Neptune

**Neptune** is the eighth and farthest-known planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. 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 named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident.

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. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. After Bouvard's death, the position of Neptune was predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Le Verrier. 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. The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by *Voyager 2*, when it flew by the planet on 25 August 1989; *Voyager 2* remains the only spacecraft to visit Neptune. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.

Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with oxygen and possibly nitrogen, though it contains a higher proportion of "ices" such as water, ammonia and methane. However, similar to Uranus, its interior is primarily composed of ices and rock; Uranus and Neptune are normally considered "ice giants" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance, though an unknown component is believed to color Neptune a deeper blue compared to Uranus.

## Storms

However, on 2 November 1994 the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere. The reason for the Great Dark Spot's disappearance is unknown. One possible theory is that heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns[*citation needed*]. The **Scooter** is another storm, a white cloud group further south than the Great Dark Spot. Its nickname was bestowed when it was first detected in the months leading up to the Voyager encounter in 1989: it moved faster than the Great Dark Spot. Subsequent images showed clouds that moved even faster than Scooter. The Wizard's eye/*Dark Spot 2* is a southern cyclonic storm, the second most intensive storm during the 1989 encounter.

The closest approach of *Voyager 2* to Neptune occurred on August 25, 1989. Since this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for *Voyager 1'*s encounter with Saturn and its moon Titan. The probe also discovered the Great Dark Spot, which has since disappeared, according to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it was later postulated to be a hole in the visible cloud deck. Neptune turned out to have the strongest winds of all the solar system's gas giants. In the outer regions of the solar system, where the Sun shines over 1000 times fainter than on Earth (still very bright with a magnitude of -21), the last of the four giants defied all expectations of the scientists.

## Moons

Neptune has 14 known moons. Triton is the largest Neptunian moon, comprising more than 99.5% of the mass in orbit around Neptune, and it is the only one massive enough to be spheroidal. Triton was discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons in the Solar System, Triton has a retrograde orbit, indicating that it was captured rather than forming in place; it was probably once a dwarf planet in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous rotation, and it is slowly spiralling inward because of tidal acceleration. It will eventually be torn apart, in about 3.6 billion years, when it reaches the Roche limit.

* Triton
* Proteus
* Nereid
* Naiad
* Thalassa