List of potentially habitable exoplanets

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Artist's impression of Kepler-22b as an oceanic "super-Earth" within its star's habitable zone.

This is a list of potentially habitable exoplanets candidates ranked by similarity with Earth using the Earth Similarity Index. The list is based on methodology and estimations provided by the Habitable Exoplanets Catalog, which is maintained by the Planetary Habitability Laboratory at University of Puerto Rico at Arecibo.

Potential habitable zone status[edit]

Artist's impression of Gliese 581 d, a potentially habitable exoplanet previously thought to not have existed but new evidence suggests that it is real.
Main article: Habitable zone

In astronomy and astrobiology, the circumstellar habitable zone (CHZ or sometimes "ecosphere", "liquid-water belt", "HZ", "life zone" or "Goldilocks zone") is the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface.[1][2]

A potentially habitable planet implies a terrestrial planet within the CHZ and with conditions roughly comparable to those of Earth (i.e. an Earth analog) and thus potentially favourable to life.

Although only about a dozen[citation needed] planets have been confirmed in the habitable zone, the Kepler spacecraft has identified a further 54 candidates[citation needed] and current estimates indicate that there are at least 8.8 billion Earth-sized planets capable of life in habitable zones of Sun-like stars in the Milky Way.[3]

On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way,[4][5] 11 billion of which may be orbiting Sun-like stars.[6] The nearest such planet could be 12 light-years away, according to the scientists.[4][5]

On 26 February 2014, NASA announced several exoplanets, namely Kepler-296e and Kepler-296f, in the habitable zones of a star similar to the Sun.[7]

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.[8]

Notable ExoplanetsKepler Space Telescope
KeplerExoplanets-NearEarthSize-HabitableZone-20150106.png
Confirmed small exoplanets in habitable zones.
(Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b)
(Kepler Space Telescope; January 6, 2015).[8]

List from the Habitable Exoplanets Catalog[edit]

The data in the following table is from the Habitable Exoplanets Catalog.[9] This habitability index does not take into account the differences in the habitability of certain types of stars and star systems, such as red dwarfs and binary star systems. See habitability of red dwarf systems for more information.

Key[edit]

The planets listed below are evaluated on seven different criteria:

  • Earth Similarity Index (ESI)—Similarity to Earth on a scale from 0 to 1, with 1 being the most Earth-like. ESI depends on the planet's radius, density, escape velocity, and surface temperature.
  • Standard Primary Habitability (SPH)—Suitability for vegetation on a scale from 0 to 1, with 1 being best-suited for growth. SPH depends on surface temperature (and relative humidity if known).
  • Habitable Zone Distance (HZD)—Distance from the center of the star's habitable zone, scaled so that –1 represents the inner edge of the zone, and +1 represents the outer edge. HZD depends on the star's luminosity and temperature and the size of the planet's orbit.
  • Habitable Zone Composition (HZC)—Measure of bulk composition, where values close to zero are likely iron–rock–water mixtures. Values below –1 represent bodies likely composed mainly of iron, and values greater than +1 represent bodies likely composed mainly of gas. HZC depends on the planet's mass and radius.
  • Habitable Zone Atmosphere (HZA)—Potential for the planet to hold a habitable atmosphere, where values below –1 represent bodies likely with little or no atmosphere, and values above +1 represent bodies likely with thick hydrogen atmospheres (e.g. gas giants). Values between –1 and +1 are more likely to have atmospheres suitable for life, though zero is not necessarily ideal. HZA depends on the planet's mass, radius, orbit size, and the star's luminosity.
  • Planetary Class (pClass)—Classifies objects based on thermal zone (hot, warm, or cold, where warm is in the habitable zone) and mass (asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian).
  • Habitable Class (hClass)—Classifies habitable planets based on temperature: hypopsychroplanets (hP) = very cold (< −50 °C); psychroplanets (P) = cold; mesoplanets (M) = medium-temperature (0–50 °C; not to be confused with the other definition of mesoplanets); thermoplanets (T) = hot; hyperthermoplanets (hT) = very hot (> 100 °C). Mesoplanets would be ideal for complex life, whereas class hP or hT would only support extremophilic life. Non-habitable planets are simply given the class NH.

Table[edit]

(For comparison purposes, the four solar terrestrial planets are included in this list)

