Editing Habitable zone (section)

====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>