Editing Habitable zone (section)

====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
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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
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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/>