Short answers, pulled from the story.
Triton was discovered by English astronomer William Lassell on the 10th of October 1846, just 17 days after Neptune itself was found. Lassell spotted the moon using a 61-centimeter metal mirror reflecting telescope he had built himself. He received a suggestion from John Herschel to search for moons after Neptune's discovery, and found Triton eight days later.
Triton orbits Neptune in a retrograde direction, meaning it travels opposite to Neptune's own rotation, because it was captured from elsewhere in the Solar System rather than forming in place. A moon cannot form in a retrograde orbit around its planet. The leading hypothesis is that Triton originated in the Kuiper belt and was captured by Neptune, possibly through a binary-exchange event in which Triton's original companion body was flung away while Triton became bound to Neptune.
Triton is thought to have originated in the Kuiper belt, the same region where Pluto resides, before being captured by Neptune's gravity. A 2024 study of both bodies found nearly identical chemical compositions, including large amounts of nitrogen and trace methane and carbon monoxide, consistent with formation beyond the water-ice line in the outer Solar System. Triton is only slightly larger than Pluto and is the largest known object believed to have originated in the Kuiper belt.
The Voyager 2 spacecraft imaged approximately 40 percent of Triton's surface during its 1989 flyby from a distance of about 40,000 kilometers. It revealed an extremely young, geologically active surface with regions estimated at just 6 million years old, active nitrogen plumes erupting up to 8 kilometers high, cantaloupe terrain unique to Triton, and a nitrogen atmosphere denser than expected. The flyby also confirmed Triton's diameter at approximately 2,706 kilometers.
Triton's plumes are geyser-like eruptions of nitrogen gas and dust that reach up to 8 kilometers high, observed by Voyager 2 in 1989. The best-documented examples are the Hili and Mahilani plumes. Scientists debate whether solar heating beneath a translucent nitrogen ice layer drives them via a solid greenhouse effect, or whether internal cryovolcanic heat is responsible; the cryovolcanic hypothesis is supported by an estimated output that possibly exceeds 400 kilograms per second.
Tidal forces are causing Triton's orbit to decay slowly inward. Studies conducted in 2025 estimated Triton will not reach Neptune's Roche limit, the distance at which tidal forces would tear it apart, for another 28 billion years, a significant revision upward from earlier estimates of 3.6 billion years. When that point is reached, Triton could either collide with Neptune's atmosphere or break up and form a new ring system.