits within CHZ [9]
* May be a Sudarsky Class II based on temperature observations[ citation needed ]
Artist's impression of Proxima Centauri b shown hypothetically as an arid rocky super-earth.jpg Proxima Centauri b Proxima Centauri 1.272344.37<4.611.1862016* Orbits within the CHZ
* Likely terrestrial.
* Likely tidally locked to its star, if so, liquid water may be at terminator
* 2000 times solar wind calculated, likely to break down any water, at least on the likely fixed star-facing side
Artist's impression of Corot-7b (alternative).jpg COROT-7b [ citation needed ] COROT-7 <91300–18004891.2–2.30.852009* Not in the CHZ
* Possibly terrestrial (unknown)
* Most likely a tidally locked lava planet, [10] temperatures facing the star way too high.
* Remote possibility of volatiles, but very likely have formed too close to star. [11]
* Would need to possess cool night side with sufficient atmospheric pressure
Exoplanet Comparison CoRoT-9 b.png COROT-9b [ citation needed ] COROT-9 266.9250–4301500952010* Not in the CHZ
* Gas giant likely has no surface, liquid water if present could only be on a large satellite (none known)
* density implies water in atmosphere, but none found yet
* Possible class II ("water cloud") or class III ("clear") atmosphere planet [12]
Planet Gliese 581 c.png Gliese 581 c [ citation needed ] Gliese 581 5.5700–1000207–1112.92007* Not in the CHZ
* Possibly not rocky
* Water may be present as solid or vapour rather than liquid. [13]
Gliese 667 Cc sunset.jpg Gliese 667 Cc Gliese 667 C 3.7277.423>228.12011* Orbits in middle of CHZ, but likely hotter than Earth
* Likely terrestrial.
* Tidal heating 300 times that of Earth may superheat the planet
* Likely tidally locked, liquid water may be limited to terminator, or beyond with sufficient atmosphere
Artist's impression of GJ 1214b.jpg Enaiposha Orkaria 7393–5554261.582009* Not in the CHZ
* Super-Earth sized
* Composition unknown
* Density models predict a wet core (unlikely to possess a surface) covered by thick atmosphere composed of volatiles similar to Neptune [14] [15] [16]
HD 28185 b - Portrait.png HD 28185 b [ citation needed ] HD 28185 1811.461387.5379.62001
HD 85512 Planetary system.jpg HD 85512 b [ citation needed ] HD 85512 3.6298365.654.432011
MOA-2007-BLG-192L.jpg MOA-2007-BLG-192Lb [ citation needed ] MOA-2007-BLG-192L 3.330002008
Kepler22b-artwork.jpg Kepler-22b Kepler-22 <52.8295600~4289.82011* Orbits within the CHZ
* Orbits sun-like star
* Estimated temperature of 22 °C.
* Composition is currently unknown. [17]
This artist's concept depicts Kepler-62e, a super-Earth-size planet in the habitable zone.jpg Kepler-62e [ citation needed ] Kepler-62 5~27012007122.32013
Kepler-62f with 62e as Morning Star.jpg Kepler-62f [ citation needed ] Kepler-62 1.413–2.80~20812003-11267.22013
KOI 172.02 compared to Earth.jpg Kepler-69c [ citation needed ] Kepler 69 ~654827000.4242.42013
TRAPPIST-1d Artist's Impression.png TRAPPIST-1d [18] TRAPPIST-1 0.297 +0.035
−0.039		282.1 ± 4.0 K (8.95 ± 4.00 °C; 48.11 ± 7.20 °F)393–84.052016
TRAPPIST-1e Artist's Impression.png TRAPPIST-1e [18] TRAPPIST-1 0.77246393–86.12017
TRAPPIST-1f Artist's Impression.png TRAPPIST-1f [18] TRAPPIST-1 0.93219393–89.22017
TRAPPIST-1g Artist's Impression.png TRAPPIST-1g [18] TRAPPIST-1 1.14199393–812.352017

Planetary systems

ImageStarDistance
(ly)		Age
(gy)		Discovery
(year)		Notes
AA Tauri <1Protoplanetary disk has spectral signatures of water vapor. Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star, have solid surfaces and atmospheres. [19]
Toliman Star has some potential for a planet with liquid water if super dense planet with high gravity and atmospheric pressure is found orbiting it (none have to date). [20]
GD 61 White dwarf system that has first confirmed water-rich rocky planetary body (asteroid). No evidence of liquid water, but larger terrestrial planets, if found in the system, may be volatiles-rich. [21]
TW Hydrae Young star with protoplanetary disk containing detected clouds of water vapour cool enough to contain thousands of Earth-oceans' worth of water. [22]

Description (alphabetical order)

Gliese 581 c, d and g

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. [23] 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. [24] 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. [25]

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. [26]

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. Gliese 667 Cc sunset.jpg
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.

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. [28] 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. [29]

Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone. [30]

HD 28185 b

HD 28185 b was the first exoplanet to be detected in the habitable zone. [31] 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 and probably life. [32]

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. [33] [34]

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. [35] life here will have to look for analogues of photosynthesis.

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. [36]

Exoplanets potentially containing water (artwork; 17 August 2018) (Left to right: Kepler-22b, Kepler-69c, Kepler-452b, Kepler-62f, Kepler-186f.) NASA-Exoplanet-WaterWorlds-20180817.jpg
Exoplanets potentially containing water (artwork; 17 August 2018) (Left to right: Kepler-22b, Kepler-69c, Kepler-452b, Kepler-62f, Kepler-186f.)

However, evidence in 2021 ruled out the existence of this planet. [38]

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. [39]

Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of Earth 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. [40] Both are assumed to be rocky planets, but since the star is 1200 light-years away, it's hard to be sure.

