List of potentially habitable exoplanets

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This is a list of potentially habitable exoplanets and possible exoplanets 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 the University of Puerto Rico at Arecibo.

Potential habitable zone status[edit]

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.

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,[3][4] 11 billion of which may be orbiting Sun-like stars.[5]

A review in 2015 came to the conclusion that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable.[6] These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf.

When looking at the nearest terrestrial exoplanet candidates, Tau Ceti e is merely 11.9 light-years away. Its average surface temperature is estimated to be 68 °C (154 °F).[7]

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; 6 January 2015).[8]
Comparison of small planets found by Kepler in the habitable zone of their host stars.

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. Note that even though many planets have an HZD value similar to Venus (−0.93), including Kepler-438b, the HZD is not used to rule on whether a planet has suffered a runaway greenhouse effect or not, and therefore, Kepler-438b is currently assumed to be a mesoplanet rather than a hyperthermoplanet.
  • 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
Earth 1.00 0.72 −0.50 −0.31 −0.52 warm terran mesoplanet 0 Inhabited prehistoric
Kepler-438b 0.88 0.88 −0.93 −0.14 −0.73 warm terran mesoplanet 470 confirmed 2015
Gliese 667 Cc 0.84 0.64 −0.62 −0.15 +0.21 warm terran[10] mesoplanet 23.6 confirmed 2011
KOI-3010.01 0.84 0.63 −0.88 −0.16 −0.06 warm superterran mesoplanet 1213.4[11] Kepler candidate 2011
Kepler-442b 0.83 0.98 −0.72 −0.15 +0.28 warm superterran mesoplanet 1291.6 confirmed 2015
Kepler-62e 0.83 0.96 −0.70 −0.15 +0.28 warm superterran mesoplanet 1199.7 confirmed 2013
Kepler-452b[12][13][14] 0.83 ??? −0.49 ??? ??? warm superterran mesoplanet 1,400 confirmed 2015
Gliese 832 c 0.81 0.96 −0.72 −0.15 +0.43 warm superterran mesoplanet 16.1 confirmed 2014
Kepler-283c 0.79 0.85 −0.58 −0.14 +0.69 warm superterran mesoplanet 1496.8 confirmed 2011
KOI-2418.01 0.79 0.00 −0.40 −0.15 +0.44 warm superterran psychroplanet 996.9 Kepler candidate 2011
Kepler-436b 0.79 0.33 −0.87 −0.14 +0.47 warm superterran mesoplanet 1339.4 confirmed 2015
Tau Ceti e 0.78 0.00 −0.92 −0.15 +0.16 warm superterran mesoplanet 11.9 unconfirmed 2012
Kepler-296f 0.78 0.15 −0.90 −0.14 +0.53 warm superterran mesoplanet 1089.6 confirmed 2011
Gliese 180 c 0.77 0.42 −0.53 −0.14 +0.64 warm superterran mesoplanet 39.5 unconfirmed 2014
Gliese 667 Cf 0.77 0.00 -0.22 −0.16 +0.08 warm terran psychroplanet 23.6 dubious 2013
Gliese 581 g 0.76 1.00 -0.70 −0.15 +0.28 warm superterran mesoplanet 20.2 dubious 2010
KOI-2474.01 0.76 0.00 −0.93 −0.15 +0.25 warm superterran mesoplanet 1605.7 Kepler candidate 2011
KOI-2469.01 0.76 0.71 −0.75 −0.13 +0.99 warm superterran mesoplanet 1556.8 Kepler candidate 2011
KOI-2992.01 0.76 0.52 −0.54 −0.13 +1.06 warm superterran mesoplanet 1375.4 Kepler candidate 2011
KOI-4333.01 0.75 0.00 −0.90 −0.15 +0.32 warm superterran thermoplanet 2504.7 Kepler candidate 2011
