ASTRONOMERS are expected to add at least one new planet to the nine already known to exist in our solar system.

The new body, provisionally named Xena, is nine billion miles from the Sun, about 100 times more distant than the Earth and well beyond Pluto.

Its diameter is 1,490 miles, 70 miles greater than that of Pluto, the smallest and most distant planet, which may qualify it for planetary status.

The decision will be taken next month at a meeting of the International Astronomical Union (IAU) in Prague when 2,000 astronomers will be asked to vote on the “expansion” of the solar system.

Xena is the largest of many similar bodies recently found in orbits beyond Pluto. Some astronomers argue that declaring them all planets will devalue the term. Others warn that a scientifically “purist” approach would mean stripping Pluto of its planetary status — which could be deeply unpopular with the public.

“We have been seeking a compromise solution,” said Iwan Williams, professor of mathematics and astronomy at Queen Mary, University of London, who chairs a committee set up by the IAU to define what a planet is.

Its report will be presented to the IAU next month. Williams has not disclosed his precise proposals but said: “We had to decide whether to stick to a purely scientific approach or to consider other aspects such as culture and history.”

Some astronomers believe that the term “planet”should be limited to the four rocky orbs, Mercury, Venus, Earth and Mars, and the four gas giants, Jupiter, Saturn, Uranus and Neptune.

Pluto was accepted as the ninth planet only because Clyde Tombaugh, who discovered it in 1930, misjudged its size, claiming that it was larger than Earth. Its actual mass is 400 times less.

By the time the mistake had been corrected, Pluto had entered the textbooks as the only planet to have been discovered by an American. Any attempt to question its status since then has been fiercely opposed by American scientists.

Professor Brian Marsden, a Briton who directs the minor planet centre at the Harvard Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, says US astronomers threatened to throw him into a hotel swimming pool after a 1980 conference where he called for Pluto to be reclassified as an asteroid.

“This issue has been overlooked for far too long,” he said. “The term planet should have a proper scientific definition.”

Xena was discovered in 2005 by Mike Brown, professor of planetary astronomy at Caltech University, California. He said many more such discoveries were likely as telescopes probe deeper into space.

“This is a huge problem for astronomy,” he said. “Scientifically speaking, I agree there are really only eight planets but the word is as much a cultural as a scientific term and we have to take account of that.”

One definition of a planet to be put forward in Prague is anything in orbit around the Sun with a diameter of more than 2,000km — about 1,250 miles. This arbitrary limit would retain Pluto (diameter 1,422 miles) as a planet and add Xena but would exclude others.

Another possible definition would be any heavenly body orbiting the Sun with a “rounded shape” — increasing the tally of planets to at least 23 and making Brown the greatest planet finder of all time, with at least eight to his name.

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