Neptune

<p>Neptune is the eighth and farthest known planet from the Sun in the Solar System. </p>In the Solar System, it is the fourth-largest planet by the third-most-massive planet diameter, and the densest giant planet. Neptune is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. Neptune is physically smaller than Uranus and thicker because its mass causes gravitational compression of its atmosphere. Neptune orbits the Sun once every 164.8 years at a mean 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 forecast as opposed to by empirical observation. Changes in the orbit of Uranus directed Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation. The position of Neptune was calculated from Bouvard's observations by John Couch Adams and Urbain Le Verrier following his death. Neptune was observed with a telescope on 23 by Johann Galle within a degree of the position predicted by Le Verrier. Though none of the remaining 13 known moons of the planet were located before the 20th century its largest moon, Triton, was discovered soon thereafter. The distance from Earth of the planet gives it a very small apparent size, which makes it challenging to research with Earth-based telescopes. Voyager 2 visited neptune, 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 allowed for further observations from afar. Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, together with traces of hydrocarbons and possibly nitrogen, even though it includes 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 areas in part account for the world's blue appearance.In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has visible and active weather patterns. For example, in the time of the Voyager 2 flyby in 1989, the world's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot. These weather patterns are driven by the strongest sustained winds of almost any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h (580 m/s; 1,300 mph). Due to its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures in its cloud tops approaching 55 K (−218 °C; −361 °F). Temperatures in the world's centre are approximately 5,400 K (5,100 °C; 9,300 °F). Neptune has a faint and fragmented ring system (labelled"arcs"), which was discovered in 1984, then afterwards confirmed by Voyager 2.

Source: Wikipedia