Orbits where planets may have liquid surface water
"Goldilocks Zone" redirects here. For the more general principle, see Goldilocks principle.
This article is about the circumstellar zone. For the galactic zone, see Galactic habitable zone.
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.[1][2][3][4][5] 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.
The habitable zone is also called the Goldilocks zone, a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented in 1953,[6] many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets.[7] Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect.[8] On November 4, 2013, astronomers reported, based on Kepler 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.[9][10] About 11 billion of these may be orbiting Sun-like stars.[11] Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star.[12] The HZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the HZ might outnumber planets.[13]
In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.[14] Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System.[15] Sustained by other energy sources, such as tidal heating[16][17] or radioactive decay[18] or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons.[19] Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars,[20] or ammoniates,[21] due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.[22]
^Su-Shu Huang, American Scientist 47, 3, pp. 397–402 (1959)
^Dole, Stephen H. (1964). Habitable Planets for Man. Blaisdell Publishing Company. p. 103.
^J. F. Kasting, D. P. Whitmire, R. T. Reynolds, Icarus 101, 108 (1993).
^Kopparapu, Ravi Kumar (2013). "A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs". The Astrophysical Journal Letters. 767 (1): L8. arXiv:1303.2649. Bibcode:2013ApJ...767L...8K. doi:10.1088/2041-8205/767/1/L8. S2CID 119103101.
^Cruz, Maria; Coontz, Robert (2013). "Exoplanets - Introduction to Special Issue". Science. 340 (6132): 565. doi:10.1126/science.340.6132.565. PMID 23641107.
^Huggett, Richard J. (1995). Geoecology: An Evolutionary Approach. Routledge, Chapman & Hall. p. 10. ISBN 978-0-415-08689-9.
^Overbye, Dennis (January 6, 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times. Retrieved January 6, 2015.
^Peale, S. J. (January 2021). "Probability of Detecting a Planetary Companion during a Microlensing Event". The Astrophysical Journal. 552 (2): 889–911. arXiv:astro-ph/0101316. doi:10.1086/320562. S2CID 17080374.
^Overbye, Dennis (November 4, 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Retrieved November 5, 2013.
^Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (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. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
^Khan, Amina (November 4, 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved November 5, 2013.
^Anglada-Escudé, Guillem; Amado, Pedro J.; Barnes, John; et al. (2016). "A terrestrial planet candidate in a temperate orbit around Proxima Centauri". Nature. 536 (7617): 437–440. arXiv:1609.03449. Bibcode:2016Natur.536..437A. doi:10.1038/nature19106. PMID 27558064. S2CID 4451513.
^Schirber, Michael (26 Oct 2009). "Detecting Life-Friendly Moons". Astrobiology Magazine. NASA. Archived from the original on 29 October 2009. Retrieved 9 May 2013.
^Lammer, H.; Bredehöft, J. H.; Coustenis, A.; Khodachenko, M. L.; et al. (2009). "What makes a planet habitable?" (PDF). The Astronomy and Astrophysics Review. 17 (2): 181–249. Bibcode:2009A&ARv..17..181L. doi:10.1007/s00159-009-0019-z. S2CID 123220355. Archived from the original (PDF) on 2016-06-02. Retrieved 2016-05-03.
^Edwards, Katrina J.; Becker, Keir; Colwell, Frederick (2012). "The Deep, Dark Energy Biosphere: Intraterrestrial Life on Earth". Annual Review of Earth and Planetary Sciences. 40 (1): 551–568. Bibcode:2012AREPS..40..551E. doi:10.1146/annurev-earth-042711-105500. ISSN 0084-6597.
^Cowen, Ron (2008-06-07). "A Shifty Moon". Science News. Archived from the original on 2011-11-04. Retrieved 2013-04-22.
^Bryner, Jeanna (24 June 2009). "Ocean Hidden Inside Saturn's Moon". Space.com. TechMediaNetwork. Retrieved 22 April 2013.
^Abbot, D. S.; Switzer, E. R. (2011). "The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space". The Astrophysical Journal. 735 (2): L27. arXiv:1102.1108. Bibcode:2011ApJ...735L..27A. doi:10.1088/2041-8205/735/2/L27. S2CID 73631942.
^"Rogue Planets Could Harbor Life in Interstellar Space, Say Astrobiologists". MIT Technology Review. MIT Technology Review. 9 February 2011. Archived from the original on 7 October 2015. Retrieved 24 June 2013.
^Wall, Mike (28 September 2015). "Salty Water Flows on Mars Today, Boosting Odds for Life". Space.com. Retrieved 2015-09-28.
^Villard, Ray (November 18, 2011). "Alien Life May Live in Various Habitable Zones: Discovery News". News.discovery.com. Discovery Communications LLC. Retrieved April 22, 2013.
In astronomy and astrobiology, the habitablezone (HZ), or more precisely the circumstellar habitablezone (CHZ), is the range of orbits around a star...
the galactic habitablezone is the region of a galaxy in which life might most likely develop. The concept of a galactic habitablezone analyzes various...
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atmosphere and geology. Of the natural satellites in the Solar System's habitablezone – the Moon, two Martian satellites (though some estimates put those...
zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone...
the only ones in which the habitablezone coincides completely with the ultraviolet habitablezone. Since the habitablezone is farther away in more massive...
such as the extreme temperature differences created by one side of habitable-zone planets permanently facing the star, and the other perpetually turned...
difficult to observe. Of particular interest to astrobiology is the habitablezone of planetary systems where planets could have surface liquid water,...
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Gliese 581d, which is also potentially habitable, was discovered. Gliese 581c and d are within the habitablezone of the host star, and are two of the most...
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has two confirmed planets: Proxima b, an Earth-sized planet in the habitablezone discovered in 2016, and Proxima d, a candidate sub-Earth which orbits...
probability, some of these stars have Earth-like planets in a circumstellar habitablezone. Many of these stars, and hence their planets, are much older than the...
Earth-sized exoplanet, likely rocky, that is orbiting within the inner habitablezone of the red dwarf star Ross 128, at a distance of around 11 light-years...