Kepler-69c

This large rocky planet is one of two known to be orbiting the star Kepler 69, which is similar to the Sun. It's believed to be in the star's habitable zone.

It's 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. [40]

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. [41] 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. [42]

Proxima b

Proxima Centauri b, the nearest known exoplanet, is in the habitable zone of its host star, and might contain liquid water. [43]

[44] More details about the planet's physical conditions are needed for a proper evaluation of its habitability. [45] [46] [47]

TOI-1452 b

In August 2022, water was detected on the exoplanet TOI-1452 b based on studies with data from the Transiting Exoplanet Survey Satellite (TESS). [48] [49]

TRAPPIST-1 - four planets

In February 2017, seven planets were discovered in the star system TRAPPIST-1, previously unknown to host any planets. Four of the discovered planets, called TRAPPIST-1d, TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g, are candidates for liquid water. They are all located towards the outer system (with the closest to the star, TRAPPIST-1d, being within or slightly outside the inner edge of the habitable zone), making them cool planets. TRAPPIST-1e and f are probably tidally locked planets, and if liquid water exists in them, it is located in their respective terminator lines. However, if the planets support a thick enough atmosphere to transfer heat to the sides facing away from the star, much larger portions of them may be habitable.

WASP-96b

In July 2022, water was detected on the exoplanet WASP-96b based on spectrum studies with the James Webb Space Telescope. [50]

Water detected on exoplanet WASP-96b based on James Webb Space Telescope spectral studies. Exoplanet WASP-96 b (NIRISS Transmission Spectrum) (weic2206a).jpeg
Water detected on exoplanet WASP-96b based on James Webb Space Telescope spectral studies.

See also

Notes

  1. ↑ Less massive objects' gravity may be too weak to retain water

Related Research Articles

<span class="mw-page-title-main">Exoplanet</span> Planet outside the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. According to statistics from the NASA Exoplanet Archive, As of 25 July 2024, there are 5,741 confirmed exoplanets in 4,285 planetary systems, with 960 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Terrestrial planet</span> Planet that is composed primarily of silicate rocks or metals

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 accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.

<span class="mw-page-title-main">Habitable zone</span> Orbits where planets may have liquid surface water

In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

<span class="mw-page-title-main">Gliese 876 b</span> Extrasolar planet orbiting Gliese 876

Gliese 876 b is an exoplanet orbiting the red dwarf 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.

<span class="mw-page-title-main">Ocean world</span> Planet containing a significant amount of water or other liquid

An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.

<span class="mw-page-title-main">Gliese 581d</span> Contested super-Earth orbiting Gliese 581

Gliese 581d is a doubtful, and frequently disputed, exoplanet candidate orbiting within the Gliese 581 system, approximately 20.4 light-years away in the Libra constellation. It was the third planet claimed in the system and the fourth or fifth in order from the star. Multiple subsequent studies found that the planetary signal in fact originates from stellar activity, and thus the planet does not exist, but this remains disputed.

<span class="mw-page-title-main">Super-Earth</span> Type of exoplanet

A Super-Earth is a type of exoplanet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.

Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.

<span class="mw-page-title-main">Discoveries of exoplanets</span> Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016; no planet discovery has yet come from that evidence. What turned out to be the first detection of an exoplanet was published among a list of possible candidates in 1988, though not confirmed until 2003. The first confirmed detection came in 1992, with the discovery of terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems, with 1007 systems having more than one planet. This is a list of the most notable discoveries.

<span class="mw-page-title-main">Kepler-22b</span> Super-Earth exoplanet orbiting Kepler-22

Kepler-22b is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about 640 light-years from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface. Kepler-22 is too dim to be seen with the naked eye.

<span class="mw-page-title-main">Kepler-69c</span> Super-Earth orbiting Kepler-69

Kepler-69c is a confirmed super-Earth extrasolar planet, likely rocky, orbiting the Sun-like star Kepler-69, the outermore of two such planets discovered by NASA's Kepler spacecraft. It is located about 2,430 light-years from Earth.

<span class="mw-page-title-main">Kepler-62</span> K-type star in the constellation Lyra

Kepler-62 is a K-type main sequence star cooler and smaller than the Sun, located roughly 980 light-years from Earth in the constellation Lyra. It resides within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets that may be transiting their stars. On April 18, 2013, it was announced that the star has five planets, two of which, Kepler-62e and Kepler-62f are within the star's habitable zone. The outermost, Kepler-62f, is likely a rocky planet.

Kepler-296e is a confirmed super-Earth exoplanet orbiting within the habitable zone of Kepler-296. The planet was discovered by NASA's Kepler spacecraft using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. NASA announced the discovery of the exoplanet on 26 February 2014.

<span class="mw-page-title-main">TRAPPIST-1f</span> Earth-size exoplanet orbiting TRAPPIST-1

TRAPPIST-1f, also designated as 2MASS J23062928-0502285 f, is an exoplanet, likely rocky, orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

<span class="mw-page-title-main">TRAPPIST-1e</span> Earth-size exoplanet orbiting TRAPPIST-1

TRAPPIST-1e, also designated as 2MASS J23062928-0502285 e, is a rocky, close-to-Earth-sized exoplanet orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. Astronomers used the transit method to find the exoplanet, a method that measures the dimming of a star when a planet crosses in front of it.

<span class="mw-page-title-main">Kepler-1649c</span> Earth-size exoplanet orbiting Kepler-1649

Kepler-1649c is an Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star Kepler-1649, the outermost planet of the planetary system discovered by Kepler’s space telescope. It is located about 301 light-years (92 pc) away from Earth, in the constellation of Cygnus.

Kepler-737b is a super-Earth exoplanet 669 light years away. There is a chance it could be on the inner edge of the habitable zone.

References

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