Capture of Triton

The crescents of Neptune and Triton, as imaged by the Voyager 2 spacecraft.

Triton, the largest moon of Neptune, is hypothesized to have been captured from heliocentric orbit early in the Solar System's history. Triton is unusual as it is the only known large moon with a retrograde, highly inclined orbit; that is, Triton orbits in the opposite direction Neptune rotates, and its orbit is not aligned with Neptune's equatorial plane. This conflicts with the conventional theory of moon formation, where large moons tend to form from discs of debris and thus orbit prograde. As a result, astronomers have proposed various hypotheses on how Triton acquired its unusual orbital configuration, with capture hypotheses currently being the tentative consensus.

The capture of Triton would have been a cataclysmic event, severely disrupting any pre-existing moons around Neptune and reducing them to rubble. Triton itself experienced extreme tidal heating, possibly to the point of melting entirely, and may have collided with other moons. Remaining debris then accreted into the small regular moons of Neptune that lie inside Triton's orbit.