Pluto was discovered in 1930 by Clyde W.
Tombaugh. He was searching for the planet based on predictions that we now know
were false, so he found Pluto by accident. The predictions were based on the
motions of Uranus and Neptune, which were later explained by errors in
calculating Neptune's mass that were discovered through the Voyager 2
encounter.
Knowledge of Pluto is scant because it is so far away from Earth. No spacecraft have visited Pluto and the Hubble Telescope is only able to show the largest features. Pluto does have a moon, Charon which allowed for mapping of light and dark spots on Pluto by predicting which part of Pluto or Charon would be covered at certain points in time due to Charon orbiting Pluto, and by studying brightness curves. The radius and mass of Pluto are not known as they are difficult to determine. The surface temperature and composition of Pluto are also unknown. Little is known about the atmosphere, however it is probably frozen throughout most of its orbit and possibly gaseous while at perihelion, the closest point to the Sun.
Pluto is very different from the other planets in the solar system. Its orbit is very eccentric, so that while most of the time it is the farthest planet from the Sun, it sometimes crosses the path of Neptune and Neptune becomes the farthest planet from the Sun. The origin of Pluto is also uncertain, because it has similar characteristics to Triton, a moon of Neptune. This may have resulted from Pluto, Triton, and Charon (Pluto's moon) belonging to a separate class of objects. Triton may have been captured by Neptune, but Pluto and Charon continued to orbit the sun.
moons: 1; name=Charon
***Recently Pluto was named a Dwarf Planet instead of a traditional planet*** Proposed Aug 24th, 2006 http://www.msnbc.msn.com/id/14489259/
