Triton (moon) - Capture

Capture

Because moons in retrograde orbits cannot have formed out of the same region of the solar nebula as the planets they orbit, it must have been captured from elsewhere. It is suspected that Triton was captured from the Kuiper belt, a ring of small icy objects extending outward from just inside the orbit of Neptune to about 50 AU from the Sun. Thought to be the point of origin for the majority of short-period comets observed from Earth, it 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 plutinos) locked in orbital step with Neptune. Triton is only slightly larger than Pluto and nearly identical in composition, which has led to the hypothesis that the two share a common origin.

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely eccentric orbit of Neptune's moon Nereid and the scarcity of moons as compared to the other gas giants. Triton's initially eccentric orbit would have intersected orbits of irregular moons and disrupted those of smaller regular moons, dispersing them through gravitational interactions.

Triton's eccentric post-capture orbit would have also resulted in tidal heating of the moon's interior. This would have kept Triton liquid for a billion years, which is supported by evidence of differentiation in the moon's interior. This source of internal heat disappeared following circularization of the orbit.

Two types of mechanisms have been proposed for Triton's capture. In order 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. An early theory 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). A more recent and now favored hypothesis suggests that, before its capture, Triton had a massive companion similar to Pluto's moon Charon with which it formed a binary. When the binary encountered Neptune, it interacted in such a way that orbital energy was transferred from Triton to its companion; the latter was expelled, while Triton became bound to Neptune. This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects. 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 migrated outward.