Neptune is the eighth planetfrom the Sun and the farthest known solar planet. In the SolarSystem, it is the fourth-largest planet by diameter, thethird-most-massive planet, and the densest giant planet. It is 17times the mass of Earth, and slightly more massive than its near-twinUranus. Neptune is denser and physically smaller than Uranus becauseits greater mass causes more gravitational compression of itsatmosphere. It is referred to as one of the solar system's two icegiant planets (the other one being Uranus).
Being composed primarily of gases andliquids, it has no well-defined "solid surface". Theplanet orbits the Sun once every 164.8 years at an average distanceof 30.1 AU (4.5 billion km; 2.8 billion mi). It is named after theRoman god of the sea and has the astronomical symbol ♆,representing Neptune's trident.
Neptune is not visible to the unaidedeye and is the only planet in the Solar System found by mathematicalprediction rather than by empirical observation. Unexpected changesin the orbit of Uranus led Alexis Bouvard to hypothesize that itsorbit was subject to gravitational perturbation by an unknown planet.After Bouvard's death, the position of Neptune was predicted from hisobservations, independently, by John Couch Adams and Urbain LeVerrier. Neptune was subsequently observed with a telescope on 23September 1846 by Johann Galle within a degree of the positionpredicted by Le Verrier. Its largest moon, Triton, was discoveredshortly thereafter, though none of the planet's remaining 13 knownmoons were located telescopically until the 20th century. Theplanet's distance from Earth gives it a very small apparent size,making it challenging to study with Earth-based telescopes. Neptunewas visited by Voyager 2, when it flew by the planet on 25 August1989; Voyager 2 remains the only spacecraft to have visited Neptune.The advent of the Hubble Space Telescope and large ground-basedtelescopes with adaptive optics has recently allowed for additionaldetailed observations from afar.
Like Jupiter and Saturn, Neptune'satmosphere is composed primarily of hydrogen and helium, along withtraces of hydrocarbons and possibly nitrogen, though it contains ahigher proportion of ices such as water, ammonia and methane.However, similar to Uranus, its interior is primarily composed ofices and rock; Uranus and Neptune are normally considered "icegiants" to emphasize this distinction. Along with Rayleighscattering, traces of methane in the outermost regions in partaccount for the planet's blue appearance. Newest data from the Geminiobservatory shows the blue color is more saturated than the onepresent on Uranus due to thinner haze of Neptune's more activeatmosphere.
In contrast to the hazy, relativelyfeatureless atmosphere of Uranus, Neptune's atmosphere has active andvisible weather patterns. For example, at the time of the Voyager 2flyby in 1989, the planet's southern hemisphere had a Great Dark Spotcomparable to the Great Red Spot on Jupiter. More recently, in 2018,a newer main dark spot and smaller dark spot were identified andstudied. In addition, these weather patterns are driven by thestrongest sustained winds of any planet in the Solar System, withrecorded wind speeds as high as 2,100 km/h (580 m/s; 1,300 mph).Because of its great distance from the Sun, Neptune's outeratmosphere is one of the coldest places in the Solar System, withtemperatures at its cloud tops approaching 55 K (−218 °C; −361°F). Temperatures at the planet's centre are approximately 5,400 K(5,100 °C; 9,300 °F). Neptune has a faint and fragmented ringsystem (labeled "arcs"), which was discovered in1984, then later confirmed by Voyager 2.
History
Discovery
