General Information

In 1930 calculations were made that predicted the existence of a ninth planet. These calculations were based on the orbits of Uranus and Neptune. Clyde W. Tombaugh is credited with discovering Pluto after surveying the sky from Lowell observatory in Arizona. The calculations however, were later discovered to be erroneous and Voyagers' measurements of Neptune's mass actually accounted for the perturbations that Pluto's existence was to account for. Nevertheless, Pluto exists, though it is no longer considered a planet.

In 2006 the IAU (International Astronomical Union) refined the definition of a planet and Pluto is now considered a "dwarf planet", having lost its distinction as a major planet. This was prompted mainly due to the discovery of several other objects such as Sedna, Quaoar, and Xena. Xena, (officially 2003 UB313) has a diameter of 2400km — slightly larger than Pluto's. As each of these objects was discovered there were news stories declaring each of them as the solar system's new tenth planet. It has since been declared that all of them (including Pluto) shall be known as "dwarf planets", thus reducing the official count of planets to eight.

All of these objects (Pluto, Sedna, Quaoar, and Xena) as well as many other smaller objects orbit in a zone called the "Kuiper Belt". The Kuiper Belt is believed to be the origin of many of our short period comets.