Planet count may reach 12 or more
The solar system may soon be home to a dozen planets, with three new additions to the club and more to come, if astronomers approve a new definition of a planet.
The proposal before the 26th general assembly of the International Astronomical Union (IAU) in Prague distinguishes between planets and smaller celestial bodies such as comets and asteroids.
It leaves Pluto's status as a planet intact, but modified.
The proposed new definition of a planet would result in a 12-planet solar system with eight classical planets; three plutons, a new growing category of Pluto-like objects, including Pluto, 2003 UB313 (dubbed Xena) and Charon; and Ceres, a former asteroid.
Powerful new telescopes that have discovered large objects in the outer regions of the solar system present a challenge to the historically based definition of a planet, which comes from the Greek word meaning wanderer.
"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," says IAU president Professor Ron Ekers.
"These discoveries have rightfully called into question whether or not they should be considered as new planets."
The planetary debate blasted off in July 2005 when a US team of astronomers announced that Pluto was much smaller than an enigmatic object, 2003 UB313, which its discoverers said was the solar system's 10th planet.
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, a Kuiper belt object.
A KBO is one of about 100,000 pieces of icy, primeval debris encircling the Sun on the outskirts of the solar system.
"Had astronomers realised in 1930 that Pluto was smaller than our Moon and with a mass well under 1% that of the Earth, perhaps some special designation would have been devised for it," say Professor Owen Gingerich, head of the IAU committee that came up with the proposed definition.
Since it was founded 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 2500 astronomers gathered in the Czech capital.
According to the draft definition, a planet must orbit 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.
"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," says Professor Richard Binzel, a member of the defining committee.
That puts about a dozen "candidate planets" on the IAU watchlist, which means even more planets could be named in the future.
If the current proposal is passed by the IAU assembly, the solar system would consist of the planets Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313 (a 'real' name to be chosen later).
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 they are in orbit beyond Neptune.
The draft planet definition will be discussed and refined during the meeting and put up for a vote on 24 August.