Neptune

Neptune is the eighth and outermost planet — the first found by mathematical prediction rather than observation, in 1846. Like Uranus, it is an ice giant of water, methane, and ammonia surrounding a small rocky core. Its deep azure colour is partly methane absorption and partly an unknown additional absorber. Despite receiving less than 0.1% of the sunlight Earth does, Neptune is dynamically restless: Voyager 2 measured wind speeds over 2,100 km/h, the fastest ever recorded on any planet. It has not yet completed a full orbit since its discovery.

A rocky core of roughly Earth's mass, wrapped in a hot, dense mantle of water, ammonia, and methane ices, then a hydrogen-helium-methane atmosphere. Neptune radiates 2.6 times the energy it receives from the Sun — a stronger internal heat output than Uranus, and the source of its violent weather. The interior pressures may produce diamond rain, as suspected for Uranus.

Bands of rapidly evolving cloud features change on weekly timescales. Voyager 2 imaged a Great Dark Spot in 1989 that had vanished by 1994; Hubble has since seen new dark spots come and go. White cirrus clouds of methane ice cast shadows on lower decks. A persistent storm seen at the south pole, called the South Polar Vortex, hosts the planet's warmest stratospheric temperatures.

Like Uranus, Neptune has been visited only once. Voyager 2 flew past on 25 August 1989, the final encounter of its grand tour, revealing six new moons, the rings, the Great Dark Spot, and the fact that Triton — Neptune's largest moon — has active nitrogen geysers. No follow-up mission has been approved, though concept studies for a Neptune-Triton orbiter ("Trident", "Neptune Odyssey") have been proposed.