Editing Habitable zone (section)

==Determination==
[[File:Triple_point_diagram_indicating_planets_within_Solar_System_habitable_zone.png|thumb|Thermodynamic properties of water depicting the conditions at the surface of the terrestrial planets: Mars is near the triple point, Earth in the liquid; and Venus near the critical point.]]

Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the [[radiative flux]] of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of [[super-Earth]] planets that can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger [[greenhouse effect]]s, can have the right temperature for liquid water to exist at the surface.<ref name=kasting-1988>{{cite journal| title=Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus |journal=Icarus |date=June 1988|author=Kasting, James F.|volume = 74|issue = 3|pages = 472–494| doi=10.1016/0019-1035(88)90116-9|pmid = 11538226|bibcode=1988Icar...74..472K |url = https://zenodo.org/record/1253896}}</ref>

===Solar System estimates===
{{seealso|Water on terrestrial planets of the Solar System}}
[[File: Estimated extent of the Solar Systems habitable zone.png|thumb|The range of published estimates for the extent of the Sun's HZ. The conservative HZ<ref name=dole-1964 /> is indicated by a dark-green band crossing the inner edge of the [[Apsis#Perihelion and aphelion|aphelion]] of [[Venus]], whereas an extended HZ,<ref name=fogg-1992 /> extending to the orbit of the [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]], is indicated by a light-green band.]]
[[File:Terrestrial planet size comp 2024.png|thumb|Solar System's [[Planetary-mass object]]s with partial or full orbits within the Extended Habitable Zone from left to right: Mercury, Venus, Earth & Moon, Mars, and Ceres. While many possess surface water in solid state, only Earth has liquid water on the surface. This is mainly due to a combination of low mass and an inability to mitigate evaporation and atmosphere loss against the [[solar wind]].]]
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 [[astronomical unit]]s,<ref name=zsom-2013 /><ref name= rayeric-2011 /><ref name= rk-2017 /><ref>{{cite web| url=http://depts.washington.edu/naivpl/sites/default/files/hz.shtml| title=Stellar habitable zone calculator| publisher=[[University of Washington]]| access-date=17 December 2015}}</ref> though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However, their atmospheric conditions vary substantially.

The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and, while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to {{convert|462|C|F}} at which water can only exist as vapor.<ref name=venus-2006>{{cite web|url=http://burro.cwru.edu/stu/advanced/venus.html |title=Venus |publisher=Case Western Reserve University |date=13 September 2006 |access-date=2011-12-21 |archive-url=https://web.archive.org/web/20120426064658/http://burro.cwru.edu/stu/advanced/venus.html |archive-date=2012-04-26 }}</ref> The entire orbits of the [[Moon]],<ref name=sharp>{{cite web |url=http://www.space.com/18067-moon-atmosphere.html |title=Atmosphere of the Moon |publisher=TechMediaNetwork |work=Space.com |access-date=April 23, 2013 |author=Sharp, Tim}}</ref> [[Mars]],<ref name="bolonkin09">{{Cite book|first=Alexander A.|last=Bolonkin|date=2009|title=Artificial Environments on Mars|publisher=Springer |place=Berlin Heidelberg|pages=599–625|isbn=978-3-642-03629-3}}</ref> and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods.<ref name="HaberleMcKay2001">{{cite journal|last1=Haberle|first1=Robert M.|last2=McKay|first2=Christopher P.|last3=Schaeffer|first3=James|last4=Cabrol|first4=Nathalie A.|last5=Grin|first5=Edmon A.|last6=Zent|first6=Aaron P.|last7=Quinn|first7=Richard|title=On the possibility of liquid water on present-day Mars|journal=Journal of Geophysical Research|volume=106|issue=E10|year=2001|pages=23317|issn=0148-0227|doi=10.1029/2000JE001360|bibcode = 2001JGR...10623317H |doi-access=free}}</ref> At [[Hellas Basin]], for example, atmospheric pressures can reach 1,115&nbsp;Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year.<ref name = "HaberleMcKay2001"/> Despite indirect evidence in the form of [[seasonal flows on warm Martian slopes]],<ref name="WRD-20140218">{{cite magazine |last=Mann |first=Adam  |title=Strange Dark Streaks on Mars Get More and More Mysterious|url=https://www.wired.com/wiredscience/2014/02/flowing-lineae-water-mars/ |date=February 18, 2014 |magazine=[[Wired (magazine)|Wired]]  |access-date=February 18, 2014 }}</ref><ref name=voanews>{{cite web|url=https://www.voanews.com/a/nasa-finds-possible-signs-of-flowing-water-on-mars-126807133/143341.html|title=NASA Finds Possible Signs of Flowing Water on Mars|date=3 August 2011 |publisher=Voice of America|access-date=August 5, 2011|url-status=live|archive-url=https://web.archive.org/web/20110917071451/http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html|archive-date=September 17, 2011}}</ref><ref name=mag>{{cite web|url=http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|title=Is Mars Weeping Salty Tears?|publisher=news.sciencemag.org|access-date=August 5, 2011|archive-url=https://web.archive.org/web/20110814065220/http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|archive-date=August 14, 2011}}</ref><ref name="NASA-20131210">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Mars Spacecraft Reveals a More Dynamic Red Planet |url=http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |date=December 10, 2013 |work=[[NASA]] |access-date=December 10, 2013 }}</ref> no confirmation has been made of the presence of liquid water at the surface. While other objects orbit partly within this zone, including comets, [[Ceres (dwarf planet)|Ceres]]<ref name=ahearn-1992>{{cite journal|last=A'Hearn|first=Michael F.|author2=Feldman, Paul D.|title=Water vaporization on Ceres|journal=Icarus|volume=98|issue=1|pages=54–60|date=1992|doi=10.1016/0019-1035(92)90206-M|bibcode= 1992Icar...98...54A}}</ref> is the only one of planetary mass. 

