Editing Exoplanet (section)

== Planetary system ==
{{Main|Planetary system}}

Exoplanets are often members of planetary systems of multiple planets around a star. The planets interact with each other gravitationally and sometimes form resonant systems where the orbital periods of the planets are in integer ratios. The [[Kepler-223]] system contains four planets in an 8:6:4:3 [[orbital resonance]].<ref>{{cite web|last=Emspak|first=Jesse|title=Kepler Finds Bizarre Systems|url=http://www.ibtimes.com/articles/117984/20110302/kepler-finds-strange-worlds-fastest-planet.htm|work=International Business Times|date=March 2, 2011|publisher=International Business Times Inc.|access-date=March 2, 2011}}</ref>

Some [[hot Jupiter]]s orbit their stars in the [[Retrograde and prograde motion#Exoplanets|opposite direction]] to their stars' rotation.<ref name="2010question">{{cite web|url=http://www.astro.gla.ac.uk/nam2010/pr10.php|title=NAM2010 at the University of Glasgow|access-date=2010-04-15|archive-date=2011-07-16|archive-url=https://web.archive.org/web/20110716051715/http://www.astro.gla.ac.uk/nam2010/pr10.php|url-status=dead}}</ref> One proposed explanation is that hot Jupiters tend to form in dense clusters, where [[Perturbation (astronomy)|perturbations]] are more common and [[gravitational capture]] of planets by neighboring stars is possible.<ref>{{cite web |author=Sutter |first=Paul M. |date=December 9, 2022 |title=Trading spaces: How swapping stars create hot Jupiters |url=https://phys.org/news/2022-12-spaces-swapping-stars-hot-jupiters.html |agency=Universe Today}}</ref>