Normally the eighth planet from the Sun, although the dwarf planet Pluto, because of its elongated orbit, occasionally comes closer for periods of 20 years around its perihelion. Neptune is the most distant of the gas giants. It is also marginally the smallest of the gas giants in the solar system, after Uranus, although its volume is still 60 times greater than that of the Earth and it is slightly more massive than Uranus. Neptune and Uranus are more appropriately referred to as ice giants because their composition is substantially different from that of Jupiter and Saturn. Neptune was discovered in 1846 by Johann Galle and Louis d'Arrest following predictions by Urbain Leverrier.
Neptune's deep atmosphere extends 10-20% of the way to the center of the planet. At high altitudes it consists mostly of hydrogen (80%) and helium (19%). Increasing concentrations of methane, other hydrocarbons (including ethane, acetylene, and diacetylene), ammonia, and water vapor occur in the dark, warmer, lower regions of the atmosphere. Finally the atmosphere blends into the superheated liquid interior. Neptune has 13 known moons.

(For further details see Neptune »)

(Source: The Internet Encyclopedia of Science »)