Despite this, studies indicate the strong possibility of past liquid water on the surface of [[Water on Venus|Venus]],<ref name="SalvadorMassol2017">{{cite journal|last1=Salvador|first1=A.|last2=Massol|first2=H.|last3=Davaille|first3=A.|last4=Marcq|first4=E.|last5=Sarda|first5=P.|last6=Chassefière|first6=E.|title=The relative influence of H2 O and CO2 on the primitive surface conditions and evolution of rocky planets|journal=Journal of Geophysical Research: Planets|volume=122|issue=7|year=2017|pages=1458–1486|issn=2169-9097|doi=10.1002/2017JE005286|bibcode=2017JGRE..122.1458S|s2cid=135136696 |url=https://hal-insu.archives-ouvertes.fr/insu-01540209/file/2017JE005286.pdf}}</ref> [[Lunar water|the Moon]],<ref>{{cite web |url=https://news.wsu.edu/2018/07/23/possibility-of-moon-life/ |title=Mysteries from the moon's past |date=23 July 2018 |publisher=[[Washington State University]] |access-date=22 August 2020}}</ref><ref>{{Cite journal|doi=10.1089/ast.2018.1844|title=Was There an Early Habitability Window for Earth's Moon?|year=2018|last1=Schulze-Makuch|first1=Dirk|last2=Crawford|first2=Ian A.|journal=Astrobiology|volume=18|issue=8|pages=985–988|pmid=30035616|pmc=6225594|bibcode=2018AsBio..18..985S}}</ref> [[Water on Mars|Mars]],<ref>{{cite web |url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2000| access-date=December 19, 2010}}</ref><ref name='Willson 2018'>{{cite web|url=https://www.space.com/8642-flashback-water-mars-announced-10-years.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2010| access-date=May 13, 2018}}</ref><ref>{{cite web| url=https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| title=Science@NASA, The Case of the Missing Mars Water| access-date=March 7, 2009| archive-url=https://web.archive.org/web/20090327234049/https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| archive-date=March 27, 2009}}</ref> [[4 Vesta|Vesta]]<ref name="ScullyRussell2015">{{cite journal|last1=Scully|first1=Jennifer E.C.|last2=Russell|first2=Christopher T.|last3=Yin|first3=An|last4=Jaumann|first4=Ralf|last5=Carey|first5=Elizabeth|last6=Castillo-Rogez|first6=Julie|last7=McSween|first7=Harry Y.|last8=Raymond|first8=Carol A.|last9=Reddy|first9=Vishnu|last10=Le Corre|first10=Lucille|title=Geomorphological evidence for transient water flow on Vesta|journal=Earth and Planetary Science Letters|volume=411|year=2015|pages=151–163|issn=0012-821X|doi=10.1016/j.epsl.2014.12.004|bibcode=2015E&PSL.411..151S}}</ref> and Ceres,<ref name="RaponiDe Sanctis2018">{{cite journal|last1=Raponi|first1=Andrea|last2=De Sanctis|first2=Maria Cristina|last3=Frigeri|first3=Alessandro|last4=Ammannito|first4=Eleonora|last5=Ciarniello|first5=Mauro|last6=Formisano|first6=Michelangelo|last7=Combe|first7=Jean-Philippe|last8=Magni|first8=Gianfranco|last9=Tosi|first9=Federico|last10=Carrozzo|first10=Filippo Giacomo|last11=Fonte|first11=Sergio|last12=Giardino|first12=Marco|last13=Joy|first13=Steven P.|last14=Polanskey|first14=Carol A.|last15=Rayman|first15=Marc D.|last16=Capaccioni|first16=Fabrizio|last17=Capria|first17=Maria Teresa|last18=Longobardo|first18=Andrea|last19=Palomba|first19=Ernesto|last20=Zambon|first20=Francesca|last21=Raymond|first21=Carol A.|last22=Russell|first22=Christopher T.|title=Variations in the amount of water ice on Ceres' surface suggest a seasonal water cycle|journal=Science Advances|volume=4|issue=3|year=2018|pages=eaao3757|issn=2375-2548|doi=10.1126/sciadv.aao3757|pmid=29546238|pmc=5851659|bibcode=2018SciA....4.3757R }}</ref><ref>[https://photojournal.jpl.nasa.gov/catalog/PIA21471 NASA.gov] PIA21471: Landslides on Ceres</ref> suggesting a more common phenomenon 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 the extended habitable zone concept, 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&nbsp;kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced [[albedo]], anti-greenhouse or other possible heat sources.

For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39&nbsp;AU; Venus, 0.72&nbsp;AU; Earth, 1.00&nbsp;AU; Mars, 1.52&nbsp;AU; Vesta, 2.36&nbsp;AU; Ceres and Pallas, 2.77&nbsp;AU; Jupiter, 5.20&nbsp;AU; Saturn, 9.58&nbsp;AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.

