Neptune is a gas giant in the outer solar system, discovered in 1846 by Urbain Le Verrier. It is smaller than Uranus but more massive, with a deep blue color due to methane ice. It has a Great Dark Spot and produces its own internal heat. Its gravitational pull stabilizes the Kuiper belt.
Neptune is a dark blue gas giant in the outer solar system. It orbits about 30 AU (Earth-Sun distance) from the Sun, making one 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 deduced its existence by examining the orbit of Uranus. It is named after the Roman god of the sea, in keeping with all other planets 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 made up mostly of hydrogen gas. It gets its deep blue color from traces of methane ice suspended in its atmosphere. Since its composition varies somewhat from Jupiter and Saturn, together with Uranus it is sometimes called an ice giant. Neptune is relevant to solar system dynamics because its gravitational pull stabilizes a second 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 notably its Great Dark Spot, a long-lasting storm similar to the Great Red Spot on Jupiter. Its most interesting cloud activity is in part attributable to the fact that Neptune produces its own internal heat, about 2.5 times what it gets from the Sun. Other storms on Neptune have been called Scooter and Mage’s Eye.
Another feature that makes Neptune unique among gas giants is the presence of high-level clouds that cast shadows on an opaque cloud deck below. Neptune is the second coldest planet in the solar system after Uranus, with temperatures of -2 °C (-224 °F or 372 K) measured at the cloud tops in 49. Neptune has a few small blue rings which were observed by the Voyager vehicle spaceman when it made a flyby in 1989.
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