Editing Axial tilt (section)

== Extrasolar planets ==
The stellar obliquity {{math|''ψ''<sub>s</sub>}}, i.e. the axial tilt of a star with respect to the orbital plane of one of its planets, has been determined for only a few systems. By 2012, 49 stars have had sky-projected spin-orbit misalignment {{math|''λ''}} has been observed,<ref name=HRM>
{{cite web
 |last=Heller |first=R.
 |title=Holt-Rossiter-McLaughlin Encyclopaedia
 |url=http://www.aip.de/People/RHeller
 |publisher=René Heller
 |access-date=24 February 2012
}}</ref> which serves as a lower limit to {{math|''ψ''<sub>s</sub>}}. Most of these measurements rely on the [[Rossiter–McLaughlin effect]]. Since the launch of space-based telescopes such as [[Kepler space telescope]], it has been made possible to determine and estimate the obliquity of an extrasolar planet. The rotational flattening of the planet and the entourage of moons and/or rings, which are traceable with high-precision photometry provide access to planetary obliquity, {{math|''ψ''<sub>p</sub>}}. Many extrasolar planets have since had their obliquity determined, such as [[Kepler-186f]] and [[Kepler-413b]].<ref>{{Cite web |last=Grossman |first=David |date=2018-06-29 |title=Study Shows Exoplanet Has a Stable Axis Just Like Earth |url=https://www.popularmechanics.com/space/deep-space/a22002151/study-shows-exoplanet-has-a-stable-axis-just-like-earth/ |access-date=2024-02-26 |website=Popular Mechanics |language=en-US}}</ref><ref>{{Cite web |date=2014-02-04 |title=Kepler Finds a Very Wobbly Planet - NASA |url=https://www.nasa.gov/news-release/kepler-finds-a-very-wobbly-planet/ |access-date=2024-02-26 |language=en-US}}</ref>

Astrophysicists have applied tidal theories to predict the obliquity of [[extrasolar planets]]. It has been shown that the obliquities of exoplanets in the [[habitable zone]] around low-mass stars tend to be eroded in less than 10<sup>9</sup> years,<ref name=Heller_2011>
{{cite journal
 |last1=Heller |first1=R.
 |last2=Leconte |first2=J.
 |last3=Barnes |first3=R.
 |title=Tidal obliquity evolution of potentially habitable planets
 |journal=[[Astronomy and Astrophysics]]
 |date=2011
 |volume=528 |pages=A27
 |bibcode=2011A&A...528A..27H
 |doi=10.1051/0004-6361/201015809
 |arxiv=1101.2156
 |s2cid=118784209
}}</ref><ref name=Heller_2011b>
{{cite journal
 |last1=Heller |first1=R.
 |last2=Leconte |first2=J.
 |last3=Barnes |first3=R.
 |date=2011
 |title=Habitability of Extrasolar Planets and Tidal Spin Evolution
 |journal=Origins of Life and Evolution of Biospheres
 |volume= 41 |issue= 6 |pages=539–43
 |bibcode=2011OLEB...41..539H
 |doi=10.1007/s11084-011-9252-3
 |pmid=22139513
 |arxiv=1108.4347
 |s2cid=10154158
}}</ref> which means that they would not have tilt-induced seasons as Earth has.