{| class="wikitable sortable"
|- style="text-align:center; align:center; background:#90b0f0;"
|+ Estimates of the circumstellar habitable zone boundaries of the Solar System
! Inner edge ([[Astronomical unit|AU]]) !! The outer edge (AU) !! Year !! Notes
|-
| 0.725 || 1.24 || 1964, Dole<ref name=dole-1964 />|| Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
|-
| || 1.005–1.008 || 1969, Budyko<ref name=budyko-1969>{{Cite journal | last1 = Budyko | first1 = M. I. | title = The effect of solar radiation variations on the climate of the Earth | doi = 10.1111/j.2153-3490.1969.tb00466.x | journal = Tellus | volume = 21 | issue = 5 | pages = 611–619 | year = 1969 | bibcode = 1969Tell...21..611B| citeseerx = 10.1.1.696.824 }}</ref>|| Based on studies of ice-albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969<ref>{{cite journal |title=A Global Climatic Model Based on the Energy Balance of the Earth-Atmosphere System |author=Sellers, William D. |journal=Journal of Applied Meteorology |date=June 1969 |volume=8 |issue=3 |pages=392–400 |doi=10.1175/1520-0450(1969)008<0392:AGCMBO>2.0.CO;2 |doi-access=free |bibcode=1969JApMe...8..392S}}</ref> and North 1975.<ref>{{cite journal
 |last1 = North
 |first1 = Gerald R.
 |date=November 1975
 |title = Theory of Energy-Balance Climate Models
 |journal = Journal of the Atmospheric Sciences
 |volume = 32
 |issue = 11
 |pages = 2033–2043
 |doi = 10.1175/1520-0469(1975)032<2033:TOEBCM>2.0.CO;2
|doi-access=free
 |bibcode = 1975JAtS...32.2033N
 }}</ref>
|-
| 0.92–0.96 || || 1970, Rasool and De Bergh<ref name=rasool-1970>{{cite journal | pages = 1037–1039 | issue = 5250 | volume = 226 | date = 13 Jun 1970 | doi = 10.1038/2261037a0 | pmid = 16057644 | issn = 0028-0836 | journal = Nature | first2 = C. | first1 = I. | title = The Runaway Greenhouse and the Accumulation of CO<sub>2</sub> in the Venus Atmosphere | url = https://pubs.giss.nasa.gov/docs/1970/1970_Rasool_ra00500s.pdf | archive-url = https://web.archive.org/web/20231114213658/https://pubs.giss.nasa.gov/docs/1970/1970_Rasool_ra00500s.pdf | archive-date = 14 November 2023 | url-status = live | last1 = Rasool | last2 = De Bergh | bibcode = 1970Natur.226.1037R | s2cid = 4201521 }}</ref>|| Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
|-
| 0.958 || 1.004 || 1979, Hart<ref name=hart-1979>{{Cite journal | last1 = Hart | first1 = M. H. | doi = 10.1016/0019-1035(79)90141-6 | title = Habitable zones about main sequence stars | journal = Icarus | volume = 37 | issue = 1 | pages = 351–357 | year = 1979 |bibcode = 1979Icar...37..351H }}</ref>|| Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
|-
| || 3.0 || 1992, Fogg<ref name=fogg-1992>{{cite journal |title=An Estimate of the Prevalence of Biocompatible and Habitable Planets |author=Fogg, M. J. |journal=Journal of the British Interplanetary Society |date=1992 |volume=45 |pages=3–12 |bibcode=1992JBIS...45....3F |pmid=11539465 |issue=1}}</ref>|| Used the [[carbon cycle]] to estimate the outer edge of the circumstellar habitable zone.
|-
| 0.95 || 1.37 || 1993, Kasting et al.<ref name=kasting-1993 />|| Founded the most common working definition of the habitable zone used today. Assumes that CO<sub>2</sub> and H<sub>2</sub>O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the [[carbonate–silicate cycle]]. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
|-
| || 2.0 || 2010, Spiegel et al.<ref name="speigel-2010">{{Cite journal | last1 = Spiegel | first1 = D. S. | last2 = Raymond | first2 = S. N. | last3 = Dressing | first3 = C. D. | last4 = Scharf | first4 = C. A. | last5 = Mitchell | first5 = J. L. | title = Generalized Milankovitch Cycles and Long-Term Climatic Habitability | doi = 10.1088/0004-637X/721/2/1308 | journal = The Astrophysical Journal | volume = 721 | issue = 2 | pages = 1308–1318 | year = 2010 |arxiv = 1002.4877 |bibcode = 2010ApJ...721.1308S | s2cid = 15899053 }}</ref>|| Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
|-
| 0.75 || || 2011, Abe et al.<ref name="abe-2011">{{Cite journal | last1 = Abe | first1 = Y. | last2 = Abe-Ouchi | first2 = A. | last3 = Sleep | first3 = N. H. | last4 = Zahnle | first4 = K. J. | title = Habitable Zone Limits for Dry Planets | doi = 10.1089/ast.2010.0545 | journal = Astrobiology | volume = 11 | issue = 5 | pages = 443–460 | year = 2011 | pmid =  21707386|bibcode = 2011AsBio..11..443A }}</ref>|| Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
|-
|  ||10  || 2011, Pierrehumbert and Gaidos<ref name= rayeric-2011 />||Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in the Solar System.
|-
| 0.77–0.87 || 1.02–1.18 || 2013, Vladilo et al.<ref name=vladilo-2013>{{cite journal |title=The habitable zone of Earth-like planets with different levels of atmospheric pressure |author1=Vladilo, Giovanni |author2=Murante, Giuseppe |author3=Silva, Laura |author4=Provenzale, Antonello |author5=Ferri, Gaia |author6=Ragazzini, Gregorio |journal=The Astrophysical Journal |date=March 2013 |volume=767 |issue=1 |pages=65–? |doi=10.1088/0004-637X/767/1/65 |arxiv=1302.4566|bibcode = 2013ApJ...767...65V |s2cid=49553651 }}</ref>|| Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar.
|-
| 0.99 || 1.67 || 2013, Kopparapu et al.<ref name=kopparapu-2013 /><ref name="Kopparapu2013b" />|| Revised estimates of the Kasting et al. (1993) formulation using updated moist greenhouse and water loss algorithms. According to this measure, Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapor. Once the stratosphere becomes wet, water vapor photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a {{CO2}} dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in {{CO2}} will not create enough warming to prevent {{CO2}} catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.67 AU. This definition does not take into account possible radiative warming by {{CO2}} clouds.
|-
| 0.38 || || 2013, Zsom et al.<br /><ref name=zsom-2013>{{cite journal |title=Towards the Minimum Inner Edge Distance of the Habitable Zone |last1=Zsom |first1=Andras |date=2013 |arxiv=1304.3714 |last2=Seager |first2=Sara |last3=De Wit |first3=Julien |doi=10.1088/0004-637X/778/2/109 |volume=778 |issue=2 |journal=The Astrophysical Journal |page=109 |bibcode=2013ApJ...778..109Z|s2cid=27805994 }}</ref>|| Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.
|-
| 0.95  ||  || 2013, Leconte et al.<ref name= leconte-2013>{{cite journal |title=Increased insolation threshold for runaway greenhouse processes on Earth-like planets |last1=Leconte |first1=Jeremy |date=2013 |arxiv=1312.3337 |last2=Forget |first2=Francois |last3=Charnay |first3=Benjamin |last4=Wordsworth |first4=Robin |last5=Pottier |first5=Alizee |doi=10.1038/nature12827 |pmid=24336285 |volume=504 |issue=7479 |pages=268–71 |journal=Nature |bibcode=2013Natur.504..268L|s2cid=2115695 }}</ref>||Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System.
|-
| 0.95 ||2.4 || 2017, Ramirez and Kaltenegger<br /><ref name=rk-2017>{{cite journal |title=A Volcanic Hydrogen Habitable Zone |last1=Ramirez |first1=Ramses |date=2017 |arxiv=1702.08618|last2=Kaltenegger |first2=Lisa |doi=10.3847/2041-8213/aa60c8 |volume=837 |issue=1 |pages=L4 |journal=The Astrophysical Journal Letters|bibcode=2017ApJ...837L...4R|s2cid=119333468 |doi-access=free }}</ref>|| An expansion of the classical carbon dioxide-water vapor habitable zone<ref name=kasting-1993 /> assuming a volcanic hydrogen atmospheric concentration of 50%.
|-
| 0.93–0.91|| || 2019, Gomez-Leal et al.<br /><ref name=gomez-2019>{{cite journal |title=Climate sensitivity to ozone and its relevance on the habitability of Earth-like planets|last1=Gomez-Leal |first1=Illeana |date=2019 |arxiv=1901.02897|last2=Kaltenegger |first2=Lisa |last3=Lucarini |first3=Valerio |last4=Lunkeit |first4=Frank |doi=10.1016/j.icarus.2018.11.019 |volume=321 |pages=608–618 |journal=Icarus|bibcode=2019Icar..321..608G|s2cid=119209241 }}</ref>|| Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of climate sensitivity in both cases. 
|-
! 0.99 !! 1.004 !! !! The tightest bounded estimate from above
|- 
! 0.38 !! 10 !! !! The most relaxed estimate from above
|-
|}

