Astronomers have found a new world orbiting the Sun. The giant lump of rock and ice is larger than the planet Pluto and is now the farthest known object in the solar system.
The discovery was announced by US scientists yesterday and the object has unofficially been named Xena, after the TV series starring Lucy Lawless. 'We have always wanted to name something Xena,' said Michael Brown, a member of the team that made the discovery using telescopes at the Palomar Observatory, outside San Diego, California.
Preliminary observations suggest Xena - officially known as 2003 UB313 - is an extremely strange world. It is currently 9 billion miles away from the Sun, roughly 100 times more distant than the Earth, and is now about three times more remote than Pluto. At its present distance, the Sun will appear so small in the sky it will almost be indistinguishable from other stars.
Xena will also be incredibly cold. Its surface temperature is likely to be only a few degrees above absolute zero, while a year there - the time Xena takes to make one passage round the Sun on its highly elliptical orbit - will be the equivalent of 560 Earth years.
Despite its distance, the little world is also proving to be highly controversial. Astronomers cannot agree whether it is a planet or just a jumped-up asteroid. Its discoverers are claiming Xena is the 10th planet. Other astronomers say it is just another of the Sun's minor planets. There are thousands of minor planets in the solar system, but only nine fully fledged major planets.
The last full planet to be discovered - in 1930 by US astronomer Clyde Tombaugh - was Pluto. But recently some astronomers have campaigned to have Pluto downgraded to 'minor planet' status. It is so small - its diameter is a mere 2,200 kilometres - that it is unworthy of the status of full planet, it was argued. This bid was finally rejected after heated scientific debate.
But now the discovery of Xena, which is only slightly bigger than Pluto, will re-ignite that row. Both Pluto and Xena are components of the Kuiper Belt, which is made up of thousands of small asteroid-like objects, many mere lumps of rock, that sweep the outermost depths of the solar system. As members of the Kuiper Belt, neither Pluto nor Xena should be rated full-fledged planets, it is argued.
The trouble for astronomers is that they do not have an exact definition of a planet. Many say that, if Pluto had been discovered today, it would not have been called a proper planet. In 1999 one group from the US Minor Planet Centre proposed that Pluto be given a new joint classification so that it would keep its position among the major planets, but also be given a designation as a minor planet. The centre dropped the proposal after outcry from those who saw it as a demotion.
Gareth Williams of the centre said he still supported dual status for Pluto, but did not think Xena should be added to the registry of major planets. It should be left as as a minor planet 'permanently', he said.
But Mark Sykes, director of the Planetary Sciences Institute in Tucson, Arizona, disagreed. It should be classed as a full planet, he said. 'The kinds of questions we would ask about this object [Xena] would be planet-like questions,' he said. For example, does it have an atmosphere and what sort of geological processes generated its apparently bright surface?
This view was backed by the leader of the team that discovered Xena. 'It is definitely bigger than Pluto, and I would say it counts out as the 10th planet,' said Brown.
Xena was first spotted in January. Since then scientists have been checking its position and size before making their announcement.
They had hoped to hold back for longer, but a secure website containing details of the discovery was recently hacked and the hacker threatened to release the information.
For Brown, the discovery is particularly satisfying. Five and a half years ago, he bet fellow astronomer Sabine Airieau five bottles of good champagne that he would find a Kuiper Belt object larger than Pluto by the end of last year. In December, having failed, he bought the champagne to send to her. Then 2003 UB313 was spotted on 8 January.
'I lost the bet by eight days,' Brown said. 'But she graciously decided she would let that window slide and I would win the bet. That means I get to drink 10 bottles of good champagne. And I think I will.'