NASA - National Aeronautics and Space Administration

Neptune is one of the two planets that cannot be seen from Earth without a telescope. Pluto is the other. Bright blue clouds cover Neptune's surface. Because these clouds look like water, the planet was named after the ancient Roman god of the sea, Neptune.

Image to left: Bright blue clouds surround Neptune. They are made of frozen methane, a gas. Because these clouds look like water, the planet was named after Neptune, the ancient Roman god of the sea. Credit: NASA/JPL

Scientists believe that Neptune is made up mostly of gases, water, and minerals. Plants and animals that live on Earth could not live on Neptune. It is much too cold, and the air has no oxygen. People and animals need oxygen to breathe.

Neptune travels around the sun in an elliptical, or oval-shaped orbit. It takes about 165 years for Neptune to go all the way around the sun. It takes our planet Earth one year to go around the sun. Pluto is the only planet farther from the sun than Neptune. But every 248 years, Pluto moves inside Neptune's orbit for about a 20-year period. During that time, Neptune is the farthest planet from the sun.

Neptune is about four times as big as Earth. It has at least 13 moons (natural satellites), and several rings. Only two of the moons, Triton and Nereid, can be seen by telescopes on Earth. Six moons and the rings were discovered in 1989 by the United States spacecraft Voyager 2. It took the first close-up pictures of Neptune, although scientists first saw Neptune through a telescope in 1846. Astronomers using powerful telescopes found the other moons in 2002 and 2003.

How to cite this article: To cite this article, World Book recommends the following format: "Neptune." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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