Neptune
Neptune takes 164.8 years to complete a single orbit around the Sun, circling at an orbital distance of 30.1 astronomical units (about 4.5 billion kilometres; 2.8 billion miles). It is the only planet in the Solar System that was not initially observed by direct empirical observation.
What it is
Neptune is the eighth and farthest known planet orbiting the Sun. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet.
Neptune is 17 times the mass of Earth. Compared to Uranus, its neighbouring ice giant, Neptune is slightly smaller, but more massive and dense. Being composed primarily of gases and liquids, it has no well-defined solid surface. Its name comes from the Roman god of the sea, and its astronomical symbol represents Neptune’s trident.
How we know
Neptune entered human knowledge first as a discrepancy: unexpected changes in the orbit of Uranus. Alexis Bouvard hypothesised that Uranus’s path was being altered by the gravitational perturbation of an unknown planet—gravity, acting at a distance, leaving its signature not in light but in motion.
After Bouvard’s death, Neptune’s position was mathematically predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Only then was Neptune directly observed with a telescope on 23…—a planet first found as a calculation, and then confirmed as a point of light.
- Unseen
- Neptune is not visible to the unaided eye; it requires a telescope to be directly observed.