Neptune (IPA: /ˈnɛp.tjun/), (IPA: /ˈnɛp.tʰun/) is the eighth and farthest known 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 14 Earth masses, but Neptune is slightly smaller in volume due to its higher density. The planet is named after the Roman god of the sea. Its astronomical symbol is . The symbol is a stylised version of Poseidon's Trident, as well as the alchemical symbol for Quicklime (Calcium oxide).
Neptune's atmosphere is primarily composed of hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune's blue colour is much more vivid than that of Uranus, which has a similar amount of methane, so an unknown component is presumed to cause Neptune's intense colour. Neptune also has the strongest winds of any planet in the solar system, measured as high as 2,100 km/h or 1,300 mph. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere 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 (−360 °F), one of the coldest in the solar system, due to its long distance from the sun. However, Neptune's centre is about 7,000 °C (13,000 °F), hotter than the sun's surface. This is due to extremely hot gases and rock in the centre.
Discovered on September 23, 1846, Neptune is notable for being the first planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. It has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.