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".