Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and less dense.The planet is named after the Roman god of the sea. Its astronomical symbol is 
, a stylized version of the god Neptune’s trident.
Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet’s other 12 moons were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
Neptune is similar in composition to Uranus, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the “ice giants“. Neptune’s atmosphere, while similar to Jupiter’s and Saturn’s in being composed primarily of hydrogen and helium, contains a higher proportion of “ices” such as water, ammonia, and methane, along with the usual traces of hydrocarbons and possibly nitrogen. In contrast the interior of Neptune is mainly composed of ices and rocks like that of Uranus.Traces of methane in the outermost regions, in part, account for the planet’s blue appearance.
Neptune has the strongest winds of any planet in the solar system, measured as high as 2100 km/h.At the time of the 1989 Voyager 2 flyby, its southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune’s temperature at its cloud tops is usually close to −218 C (55.1 K), one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune’s centre is about 7,000 °C (7,270 K), which is comparable to the Sun’s surface and similar to most other known planets. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2.
Composition and structure
A size comparison of Neptune and Earth.
With a mass of 1.0243×1026 kg,[5] Neptune is an intermediate body between Earth and the larger gas giants: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter.[10] Neptune’s equatorial radius of 24,764 km[6] is nearly four times that of the Earth. Neptune and Uranus are often considered a sub-class of gas giant termed “ice giants“, due to their smaller size and higher concentrations of volatiles relative to Jupiter and Saturn.[38] In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as “Neptunes”,just as astronomers refer to various extra-solar “Jupiters.”
Internal structure
Neptune’s internal structure resembles that of Uranus. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10 to 20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere.[40]
The internal structure of Neptune:
1. Upper atmosphere, top clouds.
2. Atmosphere consisting of hydrogen, helium, and methane gas.
3. Mantle consisting of water, ammonia, and methane ices.
4. Core consisting of rock and ice.
Gradually this darker and hotter region condenses into a superheated liquid mantle, where temperatures reach 2,000 K to 5,000 K. The mantle is equivalent to 10 to 15 Earth masses, and is rich in water, ammonia, methane, and other compounds. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean.At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth.[43] The pressure at the centre is 7 Mbar, millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.
Atmosphere
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s vivid azure differs from Uranus’s milder aquamarine. Since Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s colour.
Neptune’s atmosphere is sub-divided into two main regions; the lower thropospore, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars.The stratosphere then gives way to the thermosphere at a pressure lower than 10−4–10−5 microbars. The thermosphere gradually transitions to the exosphere.
Models suggest that Neptune’s troposphere is banded by clouds of varying compositions depending on altitude. The upper level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars, clouds of ammonia and hydrogen sulfide are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about 50 bars, where the temperature reaches 0 C. Underneath, clouds of ammonia and hydrogen sulfide may be found.
High altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km, and lie about 50–110 km above the cloud deck.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene.The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide.The stratosphere of Neptune is warmer than that of Uranus due to elevated concentration of hydrocarbons.
For reasons that remain obscure, the planet’s thermosphere is at an anomalously high temperature of about 750 K.[49][50] The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet’s magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.
