This illustration shows the comparative sizes of Earth, Pluto, Xena and other large Kuiper belt objects.

Credit: NASA/ESA/HST

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PRAGUE, 15 August 2006: Some 2,500 astronomers are meeting in the Czech capital to debate inter-galactic issues like whether or not Pluto is a planet, as the 26th congress of the International Astronomical Union opened on Monday.

"This congress will be without a doubt more important than the one held in Sydney three years ago. It also will be very open to young scientists," said Jan Palous, head of the national organising committee for the IAU event, which runs until August 25.

The experts from 75 countries are expected to take up the debate over the official definition of a planet, which arose after astronomers found that Pluto is much smaller than an enigmatic object, 2003 UB313, affectionately known as ‘Xena' (with it's moon nicknamed ‘Gabrielle'). Discoverers of Xena, 2400 kilometres in diameter, claim that it is the Solar System's 10th planet.

Xena, found some 15 billion kilometres from Earth, ignited a huge row after its finding was announced in July 2005 by an American team.

Pluto's defenders blasted Xena, saying it was not a planet, just a rock, or Kuiper belt object (KBO), which is the term for the estimated 100,000 pieces of icy, primeval debris that slowly encircle the sun on the outskirts of the solar system.

But considering Pluto's diminutive size, its angle of orbit, which greatly differs to the other planets of the Solar System, and its distance from the sun it may not be a planet itself.

If Pluto is indeed a planet then Xena could also be a planet, and it would be difficult to distinguish the planets from a number of other recently discovered large KBOs (the full details of the debate were covered in "Distant worlds", Cosmos, Issue 9 p74). A recent Cosmos Online poll showed a slim majority of readers think Pluto is a planet (28 per cent), but the alternatives also polled strongly.

The 11-day congress will host six symposiums, 17 debate sessions and 52 specialised conferences to discuss this and a number of other issues.

A special theme of this year's event will be women and astronomy with some 300 women astronomers joining the discussion, Palous said.

The IAU, which was founded in 1919, meets every three years to promote and safeguard the science of astronomy through international cooperation.