NEPTUNE

       Neptune 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. Among the gaseous planets in the Solar System, Neptune is the densest. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. Neptune orbits the Sun at an average distance of 30.1 astronomical units.

       Neptune was the first planet 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. Neptune was subsequently observed on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 moons were located telescopically until the 20th century.

       Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter, and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia, and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" to emphasise these distinctions. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing. Traces of methane in the outermost regions in part account for the planet's blue appearance.

       With a mass of 1.0243×1026 kg, Neptune is an intermediate body between Earth and the larger gas giants: its mass is 17 times that of Earth but just 1/19th that of Jupiter. Its surface gravity is surpassed only by Jupiter. Neptune's equatorial radius of 24,764 km is nearly four times that of Earth. Neptune and Uranus are often considered a subclass of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles relative to Jupiter and Saturn. In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes", just as astronomers refer to various extra-solar bodies as "Jupiters".

       Neptune's internal structure resembles that of Uranus. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere.

       The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The mantle may consist of a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones. Very-high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the base of the mantle may comprise an ocean of liquid diamond, with floating solid 'diamond-bergs'.

       The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of Earth. The pressure at the centre is about twice as high as that at the centre of Earth, and the temperature may be 5,126.85 ºC (9260.33 ºF)

Combined colour and near-infrared image of Neptune, showing bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina.

       At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.

      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. The three main rings are the narrow Adams Ring, 63,000 km from the centre of Neptune, the Le Verrier Ring, at 53,000 km, and the broader, fainter Galle Ring, at 42,000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.