Astronomers attending the International Astronomical Union's (IAU) general assembly in Prague are due to vote on whether to distinguish between planets and smaller objects such as comets and asteroids.

The result could be a 12-planet solar system comprising eight classical planets such as Earth and three "plutons", a new category that refers to Pluto-like objects and includes Ceres, which was a planet in the 1800s before it was demoted to an asteroid.

The debate over what actually constitutes a planet has been caused by the discovery of large objects in the outer regions of the solar system.

"These discoveries have rightfully called into question whether or not they should be considered as new 'planets'," Ron Ekers, the president of the IAU, said in a statement on Wednesday.

Celestial line-up

Since its founding in 1919, the IAU has been the arbiter in astronomical debates and after two years of work a committee has come up with a new planet definition to present to some 2,500 astronomers gathered in Prague.

According to the draft definition, a "planet" must be in orbit around a star, while not being itself a star, and it must be massive enough for its own gravity to pull it into a nearly spherical shape.

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

Richard Binzel, IAU member

The new planetary line-up would include 2003 UB313, the farthest-known object in the solar system and nicknamed Xena, as well as Pluto's largest moon, Charon and the asteroid Ceres.

The panel also proposed a new category of planets called "plutons", referring to Pluto-like objects that reside in the Kuiper Belt, a little-explored disc-shaped zone beyond Neptune containing thousands of comets and planetary objects.

Pluto itself and two of the potential newcomers - Charon and 2003 UB313 - would be plutons.

Further candidates

UB313, found some 15 billion kilometres from Earth, ignited a huge row as Pluto's defenders said UB313 was not a planet, just a rock, or KBO - Kuiper Belt Object.

Richard Binzel, a member of the defining committee, said: "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

That puts about a dozen "candidate planets" on the IAU watchlist, which means that even more planets may be named in the future.

The new "plutons" are distinguished from classical planets in  that they reside in orbits around the sun that take longer than 200 years to complete, meaning that they are in orbit beyond Neptune.

The draft "planet definition" resolution will be discussed and refined during the remainder of the assembly and put to a vote on August 24.