Editing Planetary system (section)

===Orbital configurations===
Unlike the Solar System, which has orbits that are nearly circular, many of the known planetary systems display much higher [[orbital eccentricity]].<ref>Dvorak, R.; Pilat-Lohinger, E.; Bois, E.; Schwarz, R.; Funk, B.; Beichman, C.; Danchi, W.; Eiroa, C.; Fridlund, M.; Henning, T.; Herbst, T.; Kaltenegger, L.; Lammer, H.; Léger, A.; Liseau, R.; Lunine, Jonathan I.; Paresce F, Penny, A.; Quirrenbach, A.; Röttgering, H.; Selsis, F.; Schneider, J.; Stam, D.; Tinetti, G.; White, G.; "Dynamical habitability of planetary systems", Institute for Astronomy, University of Vienna, Vienna, Austria, January 2010</ref> An example of such a system is [[16 Cygni]].

====Mutual inclination====
The mutual [[Exoplanetology#Inclination vs. spin–orbit angle|inclination]] between two planets is the angle between their [[orbital plane]]s. Many compact systems with multiple close-in planets interior to the equivalent orbit of [[Venus]] are expected to have very low mutual inclinations, so the system (at least the close-in part) would be even flatter than the Solar System. Captured planets could be captured into any arbitrary angle to the rest of the system. {{As of|2016}} there are only a few systems where mutual inclinations have actually been measured<ref>{{cite journal | arxiv=1606.04485 | doi=10.3847/1538-3881/153/1/45 | title=Kepler-108: A Mutually Inclined Giant Planet System | year=2017 | last1=Mills | first1=Sean M. | last2=Fabrycky | first2=Daniel C. | journal=The Astronomical Journal | volume=153 | issue=1 | page=45 | bibcode=2017AJ....153...45M | s2cid=119295498 | doi-access=free }}</ref> One example is the [[Upsilon Andromedae]] system: the planets c and d have a mutual inclination of about 30 degrees.<ref>{{cite journal | arxiv=1411.1059 | doi=10.1088/0004-637X/798/1/46 | title=The 3-dimensional architecture of the Upsilon Andromedae planetary system | year=2014 | last1=Deitrick | first1=Russell | last2=Barnes | first2=Rory | last3=McArthur | first3=Barbara | last4=Quinn | first4=Thomas R. | last5=Luger | first5=Rodrigo | last6=Antonsen | first6=Adrienne | last7=Fritz Benedict | first7=G. | journal=The Astrophysical Journal | volume=798 | page=46 | s2cid=118409453 }}</ref><ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/science/outofwack.html |title=NASA – Out of Whack Planetary System Offers Clues to a Disturbed Past |publisher=Nasa.gov |date=2010-05-25 |access-date=2012-08-17}}</ref>

====Orbital dynamics====
Planetary systems can be categorized according to their orbital dynamics as resonant, non-resonant-interacting, hierarchical, or some combination of these. In resonant systems 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>
Giant planets are found in mean-motion resonances more often than smaller planets.<ref>{{cite journal | arxiv=1410.4199 | doi=10.1146/annurev-astro-082214-122246 | title=The Occurrence and Architecture of Exoplanetary Systems | year=2015 | last1=Winn | first1=Joshua N. | last2=Fabrycky | first2=Daniel C. | journal=Annual Review of Astronomy and Astrophysics | volume=53 | pages=409–447 | bibcode=2015ARA&A..53..409W | s2cid=6867394 }}</ref>
In interacting systems the planets' orbits are close enough together that they perturb the orbital parameters. The Solar System could be described as weakly interacting. In strongly interacting systems [[Kepler's laws]] do not hold.<ref>{{cite arXiv |eprint=1006.3834 |last1=Fabrycky |first1=Daniel C. |title=Non-Keplerian Dynamics |class=astro-ph.EP |date=2010}}</ref>
In hierarchical systems the planets are arranged so that the system can be gravitationally considered as a nested system of two-bodies, e.g. in a star with a close-in hot Jupiter with another gas giant much further out, the star and hot Jupiter form a pair that appears as a single object to another planet that is far enough out.

Other, as yet unobserved, orbital possibilities include: [[double planet]]s; various [[co-orbital configuration|co-orbital planets]] such as quasi-satellites, trojans and exchange orbits; and interlocking orbits maintained by [[nodal precession|precessing orbital planes]].<ref>{{cite journal | arxiv=0812.2949 | doi=10.1111/j.1365-2966.2009.14552.x | title=Equilibria in the secular, non-co-planar two-planet problem | year=2009 | last1=Migaszewski | first1=Cezary | last2=Goździewski | first2=Krzysztof | journal=Monthly Notices of the Royal Astronomical Society | volume=395 | issue=4 | pages=1777–1794 | doi-access=free | bibcode=2009MNRAS.395.1777M | s2cid=14922361 }}</ref>

====Number of planets, relative parameters and spacings====
[[File:Orbits of some Kepler Planetary Systems.jpg|thumb|350px|The spacings between orbits vary widely amongst the different systems discovered by the [[Kepler space telescope]].]]

*[https://arxiv.org/abs/1212.1859 On The Relative Sizes of Planets Within Kepler Multiple Candidate Systems], [[David Ciardi|David R. Ciardi]] et al. December 9, 2012
*[https://arxiv.org/abs/1410.4192 The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets], Sarah Ballard, John Asher Johnson, October 15, 2014
*[https://arxiv.org/abs/1304.3341 Exoplanet Predictions Based on the Generalised Titius-Bode Relation], Timothy Bovaird, Charles H. Lineweaver, August 1, 2013
*[https://arxiv.org/abs/1404.2552 The Solar System and the Exoplanet Orbital Eccentricity - Multiplicity Relation], Mary Anne Limbach, Edwin L. Turner, April 9, 2014
*[https://arxiv.org/abs/1409.3320 The period ratio distribution of Kepler's candidate multiplanet systems], [[Jason Steffen|Jason H. Steffen]], Jason A. Hwang, September 11, 2014
*[https://arxiv.org/abs/1302.7190 Are Planetary Systems Filled to Capacity? A Study Based on Kepler Results], Julia Fang, Jean-Luc Margot, February 28, 2013

====Planet capture====
[[Rogue planet|Free-floating planet]]s in open clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and <!--abstract seems to have a typo 10^6: rest of paper uses 10^5-->10<sup>5</sup> AU<!--see preceding comment-->. The capture efficiency decreases with increasing cluster size, and for a given cluster size it increases with the host/primary{{Clarify|reason=Please reword to avoid the use of a slash; this is ambiguous.|date=August 2021}} mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Some planet–host metallicity correlation may still exist due to the common origin of the stars from the same cluster. Planets would be unlikely to be captured around [[neutron star]]s because these are likely to be ejected from the cluster by a [[pulsar kick]] when they form. Planets could even be captured around other planets to form free-floating planet binaries. After the cluster has dispersed some of the captured planets with orbits larger than 10<sup>6</sup> AU would be slowly disrupted by the [[galactic tide]] and likely become free-floating again through encounters with other field stars or giant [[molecular cloud]]s.<ref name="WideRecapture">{{cite journal |last1=Perets |first1=Hagai B. |title=On the Origin of Planets at Very Wide Orbits from the Recapture of Free Floating Planets |date=2012-03-13 |last2=Kouwenhoven |first2=M. B. N.|journal=The Astrophysical Journal |volume=750 |issue=1 |page=83 |doi=10.1088/0004-637X/750/1/83 |arxiv=1202.2362 |bibcode=2012ApJ...750...83P }}</ref>