Editing Exoplanet (section)

=== Tidal heating ===
If a planet has an eccentric orbit, then [[tidal heating]] can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also [[tidal circularization|circularize]] orbits over time, so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits.<ref>{{Cite journal | doi = 10.1089/ast.2012.0851| pmid = 23537135| title = Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating| journal = Astrobiology| volume = 13| issue = 3| pages = 225–250| year = 2013| last1 = Barnes | first1 = R. | last2 = Mullins | first2 = K. | last3 = Goldblatt | first3 = C. | last4 = Meadows | first4 = V. S. | last5 = Kasting | first5 = J. F. | last6 = Heller | first6 = R. | bibcode=2013AsBio..13..225B|arxiv = 1203.5104 | pmc=3612283}}</ref> Eccentric planets further out than the habitable zone would still have frozen surfaces, but the tidal heating could create a subsurface ocean similar to [[Europa (moon)|Europa]]'s.<ref name="Superhabitable">{{Cite journal | doi = 10.1089/ast.2013.1088| pmid = 24380533| title = Superhabitable Worlds| journal = Astrobiology| volume = 14| issue = 1| pages = 50–66| year = 2014| last1 = Heller | first1 = R. | last2 = Armstrong | first2 = J. | bibcode=2014AsBio..14...50H|arxiv = 1401.2392 | s2cid = 1824897}}</ref> In some planetary systems, such as in the [[Upsilon Andromedae]] system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.<ref>{{Cite journal | doi = 10.1111/j.1365-2966.2008.13868.x| title = Tidal heating of terrestrial extrasolar planets and implications for their habitability| journal = Monthly Notices of the Royal Astronomical Society| volume = 391| issue = 1| pages = 237–245| year = 2008| last1 = Jackson | first1 = B. | last2 = Barnes | first2 = R. | last3 = Greenberg | first3 = R. | doi-access = free| bibcode = 2008MNRAS.391..237J|arxiv = 0808.2770 | s2cid = 19930771}}</ref>