Neptune: Eighth planet from the Sun (usually)

Average distance from the Sun: 30.06 AU (4,500 million km)
Orbital period (Length of Year): 164.8 Earth years
Rotational period (Length of Day): 19.1 Earth hours
Orbital inclination: 1.77 deg
Average orbital speed around the Sun: 5.45 km/s
Diameter: 49,528 km
Axial inclination: 29.6 deg
Mass: 1.02 x 10 ^ 26 kg (17.1 Earth masses)
Relative surface gravity (Earth = 1): 1.125
Average temperature: -225 deg C (-373 deg F)
Atmosphere: hydrogen, helium, methane
Albedo: 0.51
Number of moons: 12

The planet Neptune was the first planet to be predicted before it was discovered. A few decades after Uranus was accidentally discovered by astronomer Sir William Herschel, it became apparent that it was not precisely following its predicted orbit. Astronomers theorized that another, unseen planet was pulling on Uranus with its gravity, altering its path. John Adams, a student in England, and Urbain Leverrier, a French mathematician, independently made calculations predicting where this invisible planet should be found in the sky. In 1846, Leverrier sent his predictions to the German astronomer Johann Galle at the Berlin Observatory, who upon receiving these coordinates located Neptune that very night!

Neptune is a sky-blue colored planet as seen through a telescope. This color is due to the methane in its atmosphere. Because Neptune is so far away and faint, it is difficult to see with any detail. But in August of 1989, the Voyager 2 probe passed by Neptune and returned a wealth of information.

Neptune has a turbulent atmosphere reminiscent of Jupiter's. A large, dark blue storm, called the "Great Dark Spot" rotated with the planet, and bright cirrus clouds appeared in the upper atmosphere. One smaller storm that remained throughout Voyager's inspection of Neptune was nicknamed "Scooter" by Voyager scientists. There are winds blowing retrograde (in the direction opposite the planet's rotation) with speeds over 600 meters per second, or 1,340 miles per hour Ð the fastest known winds in the solar system. The Hubble Space Telescope was recently pointed at Neptune. Scientists discovered that the Great Dark Spot found by Voyager has since disappeared. It is not yet known whether this storm system actually dissipated or whether it has been covered up by higher-level clouds in the atmosphere.

Although Neptune is thought to have a small, rocky core, most of the planet consists of a thick layer of water and rock, which are at such temperatures and pressures that they must be in a liquid state, resembling a watery mud.

Voyager 2 confirmed the existence of faint rings around the planet, with some parts of the rings more clumped than others. Neptune has three separate rings and one broad sheet of particles closer to the planet. Thus, ring systems have been found for all four of the giant planets: Jupiter, Saturn, Uranus, and Neptune.

Triton, Neptune's largest satellite, is as intriguing as the planet itself. Triton is 2,720 kilometers (1,690 miles) wide. Triton's surface is constantly being recoated with ice, erupting from "ice geysers." As sunlight warms the icy surface of Triton,heat builds up under the surface ice, causing some of the ice to sublimate to gas. As more gas builds up, the pressure on the overlying ice increases until at last the nitrogen bursts through the upper ice in a geyser, carrying with it black soot from below that gets blown in long trails by Triton's winds.

Triton's surface temperature is currently the coldest measured in our solar system: -236 degrees C. Astronomers believe that Triton was captured by Neptune's gravity instead of coalescing around Neptune during the formation of the solar system. This would explain Triton's retrograde ("backwards") orbit and its unusually high density.

Nereid, Neptune's outermost moon, has the most eccentric orbit of any known moon. This cometary-like orbit means that Nereid, at its greatest distance, is seven times as far away from Neptune as it is at closest approach.

Information Source:  http://www.moreheadplanetarium.org/index.cfm