Name ESI SPH HZD HZC HZA pClass hClass Distance (ly) Status Year of
discovery
0N/A !Earth 1.00 0.72 −0.50 −0.31 −0.52 warm terran mesoplanet
1 KOI-1686.01 0.89 0.42 −0.63 −0.16 −0.13 warm superterran mesoplanet 1033.8 KOI false positive[10] 2011
2 Kepler-438b 0.88 0.88 −0.93 −0.14 −0.73 warm terran mesoplanet 470 confirmed 2015
3 Gliese 667 Cc 0.84 0.64 −0.62 −0.15 +0.21 warm terran[11] mesoplanet 23.6 confirmed 2011
4 KOI-3010.01 0.84 0.63 −0.88 −0.16 −0.06 warm superterran mesoplanet 1213.4[12] Kepler candidate 2011
5 Kepler-442b 0.83 0.98 −0.72 −0.15 +0.28 warm superterran mesoplanet 1291.6 confirmed 2015
6 Kepler-62e 0.83 0.96 −0.70 −0.15 +0.28 warm superterran mesoplanet 1199.7 confirmed 2013
7 Gliese 832 c 0.81 0.96 −0.72 −0.15 +0.43 warm superterran mesoplanet 16.1 confirmed 2014
8 Kepler-283c 0.79 0.85 −0.58 −0.14 +0.69 warm superterran mesoplanet 1496.8 confirmed 2011
9 KOI-2418.01 0.79 0.00 −0.40 −0.15 +0.44 warm superterran psychroplanet 996.9 Kepler candidate 2011
10 Kepler-436b 0.79 0.33 −0.87 −0.14 +0.47 warm superterran mesoplanet 1339.4 confirmed 2015
11 Tau Ceti e 0.78 0.00 −0.92 −0.15 +0.16 warm superterran mesoplanet 11.9 unconfirmed 2012
12 Kepler-296f 0.78 0.15 −0.90 −0.14 +0.53 warm superterran mesoplanet 1089.6 confirmed 2011
13 Gliese 180 c 0.77 0.42 −0.53 −0.14 +0.64 warm superterran mesoplanet 39.5 unconfirmed 2014
14 Gliese 667 Cf 0.77 0.00 -0.22 −0.16 +0.08 warm terran psychroplanet 23.6 dubious 2013
15 Gliese 581 g 0.76 1.00 -0.70 −0.15 +0.28 warm superterran mesoplanet 20.2 dubious 2010
16 KOI-2474.01 0.76 0.00 −0.93 −0.15 +0.25 warm superterran mesoplanet 1605.7 Kepler candidate 2011
17 KOI-2469.01 0.76 0.71 −0.75 −0.13 +0.99 warm superterran mesoplanet 1556.8 Kepler candidate 2011
18 KOI-2992.01 0.76 0.52 −0.54 −0.13 +1.06 warm superterran mesoplanet 1375.4 Kepler candidate 2011
19 KOI-4333.01 0.75 0.00 −0.90 −0.15 +0.32 warm superterran thermoplanet 2504.7 Kepler candidate 2011
20 Gliese 163 c 0.75 0.02 −0.96 −0.14 +0.58 warm superterran mesoplanet 48.9 confirmed 2012
21 Gliese 180 b 0.75 0.41 −0.88 −0.14 +0.74 warm superterran mesoplanet 39.5 unconfirmed 2014
22 HD 40307 g 0.74 0.04 −0.23 −0.14 +0.77 warm superterran psychroplanet 41.7 confirmed 2012
23 KOI-854.01 0.74 1.00 −0.72 −0.13 +1.39 warm superterran mesoplanet 1077.7 Kepler candidate 2011
24 KOI-4550.01 0.74 1.00 −0.82 −0.13 +0.97 warm superterran mesoplanet 1847.5 Kepler candidate 2011
25 Kepler-61b 0.73 0.27 −0.88 −0.13 +1.24 warm superterran mesoplanet 1062.8 confirmed 2013
26 Kepler-443 0.73 0.91 −0.49 −0.13 +1.44 warm superterran mesoplanet 2564.4 confirmed 2015
27 KOI-2762.01 0.73 0.10 −0.27 −0.14 +1.01 warm superterran mesoplanet 1169.5 Kepler candidate 2011
28 KOI-1871.01 0.72 0.27 −0.88 −0.12 +1.34 warm superterran mesoplanet 1176.7 Kepler candidate 2011
29 KOI-4036.01 0.72 0.99 −0.77 −0.12 +1.49 warm superterran mesoplanet 1135.0 Kepler candidate 2011
30 Gliese 422 b 0.71 0.17 −0.41 −0.13 +1.11 warm megaterran mesoplanet 41.3 unconfirmed 2014
31 Kepler-22b 0.71 0.53 −0.64 −0.12 +1.79 warm superterran mesoplanet 619.4 confirmed 2011
32 KOI-3282.01 0.71 0.04 −0.92 −0.12 +1.43 warm superterran mesoplanet 1162.9 Kepler candidate 2011
33 KOI-4450.01 0.71 0.00 −0.83 −0.13 +1.33 warm superterran mesoplanet 2553.4 Kepler candidate 2011
34 KOI-4054.01 0.70 0.00 −0.91 −0.12 +1.30 warm superterran mesoplanet 1770.4 Kepler candidate 2011
35 Kepler-440b 0.70 0.00 +0.01 −0.15 +0.38 warm superterran psychroplanet 706.5 confirmed 2015
36 KOI-4583.01 0.69 0.07 −0.23 −0.12 +2.03 warm superterran psychroplanet 3265.0 Kepler candidate 2011
37 Kepler-298d 0.68 0.00 −0.86 −0.11 +2.11 warm superterran mesoplanet 1545 [13] confirmed 2012
38 Kepler-439b 0.68 0.00 −0.99 −0.13 +1.18 warm superterran thermoplanet 1914.8 confirmed 2015
39 Kapteyn b 0.67 0.00 +0.08 −0.15 +0.57 warm superterran psychroplanet 12.7 confirmed 2014
40 Kepler-62f 0.67 0.00 +0.45 −0.16 +0.19 warm superterran psychroplanet 1199.7 confirmed 2013
41 Kepler-186f 0.64 0.00 +0.48 −0.17 −0.26 warm terran psychroplanet 492 confirmed 2014
42 Kepler-174d 0.61 0.00 +0.32 −0.13 +1.77 warm superterran psychroplanet 878.3 confirmed 2011
43 KOI-2770.01 0.60 0.00 +0.33 −0.13 +1.77 warm superterran psychroplanet 1470.4 Kepler candidate 2011
44 Gliese 667 Ce 0.60 0.00 +0.51 −0.16 +0.23 warm terran psychroplanet 23.6 dubious 2013
45 Gliese 682 c 0.59 0.00 +0.22 −0.14 +1.19 warm superterran psychroplanet 16.6 unconfirmed 2014
46 KOI-4356.01 0.55 0.00 +0.77 −0.14 +1.22 warm superterran psychroplanet 1239.7 Kepler candidate 2011
47 Gliese 581 d 0.53 0.00 +0.78 −0.14 +0.94 warm superterran hypopsychroplanet 20.2 unconfirmed[14][15] 2007
@1N/A ~Venus 0.78 0.00 −0.93 −0.28 −0.70 warm terran hyperthermoplanet close to zero non-exoplanet prehistoric
@2N/A ~Mars 0.64 0.00 +0.33 −0.13 −1.12 warm subterran hypopsychroplanet close to zero non-exoplanet prehistoric
@3N/A ~Mercury 0.39 0.00 −1.46 −0.52 −1.37 hot mercurian non-habitable close to zero non-exoplanet prehistoric

