The nine planets of the solar system are about to be transformed into 12.

The International Astronomical Union (IAU) is planning to add three new members to the exclusive club of large celestial objects orbiting our Sun.

Astronomers are about to vote on an official proposal to extend the definition of a planet to include at least three more objects that are known to be big enough to warrant planetary status.

It will mean that astronomy textbooks will have to be rewritten with the names Ceres, Charon and UB313 being added to the more familiar names of the classical planets.

At one point it was thought that Pluto - the smallest and most distant of the planets - would be kicked out of the club, but now it appears that it is welcomed as the prototype of a new class of smaller planets known as "plutons".

The International Astronomical Union, which has been the arbiter of planetary nomenclature since 1919, has received a new definition of a planet from a special committee of seven experts set up two years ago to adjudicate on the issue.

Ron Ekers, the president of the IAU, said the ancient description of a planet as an object that wanders against a backdrop of fixed stars is no longer valid in an age of advanced telescopes.

"Modern science provides muchmore knowledge than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars," Dr Ekers said.

"Recent new discoveries have been made of objects in the outer regions of our solar system that have sizes comparable to and larger than Pluto.

These discoveries have rightfully called into question whether or not they should be considered as new planets." The three new planets are Charon, once considered a moon of Pluto but now described as its double planet; Ceres, formerly known as an asteroid or minor planet; and UB313, an object that has yet to be given a formal name (although it has been nicknamed Xena), and which was only identified last year.

There are now eight "classical"planets, three "plutons", those planets that are similar in size to Pluto withextremely wide solar orbits, and theasteroid-like Ceres.

Experts sitting on IAU's planet definition committee - composed of astronomers, historians and writers - concluded that in future a planet should be defined as a celestial body that is big enough for its gravity field to form a near-spherical shape.

The object must also be in orbit around the Sun - or another star - but not as a satellite of another planet, which rules out the Moon and the larger moons of other planets.

"Our goal was to find a scientific basis for a new definition of 'planet', and we chose gravity as the determining factor," said Professor Richard Binzel, a planetary scientist and member of the definition committee.

"Nature decides whether or not an object is a planet.".

The new definition of a planet means that there are another dozen or two dozen other known objects in the solar system that may one day be included in the planetary club.

The seven-member definition committee convened in Paris in late June and early July, and its recommendations will now go to the IAU's general assembly which will vote on the resolution as its meeting in Prague this week.

Professor Owen Gingrich, the committee chairman, said the deliberations were long and hard, but in the end a consensus was reached.

"In July we had vigorous discussions of both the scientific and the cultural-historical issues and on the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus," Professor Gingrich said.

"But by the end of a long day, the miracle had happened - we had reached a unanimous agreement."The issue came to a head after it was discovered that UB313 was bigger than Pluto, which was discovered in 1930 and was only called a planet because it was originally thought to be as big as Earth.