Pluto was demoted by the votes of 424 astronomers in Prague.
In August 2006 the International Astronomical Union defined 'planet' for the first time, and Pluto no longer qualified. The decision was taken on the last day of a conference, by a few hundred of its thousands of members. The row has never quite ended, and 'plutoed' became the Word of the Year.
Pluto's status was shaky almost from the start. Clyde Tombaugh found it in 1930 while searching for a large 'Planet X' beyond Neptune, but Pluto turned out far smaller than hoped. When its moon Charon was discovered in 1978, astronomers could finally weigh it: about a twenty-fifth of Mercury's mass, smaller even than Earth's Moon. From the 1990s, astronomers found many other icy bodies in similar orbits in the Kuiper Belt, and Pluto began to look like the biggest member of a crowd rather than a planet.
History had seen this before. Ceres was hailed as a planet when it was found in 1801, but as more similar objects turned up between Mars and Jupiter, they were reclassified as asteroids. The crisis for Pluto came with the discovery of Eris, first measured as slightly larger than Pluto. Either both were planets, and potentially many more, or neither was.
The IAU's final definition, adopted in 2006, says a planet must orbit the Sun, be massive enough to pull itself into a nearly round shape, and have 'cleared the neighbourhood' around its orbit. Pluto fails the third test and became a 'dwarf planet', leaving eight planets. An earlier draft would have added Ceres, Eris and Charon to the list instead.
Critics object on several fronts. Only 424 astronomers voted, fewer than 5 per cent of the membership. Alan Stern, who led NASA's mission to Pluto, argues that Earth, Jupiter and Neptune have not fully cleared their orbits either, pointing to the asteroids that share them. Others note the definition depends on location: a Mars-sized body far out in the Oort cloud could not clear such a vast zone. Meanwhile the American Dialect Society made 'plutoed', meaning demoted, its word of 2006.
Source: Wikipedia — IAU definition of planet · Text summarised from Wikipedia (CC BY-SA 4.0)
