1. Characteristics ↑
Neptune is the furthest planet from the Sun, orbiting at about thirty AU (one AU is the distance between the Earth and the Sun). German astronomer Johann Galle discovered Neptune in 1846, following calculations made by French mathematician Urbain Le Verrier. British astronomer William Lassell discovered its largest moon, Triton, two and a half weeks later.
Neptune, image taken by Voyager 2. Image credit: NASA/JPL/Public domain.
Neptune is the third most massive planet in the Solar System, after Jupiter and Saturn. It is the fourth largest planet, having a slightly smaller diameter than Uranus. It takes just under 60,000 days for Neptune to orbit the Sun, this is over 160 years, and one day on Neptune is just over 16 hours long. Like Mercury, Venus, Mars, Jupiter, and Saturn, but unlike Uranus, Neptune is named after a Roman god, the god of the sea.
2. Moons ↑
2.1 Triton ↑
Neptune has a faint ring system and 13 known moons. The largest of these is Triton, which contains over 90% of the mass of all the Neptunian moons, and is the only Neptunian moon that is spherical. Triton is the only moon in the Solar System to have a retrograde orbit. This mean it orbits in the opposite direction to Neptune's rotation[9a]. It's also the only known moon in the Solar System to have a surface made mainly of frozen nitrogen.
Triton is composed of a core of rock and metal, with an icy mantle, and active volcanoes[9b]. It may also contain water. Triton has a thin nitrogen atmosphere. This is the coldest atmosphere in the Solar System, at about -235 °C[9c].
Triton, image taken by Voyager 2. Image credit: NASA/JPL/USGS/Public domain.
Clouds of Neptune, image taken by Voyager 2. Image credit: NASA/JPL/Public domain.