Search this website:

This web page location:

home page  >   Solar System  >   Neptune

Solar System


source of comets, dwarf planet Pluto, largest planet, astronomical units, Kuiper Belt

Deeper web pages:

>  Observation From Earth and Space

>  Composition and Structure

>  Rings and Moons

>  Influence on Pluto and The Kuiper Belt

Neptune (planet), eighth planet in distance from the Sun, fourth largest planet in diameter, and third largest in mass in the solar system. Neptune’s gravity has a major influence on the Kuiper Belt, a region of icy bodies in the outer solar system that is a source of comets and includes the dwarf planet Pluto, formerly counted as the ninth planet.

Because of its great size and mass, scientists classify Neptune as one of the giant or Jovian (Jupiter-like) planets—along with Jupiter, Saturn, and Uranus. Like Uranus, Neptune is also classified as an ice giant planet, mainly made of the ice-forming molecules water, ammonia, and methane as a liquid mixture above what is thought to be a rocky core. Its atmosphere is mainly hydrogen and helium, along with methane gas that gives the planet a blue-green color.

Neptune orbits the Sun at an average distance of about 4,490 million km (about 2,790 million mi) in a period of 165 Earth years and only receives about 1/900th the amount of sunlight that Earth does. Neptune’s diameter at the equator is about 49,520 km (about 30,767 mi). Even though Neptune’s volume is 72 times Earth’s volume, its mass is only 17.15 times Earth’s mass.

Neptune has four rings and 13 known moons. The planet is named after the sea god Neptune in Roman mythology.

Motion of Neptune

Neptune orbits about 4,490 million km (about 2,790 million mi), or 30 astronomical units (AU) from the Sun, beyond Uranus. An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). Neptune takes 164.79 years to complete a single revolution around the Sun, so a year on Neptune is 164.79 times longer than a year on Earth. The orbit of Neptune traces out a flat region of space called the planet’s orbital plane. The orbital plane of Neptune lies close to Earth’s orbital plane. As a result, Neptune always crosses the same region of Earth’s sky.

The planet spins around its axis once every 16 hours in a counterclockwise direction, just as Earth spins once every 24 hours. The axis of rotation on Neptune tilts 29.6° into its orbital plane (the plane created by Neptune’s orbit around the Sun). This tilt gives Neptune almost Earthlike seasons. (Seasons on Earth result from our planet’s 23.5° tilt into its orbital plane.)


Hubbard, William B., B.A., Ph.D.

Professor of Planetary Sciences, University of Arizona Lunar and Planetary Laboratory. Author of "Planetary Interiors".

Article key phrases:

source of comets, dwarf planet Pluto, largest planet, astronomical units, Kuiper Belt, orbital plane, Roman mythology, methane gas, outer solar system, axis of rotation, Uranus, blue-green color, equator, helium, Saturn, Neptune, ammonia, Jupiter, hydrogen, planets, sunlight, sky, gravity, atmosphere, rings, diameter, AU, planet, mass, volume, scientists, distance, Sun, period, result, hours, years

Search this website: