The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989 Voyager 2 remains the only spacecraft to have visited Neptune.
The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 known moons were located telescopically until the 20th century. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Le Verrier. After Bouvard's death, the position of Neptune was predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Unexpected changes in the orbit of Uranus led Alexis Bouvard to hypothesise that its orbit was subject to gravitational perturbation by an unknown planet. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. It is named after the Roman god of the sea and has the astronomical symbol, representing Neptune's trident. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km 2.8 billion mi). It is referred to as one of the solar system's two ice giant planets (the other one being Uranus).īeing composed primarily of gases and liquids, it has no well-defined "solid surface". Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. It is 17 times the mass of Earth, and slightly more massive than its near-twin Uranus. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is the eighth planet from the Sun and the farthest known solar planet.
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