Solar System loses a planet after experts reject Pluto

Pluto lost its status as a full planet yesterday when astronomers voted to reclassify it as a "dwarf planet", leaving eight "classical planets" in the Solar System.

The International Astronomical Union threw out a resolution that would have increased the number of planets in the solar system to 12 - and instead opted for a tighter definition that eliminated Pluto.

Delegates to the IAU's meeting in Prague drew a clear distinction between the eight classical planets, most of which were known since antiquity, and the much smaller Pluto, which was only discovered in 1930. From now on, the planets will be restricted to Mercury, Earth, Jupiter, Saturn, Neptune, Venus, Mars and Uranus.

Just a week ago leaders of the IAU had proposed placing it in a sub-category known as "Plutons". This idea proved so unpopular that it was dropped. The scientists said that Pluto should from now on be considered a dwarf planet along with two other smaller celestial bodies - Ceres and UB313 - that orbit the Sun.

The scientists agreed that to be called a planet, a celestial body must be in orbit around a star while not itself being a star. It also must be large enough in mass for its own gravity to pull it into a nearly spherical shape and have cleared the neighbourhood around its orbit.

This last clause, which was added to the definition, meant that Pluto was disqualified because its oblong orbit overlaps with the orbit of Neptune.

The new definition - the first time the IAU has tried to define scientifically what a planet is - means a second category called "dwarf planets" has been created, and a third for all other objects, except satellites, known as "small solar system bodies".

Asteroids have been loosely referred to as "minor planets" but are now classed as small solar system bodies.

Charon, the largest of Pluto's three moons, had been included among the "Plutons", since some experts regard it as a twin planet. However, it is no longer being given any special status.

The need to define what it takes to be a planet was driven by technological advances that enable astronomers to look further into space and to measure more precisely the size of celestial bodies in our solar system.

Professor Ron Ekers, president of the IAU, said: "Modern science provides much more knowledge than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our solar system that have sizes comparable to and larger than Pluto. These discoveries have called into question whether or not they should be considered as new 'planets'."

Astronomers were divided after a team led by Professor Mike Brown at the California Institute of Technology discovered a planet larger than Pluto, UB313, nicknamed "Xena" after the Warrior Princess TV series.