Pluto faces relegation as a planet

SPACE: Astronomers from around the world have begun deliberations that could lead to the solar system being officially expanded or to Pluto being struck off the list of planets.

More than 2,000 astronomers are meeting in Prague to define exactly what counts as a planet, with the likely outcomes being Pluto's relegation to the status of "ice dwarf" or promotion to the planetary club of a similarly sized lump of rock and ice.

Either way, the world's textbooks will be rewritten and celestial wallcharts torn down if the International Astronomical Union votes on August 24th to change the 76-year-old status of Pluto as the solar system's ninth and most distant planet.

When it was spotted in 1930, Pluto was believed to be as big as Earth, only for later observations to show that it is smaller than our moon.

Pluto also maintains an elongated orbit around the sun that makes it less like the other eight planets than the thousands of icy rocks that move with it through a band of space beyond Neptune known as the Kuiper Belt.

Pluto's planetary identity crisis came to a head last year when an American astronomer, Michael Brown, observed another object in the Kuiper Belt that appears to be larger than Pluto, which has a diameter 6½ times smaller than Earth's.

Prof Brown nicknamed his discovery Xena, after the warrior princess of TV series fame, and declared it the 10th planet.

But many astronomers intend to use the conference to achieve a long-standing aim, that of striking Pluto from the list of planets. They believe our solar system would be a much tidier place with just eight planets: four "terrestrial" or rocky planets nearest the sun, Mercury, Venus, Earth and Mars, and four "gas giants" beyond, Jupiter, Saturn, Uranus and Neptune.

Then Pluto, Xena and perhaps thousands of similar distant objects could be consigned to the status of "ice dwarves", some stargazers suggest, noting that to allow Pluto to remain a planet on merit of size alone would prompt the immediate elevation of Xena to the status of 10th planet and open the door to scores of other pretenders.

The decision won't be easy, according to conference organiser Pavel Suchan, who said delegates were evenly divided over what to do with little Pluto. "So far it looks like a stalemate," he said. "One half wants Pluto to remain a planet, the other half says Pluto is not worth being called a planet."