THE solar system's biggest planets are at risk of being stripped of their status after the world's top astronomers voted to downgrade Pluto into the dwarf planet category.
Experts claim that the definition for planets adopted by the International Astronomical Union (IAU) could also see Neptune downgraded.
They say that both planets fail to meet all the criteria set by the IAU.
But other leading astronomers insi
st the decision to demote Pluto to the status of a "dwarf planet" has allowed them to deal with the difficult problem that there are several other Pluto-like objects orbiting our Sun.
Earlier this month the IAU proposed increasing the number of planets to 12 by recognising three new planets, the asteroid Ceres, Pluto's moon Charon and the distant world known as 2003 UB313.
Many scientists feared this could spark an ever-increasing number of planets being named as more and more smaller objects were discovered orbiting the Sun.
But following days of heated debate, scientists eventually took the decision to adopt guidelines that saw Pluto relegated to a secondary category of dwarf planets.
The vote has now left teachers and text book writers with the task of drastically revising the view of the solar system.
In the definition set by the astronomers, all planets must now "orbit the Sun, have sufficient mass for its gravity to form a nearly-round shape and have cleared the neighbourhood around its orbit".
Pluto was downgraded as its orbit crosses with that of Neptune, meaning that it was classed as a dwarf planet as it had not cleared its orbit.
But critics claim that the IAU vote, which was passed by a majority, should be abandoned as only around 400 of the union's 10,000 members took part in the vote in Prague last week.
Martin Hendry, a senior lecturer in astronomy at Glasgow University and member of the IAU, said: "Unless the science underlying this is rigorous, how can we expect to agree on a definition that will be not only understood by ourselves, but other forms of life if and when we encounter them?"
Planetologists have now started a campaign to have Pluto reinstated.
Astronomers leading the Nasa New Horizons mission that will explore the distant regions of the solar system including Pluto, described last week's decision as "muddled".
Harold Weaver, from the John Hopkins University Applied Physics Laboratory and a New Horizons project scientist, said: "Since many 'Plutinos', including Pluto, cross Neptune's orbit, I'd say Neptune's neighbourhood still needs some clearing."
But Professor John Brown, Scotland's Astronomer Royal, who was unable to vote due to work commitments, said: "I am a little sorry to see Pluto 'demoted', but overall it makes sense. We have always referred to four 'Terrestrial Planets', four 'Giants' and Pluto. This is now extended to three categories, adding Dwarf Planets."
