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Neptune is a darkish-blue gas giant in the outer solar system. It orbits about 30 AUs (Earth-Sun distances) from the Sun, making a revolution every 165 years. Since Pluto was demoted to a dwarf planet in 2006, Neptune has been the outermost planet in the solar system.
Neptune was discovered in 1846 by the French astronomer Urbain Le Verrier, who inferred its existence by examining the orbit of Uranus. It was named after the Roman god of the sea, in keeping with all the other planets which were named after ancient gods.
Neptune has a volume of 57.7 Earths and a mass of 17.1 Earths. Its diameter is slightly less than four Earths. Like the other gas giants, Neptune is mainly made up of hydrogen gas. It gets its deep blue color from trace methane ices suspended in its atmosphere. Because its composition varies somewhat from Jupiter and Saturn, along with Uranus it is sometimes called an ice giant. Neptune is relevant to solar system dynamics because its gravitational pull stabilizes a 2nd asteroid belt in the outer solar system, of which Pluto is a member, the Kuiper belt.
Although Neptune is more massive than its sister planet Uranus, it is actually smaller, because it is more compact, and its core is slightly larger. Unlike Uranus, Neptune has some surface features, most prominently its Great Dark Spot, a long-lived storm similar to the Great Red Spot on Jupiter. Its more interesting cloud activity is partially attributable to the fact that Neptune produces its own internal heat, about 2.5 times that it receives from the Sun. Other storms on Neptune have been named Scooter and the Wizard's Eye.
Another characteristic making Neptune unique among the gas giants is the presence of high-level clouds which cast shadows onto an opaque cloud deck below. Neptune is the 2nd coldest planet in the solar system after Uranus, with −224 °C (−372 °F or 49 K) temperatures measured at the cloud tops in 1989. Neptune has some small, faint azure rings which were observed by the Voyager spacecraft when it made a flyby in 1989.