Pluto is the most distant planet from the sun (usually). It is also the smallest, actually being smaller in diameter than seven of the solar system's moons.

Discovery

In 1905, American astronomer Percival Lowell found that the force of gravity of some unknown planet seemed to be affecting the orbits of Neptune and Uranus. He predicted the location of a new planet, and began searching for it from his observatory in Flagstaff, Arizona. Lowell used a telescope to photograph the area of the sky where he thought the planet would be found, but died in 1916 without finding it.

In 1929, Clyde W. Tombaugh, an assistant at the Lowell Observatory, used predictions made by Lowell and other astronomers and photographed the sky with a more powerful telescope. On February 18, 1930, Tombaugh found Pluto's image on three photographs; the discovery was announced on March 13. The planet was named after the Greek and Roman god of the lower world.

Exploration

Pluto has not yet been visited by a spacecraft. A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.

Satellites

Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. It is the largest moon with respect to its primary planet in the solar system. Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades.

In late 2005, a team using the Hubble Space Telescope discovered two additional moons, which have subsequently been named Nix and Hydra.

Planetary Status

Controversy about whether Pluto is or is not a true planet has been ongoing almost since Tombaugh's discovery of the body. On August 24, 2006, the International Astronomical Union decided on a new definition of "planet" which does not include Pluto, which is now classified as a "dwarf planet."

Uranus
Clyde William Tombaugh

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This page was last updated on 08/09/2011.