Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune was first observed by Galle and d' Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. Neptune is smaller in diameter but larger in mass than Uranus. Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but here is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint. Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

Orbit

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere at −218 °C (55 K). After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years. Despina is the third of Neptune's known satellites: orbit: 52,600 km from Neptune diameter: 148 km mass:. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion.

Moons

Neptune also has several tiny distant and unnamed moons which were discovered recently. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings.

Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers. Neptune's atmosphere is primarily composed of hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune was discovered that very night, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Neptune's blue color is much more vivid than that of Uranus, which has a similar amount of methane. Neptune's second known satellite (by order of distance), the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system. Neptune will return to the same location on its orbit as at its discovery in August 2011. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. Neptune's rings are believed to be relatively young and relatively short-lived.