Pluto loses status as a planet

Astronomers have voted to strip Pluto of its status as a planet.

About 2,500 scientists meeting in Prague have adopted historic new guidelines that see the small, distant world demoted to a secondary category. The researchers said Pluto failed to dominate its orbit around the Sun in the same way as the other planets.

The International Astronomical Union's (IAU) decision means textbooks will now have to describe a Solar System with just eight major planetary bodies.

Pluto, which was discovered in 1930 by the American Clyde Tombaugh, will be referred to as a "dwarf planet". There is a recognition that the demotion is likely to upset the public, who have become accustomed to a particular view of the Solar System.

The scientists agreed that for a celestial body to qualify as a planet:

• it must be in orbit around the Sun
• it must be large enough that it takes on a nearly round shape
• it has cleared its orbit of other objects

Pluto was automatically disqualified because its ighly elliptical orbit overlaps with that of Neptune. It will now join a new category of dwarf planets.

The critical blow for Pluto came with the discovery three years ago of an object currently designated 2003 UB313. After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter: it is bigger than Pluto.

2003 UB313 will now join Pluto in the dwarf category, along with the biggest asteroid in the Solar System, Ceres.

Named after the god of the underworld in Roman mythology, Pluto orbits the Sun at an average distance of 5.9 billion kilometres (3.7 billion miles) taking 247.9 Earth years to complete a single circuit of the Sun.