Is it a bird, a rock, or a new planet?

School textbooks on astronomy may have to be rewritten following official confirmation that a new object discovered at the edge of the Solar System is bigger than the ninth planet, Pluto.

The findings could mean that either there will now be 10 planets in the Solar System or that the status of Pluto will be downgraded and we will be back to having just eight planets.

German astronomers have calculated that the object officially known as 2003 UB313 is substantially bigger than Pluto which bolsters the case for conferring it with the status of a properly named planet.

The object, which has been given various nicknames such as "Xena", "Santa" and "Rudolph", will almost certainly be named after one of the Graeco-Roman deities that has so far not been used to describe other planets or asteroids.

A committee of the International Astronomical Union will meet later this year to decide whether UB313 should be considered the 10th planet and if so what name it should be given.

Following UB313's discovery, some astronomers have already proposed that Pluto should be downgraded. "What we've discovered is actually minus-one planet'. We've gone from a nine-planet Solar System to an eight-planet Solar System," said Dave Jewitt, who in 1992 discovered the first small objects beyond Pluto in a zone known as the Kuiper Belt.

However, the latest study published in the journal Nature shows that UB313 is bigger than Pluto. It lends weight to the idea that the object is a fully-fledged planet, said Professor Frank Bertoldi of the University of Bonn.

"My personal opinion is that we shouldn't downgrade Pluto for historical and cultural reasons. But then the question is what to do with these larger objects," Professor Bertoldi said.

"In my view we should consider them to be planets. We're not going to find hundreds more of them, but we may find a few more," he said.

Professor Mike Brown and his colleagues at the California Institute of Technology found UB313 in January 2005 after a survey of the sky using a wide-field digital camera that could search for faint objects beyond Pluto and Neptune, the two most distant planets.

At the time of its discovery, scientists could not be sure how big UB313 is because judging size from the brightness of optical images relies on the object's surface reflectivity as well as its overall dimensions.

Professor Bertoldi and his colleagues used a different approach of measuring the thermal emission or heat of the object which gave them an estimated diameter of 3000km which would make it about 700km bigger than Pluto.

This means that UB313 is the largest object found in the Solar System since the discovery of Neptune in 1846. It orbits the Sun at a distance of four billion kilometres, more than 30 times the distance between the Earth and the Sun.

"Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," Professor Bertoldi said.

Both Pluto and UB313 are thought to belong to the many thousands of icy objects that are known to orbit the Sun in the Kuiper Belt.

Wilhelm Altenhoff of the Max Planck Institute for Radio Astronomy in Bonn, who developed the technique used to measure UB313, said that its size suggests that Pluto is not such an unusual object as was previously thought.

"Maybe even we can find other small planets out there, which could teach us more about how the Solar System formed and evolved," Dr Altenhoff said.

"The Kuiper Belt objects are the debris from its formation, an archaeological site containing pristine remnants of the solar nebula from which the Sun and the planets formed," he said.

- INDEPENDENT