Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune.
Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU. It has an eccentricity of 0.01 and an orbital inclination of 1.8° relative to the ecliptic. Neptune is named after the Roman god of the sea, and its astronomical symbol is ♆, a stylised version of the god Neptune’s trident. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet.
Neptune was subsequently observed on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining known 14 moons were located telescopically until the 20th century. Triton is significantly larger than Pluto’s moon Charon, and the discovery of the Neptunian system has been a key development in the study of planetary formation.
Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter, and Saturn. Neptune’s atmosphere, like that of Uranus, is composed of hydrogen and helium, with a small amount of methane. The interior of Neptune, like that of Uranus, is primarily composed of ice and rock. Traces of water, ammonia, and methane have been detected in the atmosphere of Neptune, and evidence suggests that the mantle may contain some water.
The distance from the Sun to Neptune is about 4.5 billion kilometers.
A year on Neptune is about 164.8 Earth years.
A day on Neptune is about 16.1 Earth hours.
It has the strongest winds in the Solar System, reaching speeds of up to 2,100 kilometers per hour.
The temperature on Neptune can range from -210 degrees Celsius to -220 degrees Celsius.