NASA - National Aeronautics and Space Administration

Pluto is a far-off dwarf planet. From its discovery in 1930, people considered Pluto to be the ninth planet of our solar system. However, because of its small size and irregular orbit, many astronomers questioned whether Pluto should be grouped with planets like Earth and Jupiter. In 2006, this debate led the International Astronomical Union, the recognized authority in naming heavenly objects, to formally classify Pluto as a dwarf planet.

Image to right: Pluto is so far from Earth that even powerful telescopes show little detail of its surface. The Hubble Space Telescope took these pictures of Pluto. Credit: NASA

Pluto's path around the sun takes it farther away from the sun than any other planet. Pluto cannot be seen from Earth without a telescope.

Pluto is about 39 times as far from the sun as Earth is. It takes about 248 years for Pluto to make one trip around the sun. For about 20 years during each trip, it comes closer to the sun than Neptune does.

Pluto is much smaller than Earth, and it is very cold. Pluto has one moon.*

In 1996, the Hubble Space Telescope took the first good pictures of Pluto. The planet has large, bright areas that are probably frozen gases. In 2006, the U.S. National Aeronautics and Space Administration (NASA) launched the New Horizons probe. The probe was expected to fly by Pluto in 2015.

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

* Note from editor: Three moons have been discovered around Pluto.

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