Editing Triton (moon) (section)

== Capture ==
{{main|Capture of Triton}}
[[File:Outersolarsystem objectpositions labels comp.png|thumb|left|The [[Kuiper belt]] (green), in the Solar System's outskirts, is where Triton is thought to have originated.]]

The current understanding of moons in retrograde orbits means they cannot form in the same region of the [[solar nebula]] as the planets they orbit.  Therefore, Triton must have been captured from elsewhere in the Solar System. Astrophysicists believe it might have originated in the [[Kuiper belt]],<ref name="Agnor06"/> a ring of small icy objects extending from just inside the orbit of Neptune to about 50&nbsp;[[astronomical unit|AU]] from the Sun. Thought to be the point of origin for the majority of short-period [[comet]]s observed from Earth, the belt is also home to several large, planet-like bodies including [[Pluto]], which is now recognized as the largest in a population of Kuiper belt objects (the [[plutino]]s) [[orbital resonance#Pluto resonances|locked in resonant orbits]] with Neptune. Triton is only slightly larger than Pluto and is nearly identical in composition, which has led to the hypothesis that the two share a common origin.<ref name="Cruikshank2004"/>

This has been further supported in a 2024 study of the chemical composition of Pluto and Triton which suggests they originated in the same region of the outer Solar System before the latter was pulled into Neptune's orbit.<ref>{{Cite news |date=June 18, 2024 |title=Pluto and the largest moon of Neptune might be siblings |url=https://www.newscientist.com/article/2436032-pluto-and-the-largest-moon-of-neptune-might-be-siblings/ |access-date=June 27, 2024 |work=New Scientist UK edition |pages=Space section}}</ref>

Studying prior data on the two bodies, the team found that both have a large amount of nitrogen and trace amounts of methane and carbon monoxide, which could have accumulated in the outer regions of the young nebula "For some reason, Triton was then ejected from this region and ensnared by Neptune". "They had to have formed beyond the water-ice line," says Mandt, referring to the distance from the sun where water would freeze into ice or snow, which is why Triton and Pluto have similar amounts of certain key elements. "One possibility is that the giant planets moved closer to the sun early in the first 100 million years or so of the Solar System, which may have disrupted the orbits of some bodies like Triton", says Mandt.<ref name=":2">{{Cite news |date=June 18, 2024 |title=Pluto and the largest moon of Neptune might be siblings |url=https://www.newscientist.com/article/2436032-pluto-and-the-largest-moon-of-neptune-might-be-siblings/ |access-date=June 27, 2024 |work=New Scientist UK edition |pages=Space section}}</ref>

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely [[eccentric orbit]] of Neptune's moon [[Nereid (moon)|Nereid]] and the scarcity of moons as compared to the other [[giant planet]]s. Triton's initially eccentric orbit would have intersected the orbits of irregular moons and [[Perturbation (astronomy)|disrupted]] those of smaller regular moons, dispersing them through [[gravitation]]al interactions.<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/>

Triton's eccentric post-capture orbit would have also resulted in [[tidal heating]] of its interior, which could have kept Triton fluid for a billion years; this inference is supported by evidence of differentiation in Triton's interior. This source of internal heat disappeared following tidal locking and circularization of the orbit.<ref name="Ross1990"/>

Two types of mechanisms have been proposed for Triton's capture. To be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape.<ref name="EncycSolSys-Triton"/> An early model of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely).<ref name="EncycSolSys-Triton"/> A more recent hypothesis suggests that, before its capture, Triton was part of a binary system. When this binary encountered Neptune, it interacted in such a way that the binary dissociated, with one portion of the binary expelled, and the other, Triton, becoming bound to Neptune. This event is more likely for more massive companions.<ref name="Agnor06"/> This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects.<ref name="IOPorg-KuiperObjectBinaries"/><ref name="Jewitt2005"/> The event was brief but gentle, saving Triton from collisional disruption. Events like this may have been common during the formation of Neptune, or later when it [[Planetary migration|migrated outward]].<ref name="Agnor06"/>

However, simulations in 2017 showed that after Triton's capture, and before its orbital eccentricity decreased, it probably did collide with at least one other moon, and caused collisions between other moons.<ref>{{cite journal|last1=Raluca Rufu and [[Robin Canup]]|title=Triton's evolution with a primordial Neptunian satellite system|journal=The Astronomical Journal|volume=154|issue=5|page=208|arxiv=1711.01581|date=November 5, 2017|doi=10.3847/1538-3881/aa9184|pmid=31019331|pmc=6476549|bibcode=2017AJ....154..208R |doi-access=free }}</ref><ref>{{cite journal|title=Triton crashed into Neptune's moons|journal=New Scientist|date=November 18, 2017|volume=236|issue=3152|page=16|doi=10.1016/S0262-4079(17)32247-9|bibcode=2017NewSc.236...16.|url=https://www.newscientist.com/article/mg23631521-900-neptunes-other-moons-were-normal-until-triton-crashed-the-party}}</ref>