Previous candidates[edit]

HD 85512 b was initially found to be potentially habitable,[16][17] but updated models for the boundaries of the habitable zone placed the planet interior to the HZ,[18][19] and it is now considered non-habitable.[9] Kepler-69c has gone through a similar process; though initially believed to be potentially habitable,[20] it was quickly realized that the planet is more likely to be similar to Venus,[21] and is thus no longer considered habitable.[9]

Similarly, Tau Ceti f was initially considered potentially habitable,[22] but the improved model of the circumstellar habitable zone places the planet exterior to the outer limits of habitability, so it is now considered non-habitable.[9]

See also[edit]

References[edit]

  1. ^ "VPL Glossary". washington.edu. 
  2. ^ "The Goldilocks Zone". NASA. 2 October 2003. 
  3. ^ "Milky Way Teeming With Billions Of Earth-Size Planets". Huffington Post. 2 November 2014. 
  4. ^ a b Overbye, Dennis (November 4, 2013). "Far-Off Planets Like the Earth Dot the Galaxy". New York Times. Retrieved November 5, 2013. 
  5. ^ a b Petigura, Erik A. et al. (October 31, 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. Retrieved November 5, 2013. 
  6. ^ Khan, Amina (November 4, 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved November 5, 2013. 
  7. ^ Staff (26 February 2014). "715 Newly Verified Planets More Than Triples the Number of Confirmed Kepler Planets". NASA. Retrieved 8 January 2015. 
  8. ^ a b Clavin, Whitney et al. (6 January 2015). "NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones". NASA. Retrieved 6 January 2015. 
  9. ^ a b c d "PHL's Exoplanets Catalog - Planetary Habitability Laboratory @ UPR Arecibo". Retrieved February 20, 2015. 
  10. ^ http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=KOI-1686.01
  11. ^ http://phl.upr.edu/projects/habitable-exoplanets-catalog
  12. ^ http://phl.upr.edu/library/notes/thenewpotentialhabitableexoplanetscandidatesofnasakepler
  13. ^ http://phl.upr.edu/projects/habitable-exoplanets-catalog/data
  14. ^ http://astronomynow.com/2015/03/09/reanalysis-of-data-suggests-habitable-planet-gj581d-really-does-exist/
  15. ^ http://www.sciencemag.org/content/347/6226/1080.3.abstract
  16. ^ Kaltenegger, L. et al. (2011). "A Habitable Planet around HD 85512?". arXiv:1108.3561. Bibcode:2011arXiv1108.3561K. 
  17. ^ http://phl.upr.edu/press-releases/fivepotentialhabitableexoplanetsnow
  18. ^ Kopparapu, Ravi kumar et al. (2013). "Habitable Zones Around Main-Sequence Stars: New Estimates". arXiv:1301.6674. Bibcode:2013ApJ...765..131K. 
  19. ^ http://phl.upr.edu/press-releases/anewhabitablezone
  20. ^ Barclay, Thomas et al. (2013). "A super-Earth-sized planet orbiting in or near the habitable zone around Sun-like star". arXiv:1304.4941. Bibcode:2013ApJ...768..101B. 
  21. ^ Kane, Stephen R. et al. (2013). "A Potential Super-Venus in the Kepler-69 System". arXiv:1305.2933. Bibcode:2013ApJ...770L..20K. 
  22. ^ http://phl.upr.edu/press-releases/twonearbyhabitableworlds

External links[edit]