Neptune is the aicht an farthest planet frae the Sun in the Solar Seestem. It is the fowert-lairgest planet bi diameter an the third-lairgest bi mass. Amang the gaseous planets in the solar seestem, Neptune is the maist dense. Neptune is 17 times the mass o Yird an is slichtly mair massive than its near-twin Uranus, which is 15 times the mass o Earth but nae as dense.[lower-alpha 1] On average, Neptune orbits the Sun at a distance o 30.1 AU, approximately 30 times the Yird–Sun distance. Named for the Roman god o the sea, its astronomical seembol is ♆, a stylised version o the god Neptune's trident.
Neptune wis the first planet foond bi mathematical prediction rather than bi empirical observation. Unexpectit chynges in the orbit o Uranus led Alexis Bouvard tae deduce that its orbit wis subject tae gravitationalperturbation bi an unkent planet. Neptune wis subsequently observed on 23 September 1846 bi Johann Galle within a degree o the poseetion predictit bi Urbain Le Verrier, an its lairgest muin, Triton, wis discovered shortly thareefter, tho none o the planet's remainin 13 muins wur locatit telescopically till the 20t century. Neptune haes been visited bi anly ane spacecraft, Voyager 2, which flew bi the planet on 25 August 1989.
Neptune is similar in composeetion tae Uranus, an baith hae composeetions which differ frae those o the lairger gas giants, Jupiter, an Saturn. Neptune's atmosphere, while similar tae Jupiter's an Saturn's in that it is componed primarily o hydrogen an helium, alang wi traces o hydrocarbons an possibly nitrogen, contains a heicher proportion o "ices" such as watter, ammonia, an methane. Astronomers sometimes categorise Uranus an Neptune as "ice giants" in order tae emphasise these distinctions. The interior o Neptune, lik that o Uranus, is primarily componed o ices an rock. It is possible that the cor has a solit surface, but the temperatur would be thoosands o degrees an the atmospheric pressur crushin. Traces o methane in the ootermaist regions in pairt accoont for the planet's blue appearance.
In contrast tae the hazy, relatively featureless atmosphere o Uranus, Neptune's atmosphere is notable for its active an veesible wather patterns. For example, at the time o the 1989 Voyager 2flyby, the planet's soothren hemisphere possessed a Great Dark Spot comparable tae the Great Red Spot on Jupiter. These wather patterns are driven bi the strangest sustained winds o ony planet in the Solar Seestem, wi recordit wind speeds as heich as 2,100 kilometres per hour (1,300 mph). Acause o its great distance frae the Sun, Neptune's ooter atmosphere is ane o the cauldest places in the Solar Seestem, wi temperaturs at its clood taps approachin 55 K (−218 °C). Temperaturs at the planet's centre are approximately 5,400 K (5,000 °C). Neptune haes a faunt an fragmented ring seestem (labeled 'arcs'), which mey hae been detectit durin the 1960s but wis anly indisputably confirmed in 1989 bi Voyager 2.