===Extrasolar extrapolation===
{{see also|Habitability of red dwarf systems|Habitability of K-type main-sequence star systems}}
[[File:Orbit of 82 G. Eridani d.png|thumb|The orbit of [[82 G. Eridani d]] which passes through predicted conservative and optimistic habitable zones of its sun-like [[G-type main-sequence star|G-type]] parent star.]]
Astronomers use stellar flux and the [[inverse-square law]] to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun,<ref name="kopparapu-2013" /> a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at <math>\sqrt{0.25}</math>, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the HZ concept is more complex.

====Spectral types and star-system characteristics====
[[File:Circling Two Suns.ogv|thumb|300px|A video explaining the significance of the 2011 discovery of a planet in the circumbinary habitable zone of Kepler-47]]

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain [[spectral type]]s. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration.<ref>{{cite journal| arxiv=1303.6645| title=S-Type and P-Type Habitability in Stellar Binary Systems: A Comprehensive Approach. I. Method and Applications| date=2013 |last=Cuntz |first=Manfred | doi=10.1088/0004-637X/780/1/14 | volume=780 | issue=1| journal=The Astrophysical Journal | page=14 | bibcode=2014ApJ...780...14C| s2cid=118610856}}</ref> If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.<ref name="forget-1997">{{cite journal|doi=10.1126/science.278.5341.1273| title=Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation| date=1997| last1=Forget| first1=F.| journal=Science| volume=278|issue=5341|pages=1273–6| pmid=9360920| last2=Pierrehumbert|first2=RT|bibcode = 1997Sci...278.1273F| citeseerx=10.1.1.41.621}}</ref><ref name="mischna-2000">{{cite journal| doi=10.1006/icar.2000.6380| title=Influence of Carbon Dioxide Clouds on Early Martian Climate| date=2000 |last1=Mischna| first1=M| journal=Icarus| volume=145 |issue=2| pages=546–54| pmid=11543507| last2=Kasting| first2=JF| last3=Pavlov| first3=A|last4=Freedman| first4=R| bibcode = 2000Icar..145..546M }}</ref>

