Difference between revisions 917021608 and 917030155 on enwiki

{{redirect|Goldilocks Zone|the more general principle|Goldilocks principle}}
{{redirect|Habitable zone|the galactic zone|Galactic habitable zone}}
{{short description|Zone around a star with strong possibilities for stable liquid water on a suitable planet}}
(contracted; show full)referred to various "zones" in which life could emerge.<ref name=huggett-1995 /><ref name=strughold-1953>{{cite book |url=https://books.google.com/books/about/The_green_and_red_planet.html?id=zNbPAAAAMAAJ |title=The Green and Red Planet: A Physiological Study of the Possibility of Life on Mars |publisher=University of New Mexico Press |author=Strughold, Hubertus |date=1953}}</ref> In the same year, [[Harlow Shapley]] wrote "Liquid Water Belt", which described the same 
theoryconcept in further scientific detail. Both works stressed the importance of liquid water to life.<ref name="Kasting2010">{{cite book|author=Kasting, James|title=How to Find a Habitable Planet|url=https://books.google.com/books?id=xPqEeB-SRvUC|accessdate=4 May 2013|date=2010|publisher=Princeton University Press|isbn=978-0-691-13805-3|page=127}}</ref> [[Su-Shu Huang]], an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.<ref name=kasting-1993>{{cite journal |url= |title=Habitable Zones around Main Sequence Stars |author1=Kasting, James F. |author2=Whitmire, Daniel P. |author3=Reynolds, Ray T. |journal=Icarus |date=January 1993 |volume=101 |issue=1 |pages=108–118 |doi=10.1006/icar.1993.1010 |bibcode=1993Icar..101..108K |pmid=11536936}}</ref><ref name=huang-1966>{{cite book |url=https://books.google.com/books?id=D0UrAAAAYAAJ |title=Extraterrestrial life: An Anthology and Bibliography |publisher=National Academy of Sciences |author=Huang, Su-Shu |date=1966 |location=Washington, D. C. |pages=87–93 |others=National Research Council (U.S.). Study Group on Biology and the Exploration of Mars}}</ref> A major early contributor to habitable zone theoryconcept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in [[multiple star system]]s, given the gravitational instabilities of those systems.<ref name=huang-1960>{{cite journal |title=Life-Supporting Regions in the Vicinity of Binary Systems |author=Huang, Su-Shu |journal=Publications of the Astronomical Society of the Pacific |date=April 1960 |volume=72 |issue=425 |pages=106–114 |bibcode=1960PASP...72..106H |doi=10.1086/127489}}</ref>

The theoryconcept of habitable zones was further developed in 1964 by [[Stephen H. Dole]] in his book ''Habitable Planets for Man'', in which he discussed the concept of circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million.<ref name="dole-1964"/> At the same time, science-fiction author [[Isaac Asimov]] introduced the concept of a circumstellar habitable (contracted; show full)n then (and still) known as the habitable zone.<ref name=kasting-1993 /> Kasting was the first to present a detailed model for the habitable zone for exoplanets.<ref name="F. Kasting, D. P 1993"/><ref name="Seager 2013">{{cite journal |title=Exoplanet Habitability |journal=Science |year=2013 |last=Seager |first=Sara |volume=340 |issue=577 |pages=577–581 |doi=10.1126/science.1232226 |pmid=23641111 |bibcode=2013Sci...340..577S }}</ref>

An update to habitable zone 
theoryconcept came in 2000, when astronomers [[Peter Ward (paleontologist)|Peter Ward]] and [[Donald Brownlee]] introduced the idea of the "[[galactic habitable zone]]", which they later developed with [[Guillermo Gonzalez (astronomer)|Guillermo Gonzalez]].<ref name="Rare Earth" /><ref name=gonzalez-2001>{{cite journal |title=The Galactic Habitable Zone I. Galactic Chemical Evolution |author1=Gonzalez, Guillermo |author2=Brownlee, Donald |author3=Ward, Peter |journal=Icarus |date=July 2001 |volume=152 |issue=1 |pages=185–200 |doi=10.1006/icar.2001.6617 |arxiv=astro-ph/0103165|bibcode = 2001Icar..152..185G }}</ref> The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a [[galactic center]] that stars there are enriched with [[metallicity|heavier elements]], but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.<ref name="Rare Earth"/>

Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an [[alternative biochemistry]].<ref name=villard-2011 /> In 2013, further developments in habitable zone theoryconcepts were made with the proposal of a circum''planetary'' habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.<ref name=hadhazy-2013>{{cite news |url=http://www.astrobio.net/exclusive/5364/the-habitable-edge-of-exomoons |title=The 'Habitable Edge' of Exomoons |work=Astrobio(contracted; show full)lt;ref>https://photojournal.jpl.nasa.gov/catalog/PIA21471 PIA21471: Landslides on Ceres</ref> suggesting a more common phenomena than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.

According to extended habitable zone 
theoryconcept, planetary mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth ([[super-Earth]] class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmosph(contracted; show full)guardian">{{cite news| url=https://www.theguardian.com/science/2007/nov/07/spaceexploration |title=Could this be Earth's near twin? Introducing planet 55 Cancri f |newspaper=[[The Guardian]] |author=Ian Sample, science correspondent |date= 7 November 2007|accessdate=17 October 2008 |location=London| archiveurl= https://web.archive.org/web/20081002080911/http://www.guardian.co.uk/science/2007/nov/07/spaceexploration| archivedate= 2 October 2008 | url-status= live}}</ref>

Though in theory
, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been detected. Planets within the zone with the potential for solid surfaces were therefore of much greater interest.

===Habitable super-Earths===
{{Category see also|Super-Earths in the habitable zone}}
[[File:Gliese 581 - 2010.jpg|thumb|The [[habitable zone]] of Gliese 581 compared with our Solar System's habitable zone.]]
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==Habitability outside the CHZ==
[[Image:Liquid lakes on titan.jpg|thumb|upright|The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the [[carbon chauvinism]] that underpins CHZ 
theoryconcept.]]
Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example, [[Saturn]]'s moons [[Titan (moon)|Titan]] and [[Enceladus]] and [[Jupiter]]'s moons [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], all of which are outside the habitable zone, may hold large volumes of liquid water in [[subsurface ocean]]s.<ref>{{cite web |url = http://phl.upr.edu/library/media/liquidwaterinthesolarsyst(contracted; show full){{authority control}}

[[Category:Search for extraterrestrial intelligence]]
[[Category:Planetary habitability]]
[[Category:Astronomical hypotheses]]
[[Category:Extraterrestrial life]]
[[Category:Articles containing video clips]]
[[Category:Extraterrestrial water]]