Gliese 163 c 0.75 0.02 −0.96 −0.14 +0.58 warm superterran mesoplanet 48.9 confirmed 2012
Gliese 180 b 0.75 0.41 −0.88 −0.14 +0.74 warm superterran mesoplanet 39.5 unconfirmed 2014
HD 40307 g 0.74 0.04 −0.23 −0.14 +0.77 warm superterran psychroplanet 41.7 confirmed 2012
KOI-854.01 0.74 1.00 −0.72 −0.13 +1.39 warm superterran mesoplanet 1077.7 Kepler candidate 2011
KOI-4550.01 0.74 1.00 −0.82 −0.13 +0.97 warm superterran mesoplanet 1847.5 Kepler candidate 2011
Kepler-61b 0.73 0.27 −0.88 −0.13 +1.24 warm superterran mesoplanet 1062.8 confirmed 2013
Kepler-443 0.73 0.91 −0.49 −0.13 +1.44 warm superterran mesoplanet 2564.4 confirmed 2015
KOI-2762.01 0.73 0.10 −0.27 −0.14 +1.01 warm superterran mesoplanet 1169.5 Kepler candidate 2011
KOI-1871.01 0.72 0.27 −0.88 −0.12 +1.34 warm superterran mesoplanet 1176.7 Kepler candidate 2011
KOI-4036.01 0.72 0.99 −0.77 −0.12 +1.49 warm superterran mesoplanet 1135.0 Kepler candidate 2011
Gliese 422 b 0.71 0.17 −0.41 −0.13 +1.11 warm megaterran mesoplanet 41.3 unconfirmed 2014
Kepler-22b 0.71 0.53 −0.64 −0.12 +1.79 warm superterran mesoplanet 619.4 confirmed 2011
KOI-3282.01 0.71 0.04 −0.92 −0.12 +1.43 warm superterran mesoplanet 1162.9 Kepler candidate 2011
KOI-4450.01 0.71 0.00 −0.83 −0.13 +1.33 warm superterran mesoplanet 2553.4 Kepler candidate 2011
KOI-4054.01 0.70 0.00 −0.91 −0.12 +1.30 warm superterran mesoplanet 1770.4 Kepler candidate 2011
Kepler-440b 0.70 0.00 +0.01 −0.15 +0.38 warm superterran psychroplanet 706.5 confirmed 2015
KOI-4583.01 0.69 0.07 −0.23 −0.12 +2.03 warm superterran psychroplanet 3265.0 Kepler candidate 2011
Kepler-298d 0.68 0.00 −0.86 −0.11 +2.11 warm superterran mesoplanet 1545[15] confirmed 2012
Kepler-439b 0.68 0.00 −0.99 −0.13 +1.18 warm superterran thermoplanet 1914.8 confirmed 2015
Kapteyn b 0.67 0.00 +0.08 −0.15 +0.57 warm superterran psychroplanet 12.7 confirmed 2014
Kepler-62f 0.67 0.00 +0.45 −0.16 +0.19 warm superterran psychroplanet 1199.7 confirmed 2013
Kepler-186f 0.64 0.00 +0.48 −0.17 −0.26 warm terran psychroplanet 492 confirmed 2014
Kepler-174d 0.61 0.00 +0.32 −0.13 +1.77 warm superterran psychroplanet 878.3 confirmed 2011
KOI-2770.01 0.60 0.00 +0.33 −0.13 +1.77 warm superterran psychroplanet 1470.4 Kepler candidate 2011
Gliese 667 Ce 0.60 0.00 +0.51 −0.16 +0.23 warm terran psychroplanet 23.6 dubious 2013
Gliese 682 c 0.59 0.00 +0.22 −0.14 +1.19 warm superterran psychroplanet 16.6 unconfirmed 2014
KOI-4356.01 0.55 0.00 +0.77 −0.14 +1.22 warm superterran psychroplanet 1239.7 Kepler candidate 2011
Gliese 581 d 0.53 0.00 +0.78 −0.14 +0.94 warm superterran hypopsychroplanet 20.2 unconfirmed[16][17] 2007
~Venus 0.78 0.00 −0.93 −0.28 −0.70 warm terran hyperthermoplanet close to zero non-exoplanet prehistoric
~Mars 0.64 0.00 +0.33 −0.13 −1.12 warm subterran hypopsychroplanet close to zero non-exoplanet prehistoric
~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,[18][19] but updated models for the boundaries of the habitable zone placed the planet interior to the HZ,[20][21] and it is now considered non-habitable.[9] Kepler-69c has gone through a similar process; though initially believed to be potentially habitable,[22] it was quickly realized that the planet is more likely to be similar to Venus,[23] and is thus no longer considered habitable.[9]