With regard to spectral types, [[Zoltán Balog (astronomer)|Zoltán Balog]] proposes that [[O-type star]]s cannot form planets due to the [[photoevaporation]] caused by their strong [[ultraviolet]] emissions.<ref>{{cite press release| url=http://www.spitzer.caltech.edu/news/863-feature06-31-Planets-Prefer-Safe-Neighborhoods |title=Planets Prefer Safe Neighborhoods |publisher=Spitzer.caltech.edu |access-date=April 22, 2013 |author=Vu, Linda |agency=NASA/Caltech}}</ref> Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.<ref name="BuccinoLemarchand2006">{{cite journal| last1=Buccino|first1=Andrea P.|last2=Lemarchand|first2=Guillermo A.|last3=Mauas|first3=Pablo J.D.| title=Ultraviolet radiation constraints around the circumstellar habitable zones| journal=Icarus| volume=183| issue=2|date=2006|pages=491–503|doi=10.1016/j.icarus.2006.03.007|arxiv = astro-ph/0512291 |bibcode = 2006Icar..183..491B |citeseerx=10.1.1.337.8642|s2cid=2241081}}</ref> Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of [[spectral class]] [[K-type main-sequence star|K0]] or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a [[tidal locking]] radius for [[red dwarf]]s. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment for life.<ref name=barnes-2013>{{cite journal |title=Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary |journal=Astrobiology |date=March 2013 |volume=13 |issue=3 |pages=279–291 |doi=10.1089/ast.2012.0867 |arxiv=1203.5104 |last1=Barnes |first1=Rory |last2=Heller |first2=René |pmid=23537137 |pmc=3612282|bibcode = 2013AsBio..13..279B }}</ref>

Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modeling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces despite tidal locking.<ref name=yang-2013 /> Astronomy professor [[Eric Agol]] argues that even [[white dwarf]]s may support a relatively brief habitable zone through planetary migration.<ref>{{cite journal |title=Transit Surveys for Earths in the Habitable Zones of White Dwarfs |author=Agol, Eric |journal=The Astrophysical Journal Letters |date=April 2011 |volume=731 |issue=2 |pages=L31 |doi=10.1088/2041-8205/731/2/L31 |arxiv=1103.2791|bibcode = 2011ApJ...731L..31A |s2cid=118739494 }}</ref> At the same time, others have written in similar support of semi-stable, temporary habitable zones around [[brown dwarf]]s.<ref name=barnes-2013 /> Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales.<ref name=rk-2014>{{cite journal |title=Habitable Zones of Pre-Main-Sequence Stars |last1=Ramirez |first1=Ramses |date=2014 |arxiv=1412.1764|last2=Kaltenegger |first2=Lisa |doi=10.1088/2041-8205/797/2/L25 |volume=797 |issue=2 |pages=L25 |journal=The Astrophysical Journal Letters|bibcode=2014ApJ...797L..25R|s2cid=119276912 }}</ref>

====Stellar evolution====
[[File:Magnetosphere rendition.jpg|thumb|left|Natural shielding against [[space weather]], such as the magnetosphere depicted in this artistic rendition, may be required for planets to sustain surface water for prolonged periods.]]
Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the [[main sequence]] for fewer than 10&nbsp;million years,<ref name="carroll">{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |edition=2nd |date=2007 |title=An Introduction to Modern Astrophysics}}</ref> would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve.<ref name="richmond">{{cite web
 |last=Richmond |first=Michael |date=November 10, 2004
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 |access-date=2007-09-19 }}</ref><ref name="guo-2009">{{Cite journal | last1 = Guo | first1 = J. | last2 = Zhang | first2 = F. | last3 = Chen | first3 = X. | last4 = Han | first4 = Z. | title = Probability distribution of terrestrial planets in habitable zones around host stars | doi = 10.1007/s10509-009-0081-z | journal = Astrophysics and Space Science | volume = 323 | issue = 4 | pages = 367–373 | year = 2009 |arxiv = 1003.1368 |bibcode = 2009Ap&SS.323..367G | s2cid = 118500534 }}</ref> Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the [[Archean|Archaean]] as it is now,<ref name="Kasting-1968">{{Cite journal
 |last1=Kasting |first1=J.F.
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 }}</ref> and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the [[red giant]] phase.<ref name=franck-2002>{{cite conference |url=http://www.pik-potsdam.de/PLACES/publications/datenfiles/ASP_269.pdf |title=Habitable Zones and the Number of Gaia's Sisters |publisher=Astronomical Society of the Pacific |access-date=April 26, 2013 |author1=Franck, S. |author2=von Bloh, W. |author3=Bounama, C. |author4=Steffen, M. |author5=Schönberner, D. |author6=Schellnhuber, H.-J. |editor1=Montesinos, Benjamin |editor2=Giménez, Alvaro |editor3=Guinan, Edward F. |book-title=ASP Conference Series |date=2002 |conference=The Evolving Sun and its Influence on Planetary Environments |pages=261–272 |bibcode=2002ASPC..269..261F |isbn=1-58381-109-5}}</ref> In order to deal with this increase in luminosity, the concept of a ''continuously habitable zone'' has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.<ref name=franck-2002 />

