Dissenting astronomers have called the decision arbitrary, pointing out that some other planets (including Earth) share their orbits with other bodies.

In addition to orbiting the sun and being rounded by its own gravitational field, the IAU definition of a “classical planet” requires an object to be the sole occupant of its orbit. A dwarf planet must only meet the first two criteria and cannot be a satellite. All other bodies in the solar system are referred to as “small solar system bodies”. IUA members voted yesterday to adopt this classification scheme and to classify Pluto as a dwarf planet.

There are three other dwarf planets – Charon, Ceres and UB313, which is informally known as Xena. Charon was once considered a moon of Pluto, but now the two bodies are called a “double-dwarf planet system”. Pluto was discovered in 1930 and Charon in 1978. Pluto also has two moons, Nix and Hydra, which were discovered in 2005.

The controversy was sparked by the discovery in 2005 of UB313. With a diameter of about 3000 km, UB313 is larger than Pluto (2300 km diameter) and occupies an orbit well beyond that of Pluto. More objects like UB313 are expected to be discovered in the future and many in the astronomical community do not wish to call these bodies planets. Others, however, are happy to see the number of planets increase with every discovery.

UPDATE 25 September 2006: NASA and others in the astronomical community are now calling Charon a moon of Pluto, not a dwarf planet.