Neptune is a major planet in the solar system, eighth planet from the Sun and fourth largest in diameter. Neptune maintains an almost constant distance, about 4.5 billion km, from the Sun. Neptune revolves outside the orbit of Uranus and for most of its orbit moves inside the elliptical path of the outermost planet Pluto. Every 248 years, Pluto's elliptical orbit brings the planet inside Neptune's nearly circular orbit for about 20 years, temporarily making Neptune the farthest planet from the Sun. The last time Pluto's orbit brought it inside Neptune's orbit was in 1979. In 1999 Pluto's orbit carried it back outside Neptune's orbit.

Astronomers believe Neptune has an inner rocky core that is surrounded by a vast ocean of water mixed with rocky material. From the inner core, this ocean extends upward until it meets a gaseous atmosphere of hydrogen, helium, and trace amounts of methane. Neptune has four rings and 11 known moons. Even though Neptune's volume is 72 times Earth's volume, its mass is only 17 times Earth's mass. Because of its size, scientists classify Neptune, along with Jupiter , Saturn, and Uranus, as one of the giant or Jovian planets (so-called because they resemble Jupiter).