In red dwarf systems, gigantic [[stellar flare]]s which could double a star's brightness in minutes<ref>{{cite web |first=Ken| last=Croswell| url=https://www.newscientist.com/article/mg16922754.200-red-willing-and-able.html |url-access=subscription |title=Red, willing and able |access-date=August 5, 2007|date=January 27, 2001 |magazine=[[New Scientist]]}} [http://www.kencroswell.com/reddwarflife.html Full reprint]</ref> and huge [[starspot]]s which can cover 20% of the star's surface area,<ref name=alekseev-2002>{{Cite journal | last1 = Alekseev | first1 = I. Y.| last2 = Kozlova | first2 = O. V.| title = Starspots and active regions on the emission red dwarf star LQ Hydrae| journal = Astronomy and Astrophysics| volume = 396| pages = 203–211| year = 2002| bibcode = 2002A&A...396..203A| doi = 10.1051/0004-6361:20021424 | doi-access = free}}</ref> have the potential to strip an otherwise habitable planet of its atmosphere and water.<ref name=alpert-2005 /> As with more massive stars, though, stellar evolution changes their nature and energy flux,<ref name=west-2006>{{cite journal| url=http://earthsky.org/space/fewer-flares-starspots-for-older-dwarf-stars |title=Andrew West: 'Fewer flares, starspots for older dwarf stars' |journal=EarthSky |date=December 19, 2006 |access-date=April 27, 2013 |author=<!--staff writer-->}}</ref> so by about 1.2&nbsp; billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.<ref name=alpert-2005>{{cite journal| last=Alpert |first=Mark |title=Red Star Rising |journal=Scientific American |volume=293 |issue=5 |pages=28 |date=November 7, 2005 |pmid=16318021 |doi=10.1038/scientificamerican1105-28 |bibcode=2005SciAm.293e..28A }}</ref><ref>{{cite web |title=AstronomyCast episode 40: American Astronomical Society Meeting, May 2007 |work=Universe Today |last1=Cain |first1=Fraser |last2=Gay |first2=Pamela |author-link2=Pamela L. Gay |url=http://media-c02m01.libsyn.com/podcasts/c50d001e8872db18d96cd44a73adccdc/46762eec/astronomycast/AstroCast-070611.mp3 |archive-url=https://wayback.archive-it.org/all/20070926102556/http://media-c02m01.libsyn.com/podcasts/c50d001e8872db18d96cd44a73adccdc/46762eec/astronomycast/AstroCast-070611.mp3 |archive-date=2007-09-26 |date=2007 |access-date=2007-06-17 }}</ref>

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.<ref>{{cite web|url=http://www.astrobio.net/topic/solar-system/sun/living-in-a-dying-solar-system-part-1/|title=Living in a Dying Solar System, Part 1|publisher=Astrobiology|language=en|author=Ray Villard|date=27 July 2009|access-date=8 April 2016|url-status=dead|archive-date=24 April 2016|archive-url=https://web.archive.org/web/20160424143742/http://www.astrobio.net/topic/solar-system/sun/living-in-a-dying-solar-system-part-1/}}</ref> For example, the Sun is expected to engulf the previously habitable Earth as a red giant.<ref name=christensen-2005>{{cite news |url=http://www.space.com/920-red-giants-planets-live.html |title=Red Giants and Planets to Live On |work=Space.com |date=April 1, 2005 |agency=TechMediaNetwork |access-date=April 27, 2013 |author=Christensen, Bill}}</ref><ref name=rk-2016 /> However, once a red giant star reaches the [[horizontal branch]], it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.<ref name=lopez-2005>{{Cite journal | last1 = Lopez | first1 = B. | last2 = Schneider | first2 = J. | last3 = Danchi | first3 = W. C. | doi = 10.1086/430416 | title = Can Life Develop in the Expanded Habitable Zones around Red Giant Stars? | journal = The Astrophysical Journal | volume = 627 | issue = 2 | pages = 974–985 | year = 2005 |arxiv = astro-ph/0503520 |bibcode = 2005ApJ...627..974L | s2cid = 17075384 }}</ref> At such stage, Saturn's moon [[Titan (moon)|Titan]] would likely be habitable in Earth's temperature sense.<ref name="LorenzLunine1997">{{cite journal| last1=Lorenz|first1=Ralph D.|last2=Lunine|first2=Jonathan I.|last3=McKay|first3=Christopher P.|title=Titan under a red giant sun: A new kind of "habitable" moon| journal=Geophysical Research Letters|volume=24|issue=22|date=1997|pages=2905–2908|issn=0094-8276|doi=10.1029/97GL52843|bibcode=1997GeoRL..24.2905L|pmid=11542268|citeseerx=10.1.1.683.8827|s2cid=14172341 }}</ref> Given that this new equilibrium lasts for about 1 [[Byr|Gyr]], and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants.<ref name=lopez-2005 /> However, around such a helium-burning star, important life processes like [[photosynthesis]] could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide.<ref name=voisey-2011>{{cite news |url=http://www.universetoday.com/83248/plausibility-check-habitable-planet-around-red-giants/ |title=Plausibility Check – Habitable Planets around Red Giants |work=Universe Today |date=February 23, 2011 |access-date=April 27, 2013 |author=Voisey, Jon}}</ref> Moreover, as Ramirez and Kaltenegger (2016)<ref name=rk-2016>{{cite journal |title=Habitable Zones of Post-Main Sequence Stars|last1=Ramirez |first1=Ramses |date=2016 |arxiv=1605.04924|last2=Kaltenegger |first2=Lisa |doi=10.3847/0004-637X/823/1/6 |volume=823 |issue=1 |pages=6 |journal=The Astrophysical Journal|bibcode=2016ApJ...823....6R|s2cid=119225201 |doi-access=free }}</ref> showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant.<ref name=rk-2016/> Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward,  the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage.<ref name=rk-2016/>

