Neptune

Neptune the 'physical' planet, as opposed to the symbolic planet  is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and less dense. The planet is named after the Roman god of the sea.

Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Neptune is similar in composition to Uranus, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Neptune's atmosphere, while similar to Jupiter and Saturn in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons and possibly nitrogen. In contrast the interior of Neptune is mainly composed of ices and rocks like that of Uranus. Traces of methane in the outermost regions, in part, account for the planet's blue appearance.

Neptune has the strongest winds of any planet in the solar system, measured as high as 2100 km/h. At the time of the 1989 Voyager 2 flyby, its southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's temperature at its cloud tops is usually close to -218 °C (55.1 K), one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune's centre is about 7,000 °C (7,270 K), which is comparable to the Sun's surface and similar to most other known planets. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2.

Neptune  resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical centre. Before Voyager 2's arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. However, in comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors.

Neptune's bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 34.9 times the radius of the planet. The magnetopause, where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much further.

Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. In addition to the narrow Adams Ring, 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.

These rings have a clumpy structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.
The outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs was difficult to explain because the laws of motion would predict that arcs would spread out into a uniform ring over very short timescales. Astronomers now believe that the arcs are corralled into their current form by the gravitational effects of Galatea, a moon just inward from the ring.

Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. Images taken from the W. M. Keck Observatory in 2002 and 2003 show considerable decay in the rings when compared to images by Voyager 2. In particular, it seems that the Liberté arc might disappear in as little as one century

Neptune has 13 known moons. The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune and the only one massive enough to be spheroidal, is Triton, 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, and probably was once a dwarf planet in the Kuiper belt.  It is close enough to Neptune to be locked into a synchronous rotation, and is slowly spiraling inward because of tidal acceleration and eventually will be torn apart when it reaches the Roche limit.

In 1989, Triton was the coldest object that had yet been measured in the solar system, with estimated temperatures of -235 °C (38 K).

Neptune's second known satellite (by order of discovery), the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system. The eccentricity of 0.7512 gives it an apoapsis that is seven times its periapsis distance from Neptune.

From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton.

Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had occulted a star. This had been attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.