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the most distant planet from the sun (usually), as well as the smallest

Pluto as seen by the New Horizons probe
Pluto as seen by the New Horizons probe

symbol of Pluto
symbol of Pluto

Facts and Figures
Mean distance from Sun 3,675,000,000 miles
Distance from Earth  
  Least 2,700,000,000 miles
  Greatest 4,670,000,000 miles
Equatorial diameter approximately 4,000 miles
Period of rotation (day) about 6 earth days
Period of revolution (year) about 248 earth years
Orbital speed  
Mass (Earth=1)  
Mean density (water=1)  
Surface gravity (Earth=1)  
Atmospheric pressure at surface (Earth=1 bar)  
Mean surface temperature  
Natural satellites 3


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 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 Roman god of the lower world.


New Horizons was launched in January 2006 and reached Pluto in 2015.


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.

The "smudge" in this picture is Pluto and the "bulge" on the smudge is Charon

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

Kerberos, a tiny moon betwwen the orbits of Nix and Hydra was discovered in 2011.

A team of scientists searching for potential hazards to the New Horizons spacecraft flyby discovered Styx in 2012.

Pluto and its moons
Pluto and its moons

Planetary Status

Despite having five moons of its own, Pluto is smaller in diameter than seven of the Solar System's other planetary moons, and 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."

See Also

Clyde Tombaugh

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The Robinson Library >> Solar System

This page was last updated on 10/16/2018.