Neptune has 13 moons, half that of Uranus and a quarter that of Jupiter or Saturn due to its large mass. Triton is the largest and was discovered first, with Nereid discovered over 100 years later. Triton is the only spherical moon and is thought to be a captured Kuiper belt object. Proteus is the second largest and has a unique cube shape.
Neptune, the eighth and last planet from the Sun since Pluto’s regression in 2006, has 13 known moons. This is about half that of its sister planet Uranus and a quarter that of Jupiter or Saturn. Because of its large mass, about 17 times that of Earth, Neptune has relatively few moons. In approximate descending order of size, the moons are Triton, Proteus, Nereid, Larissa, Galatea, Despina, Thalassa, Naiad, Halimede, Neso, Sao, Laomedeia, and Psamathe.
Triton was the first to be discovered, by William Lassell, who also discovered a moon of Saturn and two of Uranus, in 1854, only seventeen days after the discovery of Neptune itself. Only more than a hundred years later, in 1949, another moon, Nereid, was discovered. Nereid and Triton are named after minor sea deities, a naming tradition that persists with the other moons. This naming scheme is inspired by the fact that Neptune, the Roman god of the sea. Until 1989, only Triton and Nereid were known, but six were discovered that same year by the Voyager 2 space probe, while the remaining five were discovered with ground-based telescopes in 2002 or 2003.
Neptune has only one moon large enough to be spherical: Triton. With a diameter of 2700 km (1677 mi), Triton is the seventh largest moon in the Solar System, at 78% the size of the Moon. It has similar characteristics to Pluto, with 25% water ice and the rest rock material. Triton, the largest moon in the Solar System with a retrograde (“backward”) orbit, is thought to be a captured Kuiper belt object. The Kuiper Belt is the second largest asteroid belt in the Solar System, located beyond the orbit of Neptune. With an average surface temperature of 34 K, Triton is also known as the coldest surface in the Solar System. Unlike many moons in the Solar System, Triton has very few craters and is geologically active, with a smooth, milky surface.
Proteus, the second largest moon of Neptune with a diameter of 440 km, has the shape of a cube with rounded edges. Scientists think it is as massive as any body before being pulled into a sphere by its own gravity. Mimas, a moon of Saturn, is even less massive but still spherical. Proteus is also notable for being one of the darkest surfaces in the Solar System, reflecting only 6% of incoming light.
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