A vote at the International Astronomical Union meeting in Prague has expelled Pluto from the planet club. Pluto will now join a a new catgory - the dwarf planets
The move resolves a long standing problem: althoughwhen first discovered it was claimed to be several times larger than Earth, ensuring its prompt labelling, it soon turned out to be smaller than the moon. Astronomers turned a blind eye to Pluto's status until last year when Mike Brown, an astronomer at the California Institute of Technology, discovered 2003 UB313 (Xena) which is bigger than Pluot and claimed it as the 10th planet.
The discovery of Xena brought the definition of the word "planet", to a head as it dawned on scientists that tens of other rocks hurtling around the sun could also qualify for the title. As a result of this vote, the IAU now defines a planet as a body that orbits the sun, is so large its own gravity makes it roughly spherical, and, crucially, also dominates its region of the solar system.
This definition excludesPluto because it is not big enough to clear smaller bodies close to it. Pluto, along with Xena and Ceres, an asteroid that lies between Mars and Jupiter, are now officially dwarf planets. The IAU's decision represents a U-turn on a proposal which would have seen the solar system expand to 12 planets, with four, including Pluto, being classed as new objects called plutons. The proposalwas voted down because it was still too vague.
In addition to the categories of "planet" and "dwarf planet", the definition creates a third category to encompass all other objects, except satellites, to be known as small solar system bodies.
The astronomer Patrick Moore said: "They've made it far too complex. What is a dwarf planet? I agree that Pluto is not a planet, but why not just call it a Kuiper belt object or a large planetoid? In the end, I don't suppose it matters too much. It's just a name."