The nine planets of the solar system are about to be transformed into 12.
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 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.
IAU's planet definition committee 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.
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.
The Pluto-Charon sytem will now be called a doube planet.