Editing Kuiper belt (section)

=== Triton ===
{{Main|Triton (moon)}}
[[File:Triton moon mosaic Voyager 2 (large).jpg|thumb|upright|[[Neptune]]'s moon [[Triton (moon)|Triton]]]]

During its period of migration, Neptune is thought to have [[Capture of Triton|captured]] a large KBO, [[Triton (moon)|Triton]], which is the only large moon in the Solar System with a [[retrograde orbit]] (that is, it orbits opposite to Neptune's rotation). This suggests that, unlike the large [[moons of Jupiter]], [[Moons of Saturn|Saturn]] and [[Moons of Uranus|Uranus]], which are thought to have coalesced from rotating discs of material around their young parent planets, Triton was a fully formed body that was captured from surrounding space. Gravitational capture of an object is not easy: it requires some mechanism to slow down the object enough to be caught by the larger object's gravity. A possible explanation is that Triton was part of a binary when it encountered Neptune. (Many KBOs are members of binaries. See [[#Satellites|below]].) Ejection of the other member of the binary by Neptune could then explain Triton's capture.<ref>{{cite journal |title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |author=Craig B. Agnor |author2=Douglas P. Hamilton |name-list-style=amp |journal=Nature |volume=441 |issue=7090 |pages=192–194 |url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |date=2006 |access-date=29 October 2007 |archive-url=https://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archive-date=21 June 2007 |url-status=dead|bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |pmid=16688170 |s2cid=4420518 }}</ref> Triton is only 14% larger than Pluto, and spectral analysis of both worlds shows that their surfaces are largely composed of similar materials, such as [[methane]] and [[carbon monoxide]]. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its [[Nice model|outward migration]].<ref>{{cite book |last1=Encrenaz |first1=Thérèse |author1-link=Thérèse Encrenaz|last2=Kallenbach |first2=R. |last3=Owen |first3=T. |last4=Sotin |first4=C. |title=Triton, Pluto, Centaurs, and Trans-Neptunian Bodies |url=https://books.google.com/books?id=MbmiTd3x1UcC&pg=PA421 |access-date=23 June 2007 |date=2004 |publisher=Springer |isbn=978-1-4020-3362-9}}</ref>