====Desert planets====
[[File:Tharsis and Valles Marineris - Mars Orbiter Mission (30055660701).png|thumb|Dry desert planets like Mars may be more common in the habitable zone than wet planets.]]
A planet's atmospheric conditions influence its ability to retain heat so that the location of the habitable zone is also specific to each type of planet: [[desert planet]]s (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced [[greenhouse effect]], meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.<ref>[http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets Alien Life More Likely on 'Dune' Planets] {{webarchive |url=https://web.archive.org/web/20131202223111/http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets |date=December 2, 2013 }}, 09/01/11, Charles Q. Choi, ''Astrobiology Magazine''</ref><ref>{{cite journal | doi = 10.1089/ast.2010.0545 | pmid=21707386 | volume=11 | title=Habitable zone limits for dry planets | year=2011 | journal=Astrobiology | pages=443–60 | last1 = Abe | first1 = Y | last2 = Abe-Ouchi | first2 = A | last3 = Sleep | first3 = NH | last4 = Zahnle | first4 = KJ| issue=5 | bibcode=2011AsBio..11..443A }}</ref>

====Other considerations====
[[Image:BlueMarble-2001-2002.jpg|thumb|Earth's hydrosphere. Water covers 71% of Earth's surface, with the [[global ocean]] accounting for 97.3% of the [[water distribution on Earth]].]]
{{see also|Planetary habitability|Habitability of natural satellites}}
A planet cannot have a [[hydrosphere]]—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The [[origin of water on Earth]] is still not completely understood; possible sources include the result of impacts with icy bodies, [[outgassing]], [[mineralization (geology)|mineralization]], leakage from [[hydrous]] minerals from the [[lithosphere]], and [[photolysis]].<ref name="source_mrk1">{{cite journal |title= Origin of water in the terrestrial planets |last1= Drake |first1= Michael J. |s2cid= 12808812 |journal=Meteoritics & Planetary Science |date=April 2005 |volume=40 |issue= 4 |pages= 519–527 |doi= 10.1111/j.1945-5100.2005.tb00960.x |bibcode= 2005M&PS...40..519D|doi-access= free }}</ref><ref name="source_mrk2">{{cite conference |url= http://journals.cambridge.org/action/displayFulltext?type=6&fid=415222&jid=IAU&volumeId=1&issueId=S229&aid=414784&bodyId=&membershipNumber=&societyETOCSession=&fulltextType=RA&fileId=S1743921305006861 |title= Origin of water in the terrestrial planets |display-authors=1 |last1= Drake |first1= Michael J. |last2= Humberto |first2= Campins |conference = 229th Symposium of the International Astronomical Union |date=August 2005 |location = Búzios, Rio de Janeiro, Brazil |publisher= Cambridge University Press |volume=1 |issue= 4 |pages= 381–394 |doi= 10.1017/S1743921305006861 |bibcode= 2006IAUS..229..381D |book-title= Asteroids, Comets, and Meteors (IAU S229) |isbn= 978-0-521-85200-5|doi-access= free }}</ref> For an extrasolar system, an icy body from beyond the [[frost line (astrophysics)|frost line]] could migrate into the habitable zone of its star, creating an [[ocean planet]] with seas hundreds of kilometers deep<ref name=kuchner-2003>{{Cite journal|arxiv=astro-ph/0303186|title=Volatile-rich Earth-Mass Planets in the Habitable Zone|first=Marc|last=Kuchner|journal=Astrophysical Journal|date=2003|volume=596|issue=1|pages=L105–L108|doi=10.1086/378397|bibcode=2003ApJ...596L.105K|s2cid=15999168}}</ref> such as [[GJ 1214 b]]<ref name="disco-charbonneau">{{cite journal |last1=Charbonneau |first1=David
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|author11=Ruth A. Murray-Clay
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|author13=Emilio E. Falco
|author14=Joshua N. Winn
|author15=Didier Queloz
|author16=Francesco Pepe
|author17=Michel Mayor
|author18=Xavier Delfosse
|author19=Thierry Forveille
|display-authors=8
|date=2009 |title=A super-Earth transiting a nearby low-mass star |journal=Nature |volume=462 |issue=17 December 2009 |pages=891–894 |doi=10.1038/nature08679 |pmid=20016595 |bibcode=2009Natur.462..891C|arxiv = 0912.3229 |s2cid=4360404
}}</ref><ref name="planetmodels">{{cite journal |last1= Kuchner |first1= Seager |first2=M.|last2=Hier-Majumder | first3=C. A.|last3=Militzer |date=2007 |title=Mass–radius relationships for solid exoplanets |journal=The Astrophysical Journal |volume=669 |issue= 2|pages=1279–1297 |doi=10.1086/521346 |bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 |s2cid= 8369390 }}</ref> or [[Kepler-22b]] may be.<ref name=vastag-2011>{{cite news |url=https://www.washingtonpost.com/national/health-science/newest-alien-planet-is-just-the-right-temperature-for-life/2011/12/05/gIQAPk1vWO_story.html |title=Newest alien planet is just the right temperature for life |newspaper=The Washington Post |date=December 5, 2011 |access-date=April 27, 2013 |author=Vastag, Brian}}</ref>

