Density - 1.64 g/cm³,

Neptune is a large gas- and ice-rich planet of the outer solar system with at least eight moons and a system of rings. Neptune is similar in size and density to Uranus, but has a normal rotation direction. Neptune probably has an internal structure like that of Uranus, with a relatively undifferentiated core of rocky and icy materials cloaked by a thick hydrogen- and helium-rich atmosphere. Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. Neptune's most prominent feature was the Great Dark Spot (Voyager 2 on Aug 25 1989), which had disappeared in 1994 (HST observation).

Neptune's three principal rings are narrow, and one is marked by clusters of dark particles.

Neptune has three satellites greater than 300 km in diameter. Little is known about Nereid other than its eccentric orbit. A newly discovered moon, designated 1989N1, is larger than Nereid and has an irregular shape and a cratered surface.

Triton, the largest moon of Neptune (2700 km in diameter) has a retrograde orbit and a thin atmosphere, which extends up to about 5-10 km of nitrogen and methane. The temperature at the surface of Triton is only 34.5 K (-235 C), as cold as Pluto. At this temperature methane, nitrogen and carbon dioxide all freeze solid.

The most unusually feature of Triton are the ice volcanoes. The eruptive material is probably liquid nitrogen, dust, or methane compounds from beneath the surface. One of Voyager's images shows an actual plume rising 8 km above the surface and extending 140 km "downwind". Presently, an ice cap of nitrogen covers much of the southern hemisphere. No heavily cratered terrain dating from the period of intense bombardment is preserved on Triton. Highly deformed terrains were probably formed by repeated fracturing and subsequent filling with viscous volcanic ice or flooding by mixtures of molten ices of water, methane, or nitrogen. The relative youth of its features may be the result of prolonged tidal heating after Triton was gravitationally captured by Neptune.