Neptune
Neptune
Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Vital Statistics
Orbits Sun in 164 years.
Averages 2795 million miles from the Sun.
Distance from Sun varies by 67 million miles (1% of orbit diameter).
Most circular orbit apart from Venus.
Nearly a twin to Uranus.
30,775 miles in diameter, 17.2 times as massive as Earth. Smaller than  Uranus but more massive
Densest of the four large outer planets (2.2).
Rotates in about 16 hours.
Unlike Uranus, Neptune has a dynamic atmosphere. Voyager in 1989 revealed cloud belts, multiple cloud levels and a great dark storm system which, by analogy with Jupiter, was called the Great Dark Spot. A few years later, the Hubble Space Telescope showed that the Great Dark Spot had disappeared.
The Rings of Neptune
The rings of Neptune are named for people influential in the discovery of the planet. They are:
Galle, radius 41,900 km, width 1700 km. Thin and diffuse
Leverrier, radius 53200 km, width 100 km. Thick and narrow
1989N4R, radius 53200 km, width 4000 km. Thin and diffuse
Adams, radius 62900 km, width 15 km. Thick and narrow.
The Adams ring is uneven and consists of clumpy arcs of ring material connected by a thinner ring.
The Moons of Neptune
Large areas of icy surface were melted, then refrozen.
Nereid
Small, 200 miles or so (350 km)
Orbits Neptune in a strange orbit that carries it from 870,000 to 6,000,000 miles from Neptune with a period of about a year. This is the second most eccentric orbit of any planet, asteroid or moon in the Solar System. Only one asteroid has a more eccentric orbit.
Proteus
400 km diameter (250 miles); bigger than Nereid.
118,000 km (70,000 mi) from Neptune.
Other small Moons of Neptune
Naiad, 60 km (40 mi) diameter, distance 48,000 km (30,000 mi)
Thalassa 80 km (50 mi.) diameter, distance 50,000 km
Despina 150 km (90 mi.) diameter, distance 52,500 km
Galatea 160 km (100 mi.) diameter, distance 62,000 km
Larissa 200 km (120 mi.) diameter,  distance 73,500 km
All discovered by Voyager. All, including Proteus, are inside the Roche limit of Neptune, the zone where tidal forces prevent satellite formation. Hence they must have been captured or had their orbits modified somehow. Ground observation and analysis of Voyager images  continue to turn up new satellites. The count as of 2008, almost certainly  incomplete, is 13. Two, orbiting over 25 million miles away with periods of over  25 years, are the most distant known planetary satellites.
References
Robert H. Brown and Dale P. Cruickshank, 1985, The Moons of Uranus, Neptune, and Pluto.
Scientific American
, vol. 253, no. 1, pp. 38-47
Kinoshita, June, Neptune;
Voyager 2 Neptune Encounter, Special issue of
Science
, vol. 246, No. 4936, December 15, 1989.
Voyager 2 Findings on Triton, Special issue of
Science