Neptune

Neptune Neptune (pronounced [AmE: /wiki/Media:En-us-Neptune.ogg /wiki/File:Neptune_symbol.svg, a stylized version of the god Neptune's

trident. Discovered on September 23, 1846,[1] Neptune was the only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led astronomers to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently found within a degree of its predicted position, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 12 moons were located telescopically until 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 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 categorize Uranus and Neptune as "ice giants" in order to emphasize these distinctions.[11] The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.[12] Traces of methane in the outermost regions in part account for the planet's blue appearance.[13] In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphere is notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet's southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph).[14] Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet's centre, however, are approximately 7,000 K (7,000 °C), comparable to those at the Sun's surface and similar to temperatures at the centres of most of the other planets of the Solar System. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed in 1982 by Voyager 2.[15