Neptune

The farthest planet from the Sun, Neptune was found by mathematics before anyone saw it. Supersonic winds, a captured moon and clues that the giant planets moved after they formed.

Diameter: 49,528 km (3.9 × Earth)
Mass: 17 × Earth
Gravity at cloud tops: about 1.14 × Earth
Distance from Sun: 30.1 AU (about 4.5 billion km)
Year: about 165 Earth years
Day: about 16 hours 6 minutes
Moons: 16 known
Cloud-top temperature: about −200 °C

The build: an ice giant on the frontier

Neptune is a near-twin of Uranus: slightly smaller but more massive, with the same ice-giant recipe of water, ammonia and methane around a rocky core under a hydrogen–helium atmosphere.

At Neptune's present distance, building material would have been extremely sparse, and it is hard to grow a planet this big there within the lifetime of a protoplanetary disk. Many models therefore propose that Uranus and Neptune formed closer to the Sun and moved outward later through gravitational interactions with the other giants and a disk of icy bodies. One influential version is known as the Nice model. These migration scenarios are widely discussed but still debated.

Found by calculation

In the early 1800s, astronomers noticed that Uranus was drifting from its predicted path. Urbain Le Verrier calculated where an unseen planet would have to be to cause the drift. In 1846, Johann Galle pointed a telescope at that spot and found Neptune within about a degree of the prediction — a famous triumph of physics.

The windiest planet

Despite receiving only about 1/900 of the sunlight Earth gets, Neptune has the fastest winds measured on any planet, reaching around 2,000 km/h. Neptune gives off more than twice as much energy as it receives from the Sun, so internal heat probably helps power its weather. Voyager 2 photographed a huge dark storm in 1989; later Hubble images showed it had vanished and new dark spots had appeared elsewhere.

Triton: a captured world

Neptune's largest moon, Triton, orbits backwards — opposite to Neptune's rotation. Large moons that form alongside their planet do not do that, so Triton was very likely a dwarf planet from the Kuiper Belt that Neptune captured. Voyager 2 saw dark plumes erupting from its frozen nitrogen surface. Triton's orbit is slowly shrinking; in the distant future it is expected to be torn apart and may form a ring.

Lessons for planet builders

Planets can form in one place and settle somewhere else.
Internal heat can drive powerful weather even far from a star.
Moons do not have to be built locally — they can be captured.