About Neptune
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.[9] The planet is named after the Roman god of the sea. Its astronomical symbol is , a stylized version of Poseidon's trident.
Discovered on September 23, 1846,[1] 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 prior to the twentieth century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
Neptune's atmosphere is primarily composed of hydrogen and helium along with traces of methane. The methane in the atmosphere, in part, accounts for the planet's blue appearance.[10] Neptune also has the strongest winds of any planet in the solar system, measured as high as 2,100 km/h.[11] 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, or 55 K, one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune's center is about 7,000 °C, which is comparable to the Sun's surface and similar to most other known planets.