Caltech

February 8, 2006 Since 1930 when American Clyde Tombaugh discovered Pluto, schoolchildren have been taught that Planet Earth is one of nine planets which orbit the sun, and that Pluto is the outermost planet in the solar system. Then last July 30, an American team found a more distant and quite large object circling the sun some 15 billion kilometers beyond earth. Dubbed UB313, an enormous debate has erupted over whether it should be classified as the tenth planet. More fuel was added to the debate last week when a group lead by Bonn astrophysicists determined that this putative planet is bigger than Pluto. By measuring its thermal emission, the scientists were able to determine a diameter of about 3000 km, which makes it 700 km larger than Pluto and thereby marks it as the largest solar system object found since the discovery of Neptune in 1846. For the last six months, many astronomers have argued that UB313 should be classified as a Kuiper belt object (KBO) but Pluto is also in the Kuiper belt, and the revelations about its size will weigh heavily when the special 19-member panel set up by the International Astronomical Union (IAU) determines exactly what constitutes a planet. Either way, the official planetary count will no longer be nine as a decision against UB313 will demote Pluto to the status of a KBO.  Read More