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorized to outgassing, impact degassing, and ingassing.<ref name="RobinsonCatling2012">{{cite journal|last1=Robinson|first1=Tyler D.|last2=Catling|first2=David C.| title=An Analytic Radiative-Convective Model for Planetary Atmospheres| journal=The Astrophysical Journal| volume=757|issue=1|date=2012|pages=104|doi=10.1088/0004-637X/757/1/104|arxiv = 1209.1833 |bibcode = 2012ApJ...757..104R |s2cid=54997095}}</ref> Atmospheres are thought to be maintained through similar processes along with [[biogeochemical cycle]]s and the mitigation of [[atmospheric escape]].<ref name="Shizgal, 1996">{{cite journal |last1=Shizgal |first1=B. D. |last2=Arkos |first2=G. G. |s2cid=7852371 |date=1996 |title=Nonthermal escape of the atmospheres of Venus, Earth, and Mars |journal=[[Reviews of Geophysics]] |volume=34 |issue=4 |pages=483–505 |doi=10.1029/96RG02213 |bibcode = 1996RvGeo..34..483S }}</ref> In a 2013 study led by Italian astronomer [[Giovanni Vladilo]], it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure.<ref name=vladilo-2013 /> Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained<ref name=vladilo-2013 /> because even a small shift in pressure or temperature could render water unable to form as a liquid.<ref name=chaplin-2013>{{cite web |url=http://www.lsbu.ac.uk/water/phase.html |title=Water Phase Diagram |publisher=London South Bank University |work=Ices |date=April 8, 2013 |access-date=April 27, 2013 |author=Chaplin, Martin}}</ref>

Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth),<ref name=kasting-1993 /> a study<ref name="rk-2017"/> led by Ramses Ramirez and co-author [[Lisa Kaltenegger]] has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos<ref name=rayeric-2011>{{cite journal |title=Hydrogen Greenhouse Planets Beyond the Habitable Zone |last1=Pierrehumbert |first1=Raymond |date=2011 |arxiv=1105.0021|last2=Gaidos |first2=Eric |doi=10.1088/2041-8205/734/1/L13 |volume=734 |issue=1 |pages=L13 |journal=The Astrophysical Journal Letters|bibcode=2011ApJ...734L..13P|s2cid=7404376 }}</ref> had eliminated the CO<sub>2</sub>-H<sub>2</sub>O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years.

In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause [[tidal locking]], an important factor in habitability. For a tidally locked planet, the [[sidereal day]] is as long as the [[orbital period]], causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its [[bond albedo]] and reducing significantly temperature differences between the two sides.<ref name=yang-2013>{{Cite journal | last1 = Yang | first1 = J. | last2 = Cowan | first2 = N. B. | last3 = Abbot | first3 = D. S. | doi = 10.1088/2041-8205/771/2/L45 | title = Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets | journal = The Astrophysical Journal | volume = 771 | issue = 2 | pages = L45 | year = 2013| arxiv = 1307.0515| bibcode = 2013ApJ...771L..45Y| s2cid = 14119086 }}</ref>

Planetary mass [[exomoon|natural satellites]] have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets.<ref name=hadhazy-2013 /> More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like [[Io (moon)|Io]],<ref name=hadhazy-2013 /> but must remain within the [[Hill radius]] of the planet so that they are not pulled out of the orbit of their host planet.<ref name="HamiltonBurns92">{{cite journal |author1=D.P. Hamilton |author2=J.A. Burns | title= Orbital stability zones about asteroids. II – The destabilizing effects of eccentric orbits and solar radiation| journal= Icarus| date= 1992| volume= 96 |issue= 1| pages= 43–64| bibcode= 1992Icar...96...43H |doi= 10.1016/0019-1035(92)90005-R|url=http://www.astro.umd.edu/~hamilton/research/reprints/HamBurns91.pdf|citeseerx=10.1.1.488.4329 }}</ref> Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.<ref name=hadhazy-2013 />
[[File:Eccentric Habitable Zones.jpg|thumb|Artist's concept of a planet on an eccentric orbit that passes through the HZ for only part of its orbit]]
A planetary object that orbits a star with high [[orbital eccentricity]] may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation ([[cryptobiosis]]) and/or [[hyperthermophile|hyperthermostability]]. [[Tardigrades]], for example, can survive in a dehydrated state temperature between {{convert|-273|C|K|order=flip}}<ref>{{cite journal|author=Becquerel P.|date=1950| title=La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve| journal=C. R. Acad. Sci. Paris| volume=231| pages=261–263| language=fr}}</ref> and {{convert|151|C|K|order=flip}}.<ref name="survival">{{cite book| last=Horikawa|first=Daiki D.|chapter=Survival of Tardigrades in Extreme Environments: A Model Animal for Astrobiology |series=Cellular Origin, Life in Extreme Habitats and Astrobiology | title=Anoxia Evidence for Eukaryote Survival and Paleontological Strategies.| date=2012|volume=21 |publisher=Springer Netherlands| isbn=978-94-007-1895-1|pages=205–217| edition=21|editor=Alexander V. Altenbach, Joan M. Bernhard & Joseph Seckbach|doi=10.1007/978-94-007-1896-8_12}}</ref> Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the [[apastron]] where the planet is coolest and become active on approach to the [[periastron]] when the planet is sufficiently warm.<ref name=kane-2012>{{cite journal |title=The Habitable Zone and Extreme Planetary Orbits |author1=Kane, Stephen R. |author2=Gelino, Dawn M. |journal=Astrobiology |date=2012 |volume=12 |pages=940–945 |doi=10.1089/ast.2011.0798 |arxiv=1205.2429 |issue=10 |pmid=23035897|bibcode = 2012AsBio..12..940K |s2cid=10551100 }}</ref>