Neptune
DISTANCE FROM SUN: 2.8 billion miles (4.5 billion kilometers).
REVOLUTION AROUND SUN: 164 years.
ROTATION: 17.2 hours.
DIAMETER: 31,770 miles (51,100 kilometers).
DENSITY: 1.6 x that of water.
MASS: 17 x that of Earth.
SURFACE TEMPERATURE: -392°F (-200°C) at cloud tops.
Satellites (closest to most distant):
Naiad
Thalassa
Despina
Galatea
Larissa
Proteus
Triton
Nereid
The last gas giant, Neptune orbits the sun every 164 years—but spins at a tremendous rate: A day on Neptune lasts just over 17 hours. The highest winds in the solar system have been measured on Neptune. Near a persistent storm called the Great Dark Spot—similar to Jupiter’s Great Red Spot—winds have been clocked up to 1,200 miles an hour (2,000 kilometers per hour).
Neptune has a central region of molten rock, water, liquid ammonia, and methane, wrapped in a shroud of hydrogen, helium, water, and methane. As in the case of Uranus, methane in the upper atmosphere gives the planet its blue color.
Also like Uranus, Neptune has high, bright clouds floating above its blue face. So high are these clouds that they have been seen to cast a shadow on the blue lower regions of the atmosphere.
Although Neptune is usually listed as the second most distant planet from the sun, it has been observed outside the orbit of Pluto, which periodically dips closer to the center of the solar system. The two planets’ orbits never intersect, however, so no spectacular planetary collisions can be expected.
Neptune was the first planet to be predicted by mathematical theory. Using predictions formulated by mathematicians, astronomer Johann Galle spotted Neptune in 1846.


MOONS AND RINGS
Four very narrow and extremely faint rings circle Neptune. Images from Voyager 2 reveal bright clumps or arcs of material. The rings are made up of fine dust particles, possibly the remnants of meteorites that crashed into the planet’s eight moons.
Except for Nereid, which orbits at a great distance from Neptune, the planet’s moons circle just 220,000 miles (355,000 kilometers) or less from the surface. Triton, the largest, is in an orbit that will probably shrink by up to 15 percent over the next five billion years.


SURFACE TURMOIL
The Voyager probe in 1989 discovered what was called the Great Dark Spot of Neptune, an intense storm circling the planet every 18 hours. But when the Hubble Space Telescope cast its eye on the planet five years later, it was nowhere to be seen. Then, a few months later, a new dark spot was discovered by Hubble.
The comings and goings of Neptune’s spot are utterly unlike those of Jupiter’s red spot, which has been churning away for at least 300 years. The spot on Neptune has high altitude clouds along its edge, gases that have risen from the planet’s warmer surface to colder elevations, where they form methane ice crystals.
It is possible that the dark spots are actually windows in Neptune’s upper cloud cover, giving a glance at unclouded levels lower in the planet’s atmosphere.


WISPY TRAVELERS
The Hubble Space Telescope has detected clouds on both hemispheres of Neptune, including high-level clouds that brightly reflect sunlight. The bright clouds, which resemble cirrus clouds on Earth, often form and disappear within a few hours. Their shape and size are apparently influenced by atmospheric waves. Often as thin as 30 miles (48 kilometers) wide, they can extend for thousands of miles.