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Last Updated: Wednesday, 1 February 2006, 18:26 GMT
Distant world tops Pluto for size
By Jonathan Amos
BBC News science reporter

Artist's concept of the 10th planet and its moon (R Hurt, IPAC)
At 3,000km (1,860 miles), the object is significantly bigger than Pluto
An icy, rocky world reported last year to be orbiting the Sun in the distant reaches of the Solar System really is bigger than Pluto, scientists say.

New observations of the object, which goes by the designation 2003 UB313, show it to have a diameter of some 3,000km - about 700km more than Pluto.

The measurement was undertaken by a German team using a telescope in Spain, and is published in the journal Nature.

It is likely to bolster claims for the body to be given planet status.

On the other hand, it will also give support to those who believe Pluto has an inflated position and should be downgraded to a more minor classification.

Perhaps we should just sit back and relax and see what else is discovered out there in the near future
Prof Iwan Williams, Queen Mary University of London
"I'm easy on this," said the Nature paper's lead author, Professor Frank Bertoldi, from the University of Bonn and the Max-Planck-Institute for Radioastronomy.

"I would not want to demote Pluto for historical and cultural reasons - you'd upset the schoolchildren. So, it seems only fair to call objects larger than Pluto planets as well. I think we could cope," he told the BBC News website.

Icy belt

Like Pluto, 2003 UB313 orbits beyond Neptune in a region known as the Kuiper Belt.

Scientists think there are probably many tens of thousands of freezing bodies in this ring of debris left over from the formation of the Solar System.

IRAM 30m telescope on Pico Veleta in the south of Spain (Bertoldi)
The Iram 30m telescope is sited in the south of Spain
But 2003 UB313 is the first to be identified with a girth that exceeds Pluto's.

The German team looked at the far-off world using the 30m Iram (Institut de RadioAstronomie Millimetrique) telescope at Pico Veleta in the Sierra Nevada mountains.

Using a standard astronomical approach, the group was able to arrive at a figure for the diameter by measuring the heat radiated by the object and combining this with information about the amount of sunlight reflected off its surface.

The calculation of 3,000km has some uncertainty attached to it - which means the world could be up to 300km smaller or bigger - but either way, it still represents the largest object found in the Solar System since Neptune in 1846.

August meeting

Michael Brown, the California Institute of Technology astronomer who announced 2003 UB313's discovery with colleagues last July, has nicknamed the object Xena and is pressing for it to be given full planet status.

If granted such a classification, the object would assume a name from Greek or Roman mythology.

2003 UB313 (XENA)
Xena and Gabrielle (WM Keck Obs)
First seen in 2003 but finally recognised in 2005
Highly elongated orbit around Sun lasting 558 years
Currently positioned some 14.5 billion km from Earth
Has extremely frigid surface temperature of -250C
May have thin atmosphere when closest to Sun
Has moon with unofficial 'codename' of Gabrielle
Names come from US TV series, Xena: Warrior Princess
But the request has led to heated discussions within the science of astronomy which is now facing up to the prospect of even more objects of similar size being discovered in the Kuiper Belt in the next few years. Can they all be classed as planets?

"Theories for the formation of our Solar System imply there could be objects as big as Mars out there," commented Professor Alan Fitzsimmons, from Queen's University Belfast.

"We knew the day would come when something at least as large as Pluto, if not bigger, would be discovered."

And on the matter of whether it should be called a planet: "It's very difficult; my head scientifically pulls one way and my heart another. I just don't know."

The discipline's official nomenclature duties rest with the International Astronomical Union (IAU), which will discuss the classification of planets at its General Assembly in August.

'Relax' option

It will consider an options report drawn up by a working group chaired by Professor Iwan Williams, from Queen Mary University of London.

"You could have a distinction which says, 'everything bigger than Pluto is a planet', but then you are in danger of finding four or five of these objects in the next few years and you end up asking yourself, 'did we really mean to create 15 planets?'"

Professor Williams conceded it would be extremely hard to arrive at a definition for a planet that satisfied everyone.

It was possible, he added, that the IAU could simply defer any decisions until it became clearer just how many objects might be candidates for planet classification.

"Perhaps we should just sit back and relax and see what else is discovered out there in the near future."

Mike Brown's team is expected to come forward soon with its own measurement of the diameter of 2003 UB313 using data from the Hubble Space Telescope.

Comparison of Solar System body (BBC)

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