Similarly, Tau Ceti f was initially considered potentially habitable,[24] 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]

KOI-1686-01 was also considered even the single-most potentially habitable exoplanet after its discovery in 2011, until it was proved a false positive by NASA in 2015.

See also[edit]

References[edit]

  1. ^ "VPL Glossary". washington.edu. 
  2. ^ "The Goldilocks Zone". NASA. 2 October 2003. 
  3. ^ Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". New York Times. Retrieved 5 November 2013. 
  4. ^ Petigura, Erik A. et al. (31 October 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 5 November 2013. 
  5. ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013. 
  6. ^ Paul Gilster, Andrew LePage (30 January 2015). "A Review of the Best Habitable Planet Candidates". Centauri Dreams, Tau Zero Foundation. Retrieved 24 July 2015. 
  7. ^ Giovanni F. Bignami (2015). The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space. Springer. ISBN 9783319170046. , Page 110
  8. ^ 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 "Habitable Exoplanets Catalog: Introduction". Planetary Habitability Laboratory: UPR Arecibo. 2015. Retrieved 3 June 2015. 
  10. ^ "Habitable Exoplanets Catalog". Planetary Habitability Laboratory. 2015. Retrieved 3 June 2015. 
  11. ^ http://phl.upr.edu/library/notes/thenewpotentialhabitableexoplanetscandidatesofnasakepler
  12. ^ Chou, Felicia; Johnson, Michelle (23 July 2015). "NASA’s Kepler Mission Discovers Bigger, Older Cousin to Earth" (Press release). NASA. Retrieved 23 July 2015. 
  13. ^ http://www.theweathernetwork.com/news/articles/has-nasa-spotted-earths-twin-watch-live-announcement-here/54694
  14. ^ http://cnnhit.com/new/no-kepler-452b-is-not-the-most-earth-like-planet-found-so-far/
  15. ^ "Habitable Exoplanets Catalog: Data of Potentially Habitable Worlds". Planetary Habitability Laboratory. 2015. Retrieved 3 June 2015. 
  16. ^ http://astronomynow.com/2015/03/09/reanalysis-of-data-suggests-habitable-planet-gj581d-really-does-exist/
  17. ^ http://www.sciencemag.org/content/347/6226/1080.3.abstract
  18. ^ Kaltenegger, L. et al. (2011). "A Habitable Planet around HD 85512?". arXiv:1108.3561. Bibcode:2011arXiv1108.3561K. 
  19. ^ Mendez, Abel (1 August 2012). "Five Potential Habitable Exoplanets Now" (Press release). Planetary Habitability Laboratory @ UPR Arecibo. Retrieved 4 June 2015. 
  20. ^ Kopparapu, Ravi kumar et al. (2013). "Habitable Zones Around Main-Sequence Stars: New Estimates". arXiv:1301.6674. Bibcode:2013ApJ...765..131K. doi:10.1088/0004-637X/765/2/131. 
  21. ^ Mendez, Abel (29 January 2013). "A New Habitable Zone" (Press release). Planetary Habitability Laboratory @ UPR Arecibo. Retrieved 4 June 2015. 
  22. ^ 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. doi:10.1088/0004-637X/768/2/101. 
  23. ^ Kane, Stephen R. et al. (2013). "A Potential Super-Venus in the Kepler-69 System". arXiv:1305.2933. Bibcode:2013ApJ...770L..20K. doi:10.1088/2041-8205/770/2/L20. 
  24. ^ Mendez, Abel (28 December 2012). "Two Nearby Habitable Worlds?" (Press release). Planetary Habitability Laboratory @ UPR Arecibo. Retrieved 4 June 2015. 

External links[edit]