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Pluto is safe as astronomers define planet size

Posted on: Wednesday, 16 August 2006, 13:27 CDT

By Alan Crosby

PRAGUE (Reuters) - The question of whether Pluto is a real planet, hotly debated by scientists for decades, came to a head on Wednesday when the global astronomers' body proposed a definition of a planet that raises their number to 12 from nine.

The definition set out by a committee of the International Astronomers Union (IAU) answers the key question: How small can a body be and still be called a planet? in a way that leaves Pluto's status intact -- but modified.

Some 2,500 astronomers and scientists from round the world, attending an IAU conference in the Czech capital, have to weigh the committee's two-part definition, on which IAU members will vote on August 24.

To be called a planet, a celestial body must be in orbit around a star while not itself being a star, and must be large enough in mass for its own gravity to pull it into a nearly spherical shape, the seven-member committee said.

The need to define, for the first time, what it takes to be a planet stems from technological advances that enable astronomers to look further into space and to measure more precisely the size of celestial bodies in our solar system.

Pluto would remain a planet but would fall into a newly created category called Plutons, which are distinguished from classical planets in that they take longer than 200 years to orbit the sun.

Pluto would be joined in this category by two other celestial bodies, Xena and Charon, while another, Ceres, would be known as a dwarf planet.

In all, 12 planets would be listed in our solar system, at least for the time being.

"Had astronomers realized in 1930 that Pluto was smaller than our Moon and with a mass well under one percent that of the Earth, perhaps some special designation would have been devised for it," said Owen Gingerich, head of the committee.

Debate over Pluto's status intensified in 2003 when astronomers at the California Institute of Technology discovered

UB 313.

Nicknamed Xena after the warrior princess in the television show, UB 313 is one of more than a dozen celestial bodies in our solar system found to be larger than Pluto.

"Did our committee think of everything, including extra-solar system planets? Definitely not. Science is an active enterprise, constantly bringing new surprises," Gingerich said.

"Undoubtedly some future IAU committee will have to revisit this question and define the upper limit for "planet," probably well before 2106."

Source: REUTERS

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