Solar system gas giant planets
Neptunian satellites
Neptune
The eighth planet from the Sun and the fourth largest by size in the solar system. The equatorial radius of Neptune is 24,764 km (3.883 Earths), its semi-major axis is 30.11 astronomical units (4.50 x 10^9 km), and its orbital period is 164.8 yr. Neptune has at least 14 moons, the largest ones are Triton, Proteus, and Nereid, whereas its smaller moons are: Naiad, Thalassa, Despina, Galatea, Larissa, Halimede, Sao, Laomedeia, Psamathe, and Neso. Neptune has an incredibly thick atmosphere comprised of 74% hydrogen, 25% helium, and approximately 1% methane. Particles of icy methane in its upper atmosphere give Neptune its deep blue color. Large storms whirl through Neptune's upper atmosphere, and high-speed winds track around the planet at up 600 m/s, fastest recorded in the solar system. One of the largest storms ever seen was recorded in 1989. Called the Great Dark Spot, it lasted about five years. Neptune has a very thin collection of rings. They are likely made up of ice particles mixed with dust grains and possibly coated with a carbon-based substance.