Editing Planetary system (section)

==Zones==

===Habitable zone===
{{Main|Circumstellar habitable zone}}
[[File:Habitable zone - HZ.png|thumb|Location of habitable zone around different types of stars]]
The habitable zone around a star is the region where the temperature range allows for liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star; this means the habitable zone will also vary accordingly. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet.

Habitable zones have usually been defined in terms of surface temperature; however, over half of Earth's biomass is from subsurface microbes,<ref>{{cite journal | last1 = Amend | first1 = J. P. | last2 = Teske | first2 = A. | year = 2005 | title = Expanding frontiers in deep subsurface microbiology | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 219 | issue = 1–2| pages = 131–155 | doi=10.1016/j.palaeo.2004.10.018| bibcode = 2005PPP...219..131A }}</ref> and temperature increases as depth underground increases, so the subsurface can be conducive for life when the surface is frozen; if this is considered, the habitable zone extends much further from the star.<ref>[https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-25639306 Further away planets 'can support life' say researchers], BBC, January 7, 2014 Last updated at 12:40</ref>

Studies in 2013 indicate that an estimated 22±8% of Sun-like<ref group=lower-alpha name=footnoteA>For the purpose of this 1 in 5 statistic, "Sun-like" means [[G-type star]]. Data for Sun-like stars were not available so this statistic is an extrapolation from data about [[K-type star]]s</ref> stars have an Earth-sized<ref group=lower-alpha name=footnoteB>For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii</ref> planet in the habitable<ref group=lower-alpha name=footnoteC>For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).</ref> zone.<ref name="ucb1in5">{{cite web|last=Sanders|first=R.|date=November 4, 2013|title=Astronomers answer key question: How common are habitable planets?|url=http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|work=newscenter.berkeley.edu|access-date=November 6, 2014|archive-url=https://web.archive.org/web/20141107081158/http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|archive-date=November 7, 2014|url-status=dead}}</ref><ref name="earthsunhzprev">{{cite journal
|last1=Petigura |first1=E. A.|last2=Howard |first2=A. W.|last3=Marcy |first3=G. W.
|date=2013|title=Prevalence of Earth-size planets orbiting Sun-like stars|journal=[[Proceedings of the National Academy of Sciences]]|volume= 110|issue= 48|pages=19273–19278 |arxiv= 1311.6806|bibcode= 2013PNAS..11019273P|doi=10.1073/pnas.1319909110 |pmid=24191033 |pmc=3845182|doi-access=free}}</ref>

===Venus zone===
The '''Venus zone''' is the region around a star where a [[terrestrial planet]] would have [[runaway greenhouse]] conditions like [[Venus]], but not so near the star that the atmosphere completely escapes. As with the habitable zone, the location of the Venus zone depends on several factors, including the type of star and properties of the planets such as mass, rotation rate, and atmospheric clouds. Studies of the Kepler spacecraft data indicate that 32% of [[red dwarf]]s have potentially Venus-like planets based on planet size and distance from star, increasing to 45% for [[K-type main-sequence star|K-type]] and [[G-type main-sequence star|G-type]] stars.<ref group=lower-alpha name=footnoteE>For the purpose of this, terrestrial-sized means 0.5–1.4 Earth radii, the "Venus zone" means the region with approximately 1 to 25 times Earth's stellar flux for M and K-type stars and approximately 1.1 to 25 times Earth's stellar flux for G-type stars.</ref> Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus.<ref>[http://hzgallery.org/venus.html Habitable Zone Gallery - Venus]</ref><ref>{{Cite journal |last1=Kane |first1=Stephen R. |last2=Kopparapu |first2=Ravi Kumar |last3=Domagal-Goldman |first3=Shawn D. |author-link3=Shawn Domagal-Goldman |year=2014 |title=On the Frequency of Potential Venus Analogs from Kepler Data |journal=The Astrophysical Journal |volume=794 |issue=1 |pages=L5 |arxiv=1409.2886 |bibcode=2014ApJ...794L...5K |doi=10.1088/2041-8205/794/1/L5 |s2cid=119178082}}</ref>