Neptune's largest moon orbits backwards and was probably stolen from the Kuiper belt
Triton circles Neptune the wrong way, against the planet's spin, which no other large moon in the Solar System does. Moons on backward orbits cannot form alongside their planets, so Triton was almost certainly captured, most likely a dwarf planet from the Kuiper belt, a close cousin of Pluto.
William Lassell, a brewer who built his own telescopes, found it on 10 October 1846, just 17 days after Neptune itself was discovered, having been nudged by John Herschel to hunt for moons. He used a self-built reflector with a 61-centimetre metal mirror. The name, after the sea god son of Poseidon, was suggested by Camille Flammarion in 1880 but adopted only decades later; until Nereid turned up in 1949, Triton was simply the satellite of Neptune.
At 2,710 kilometres across it is the seventh-largest moon in the Solar System and bigger than any known dwarf planet. Roughly 30 to 45 percent of its mass is water ice, the rest mainly rock and metal, and it probably hides an ocean beneath an icy crust. Its surface of frozen nitrogen is young, likely under 100 million years old on average, with few craters and signs of icy volcanism. A thin nitrogen atmosphere carries ice-crystal clouds and organic haze, and its surface pressure has varied up to threefold in 30 years.
The capture left marks on the whole system. Triton's early elongated orbit would have scattered Neptune's original moons, which may explain the planet's shortage of moons and Nereid's wildly eccentric path. Tides from that orbit could have kept Triton's interior fluid for a billion years before the orbit settled into a near-perfect circle. How it was caught remains debated: a collision with a moon, or the breakup of a binary pair that flung its partner away.
Its future is dramatic. Tides are slowly drawing it inward, and one prediction has it crossing Neptune's Roche limit in 3.6 billion years, either plunging into the atmosphere or shattering into rings. Voyager 2's 1989 flyby, which saw only about 40 percent of the surface, is still the only close visit.
Source: Triton (moon)
