Neptune is the most distant planet - so far out that the Sun looks like a very bright star and one orbit takes 165 Earth years. Despite receiving 900× less sunlight than Earth, it hosts the fastest winds in the Solar System.
Open the 3D Solar SystemUranus kept drifting off its predicted path, so astronomers computed where an unseen planet’s gravity must be pulling from. In 1846 Neptune was found within 1° of the predicted spot - gravity’s laws, proven by prediction.
| Diameter | 49,528 km |
| Mass | 1.020e+26 kg |
| Surface gravity | 11 m/s² |
| Density | 1.64 g/cm³ |
| Rotation period | 16.11 h |
| Axial tilt | 28.3° |
| Mean temperature | -200 °C |
| Geometric albedo | 0.44 |
| Known moons | 16 |
| Atmosphere | About 80% H₂, 19% He and 1.5% methane, whipped into the fastest winds in the Solar System. |
| Composition | A dense fluid of water, ammonia and methane ices around a rocky core, beneath a hydrogen-helium envelope. |
How we know: Like Uranus: one Voyager 2 flyby plus density and magnetic data. The interior has never been probed.
| Discovered by | Johann Galle, from the predictions of Urbain Le Verrier (and independently John Couch Adams) |
| Year | 1846 |
| How | Found by mathematics - computed from Uranus’ orbital wobbles, then spotted within 1° of the predicted position |