ASTRONOMERS have detected a previously unknown planet in the outermost reaches of the solar system, even further from the sun than Pluto.

The discovery was announced this weekend by Mike Brown, a planetary scientist at the California Institute of Technology.

The find will reopen the debate over what constitutes a planet since the new body is clearly part of the Kuiper belt, a swarm of rocks and asteroids orbiting the sun beyond Neptune that are probably the remains of debris that formed the solar system 5 billion years ago.

However, Brown said that its sheer size in relation to the nine known planets meant that it could only be classified as a planet.

“It’s definitely bigger than Pluto,” said Brown, a professor of planetary astronomy. The planet currently lies about 97 times further away from the sun than the Earth, so a spaceship would need decades to reach it. It will remain visible to astronomers for the next six months until its orbit swings it out of view.

Brown and his colleagues Chad Trujillo and David Rabinowitz first photographed the planet with the 48in Samuel Oschin Telescope at the Palomar observatory, San Diego, in October 2003.

However, the object was so far away that its motion was not detected until they analysed the data again this year. It is the motion of such bodies against the background of stars that tells astronomers their position, size and movement.

In the past seven months, the scientists have been studying the planet to estimate its size and motions. So far, they can tell only that it is at least 1.5 times bigger than Pluto, which itself has a diameter of around 1,400 miles.

There is, however a possibility that it is substantially larger, although it is unlikely to approach the size of Mars, which has a diameter of 4,200 miles, or Earth, which is 8,000 miles in diameter.

A name for the planet has been proposed by the discoverers to the International Astronomical Union and they are awaiting a decision.

Last year, Brown found another major solar system object, slightly smaller than Pluto, which was named Sedna, after the Innuit goddess of the sea. In 2002 Brown’s team also reported the discovery of Quaoar, a planetoid about 800 miles in diameter 1 billion miles beyond Pluto. The average distance of Pluto from the sun is 3.6 billion miles.

The discovery comes at an exciting time for planetary science. In April astronomers produced what could be the first picture of a planet orbiting another star. They photographed a reddish object apparently orbiting a star called GQ Lup, more than 400 light years from Earth.

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