Editing Exoplanet (section)

=== Ice ages and snowball states ===
{{See also|Ice age|Snowball Earth}}
The outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling, then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops [[Solution weathering|carbon dioxide weathering]], resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a [[greenhouse effect]] which thaws the planet again. Planets with a large [[axial tilt]]<ref>{{cite journal| arxiv=1401.5323|bibcode = 2015P&SS..105...43L|title = Habitability of Earth-like planets with high obliquity and eccentric orbits: Results from a general circulation model |journal = Planetary and Space Science|last1 = Linsenmeier |first1 = Manuel |last2 = Pascale |first2 = Salvatore |last3 = Lucarini |first3 = Valerio |year = 2014 |doi=10.1016/j.pss.2014.11.003 |volume=105 |pages=43–59|s2cid = 119202437}}</ref> are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt.<ref>{{Cite web|last=Kelley |first=Peter |date=15 April 2014 |title=Astronomers: 'Tilt-a-worlds' could harbor life|url=https://www.washington.edu/news/2014/04/15/astronomers-tilt-a-worlds-could-harbor-life/|access-date=2023-02-12|website=UW News|language=en}}</ref><ref>{{Cite journal | doi = 10.1089/ast.2013.1129| pmid = 24611714| title = Effects of Extreme Obliquity Variations on the Habitability of Exoplanets| journal = Astrobiology| volume = 14| issue = 4| pages = 277–291| year = 2014| last1 = Armstrong | first1 = J. C. | last2 = Barnes | first2 = R.| last3 = Domagal-Goldman | first3 = S.| last4 = Breiner | first4 = J.| last5 = Quinn | first5 = T. R. | last6 = Meadows | first6 = V. S. | bibcode=2014AsBio..14..277A|arxiv = 1404.3686 | pmc=3995117}}</ref> Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.<ref>{{Cite web|last=Kelley |first=Peter |date=18 July 2013 |title=A warmer planetary haven around cool stars, as ice warms rather than cools|url=https://www.washington.edu/news/2013/07/18/a-warmer-planetary-haven-around-cool-stars-as-ice-warms-rather-than-cools/|access-date=2023-02-12|website=UW News|language=en}}</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/785/1/L9| title = Spectrum-Driven Planetary Deglaciation Due to Increases in Stellar Luminosity| journal = The Astrophysical Journal| volume = 785| issue = 1| pages = L9| year = 2014| last1 = Shields | first1 = A. L. | last2 = Bitz | first2 = C. M. |author-link2=Cecilia Bitz| last3 = Meadows | first3 = V. S. | last4 = Joshi | first4 = M. M. | last5 = Robinson | first5 = T. D. |arxiv = 1403.3695 |bibcode = 2014ApJ...785L...9S | s2cid = 118544889}}</ref>