August 16, 2006

Pluto wins reprieve but number of planets may rise

By Alan Crosby

PRAGUE -- Learning the planets of our solar system is about to become more difficult for school children around the world.

The International Astronomers Union (IAU) put forward a definition of a planet on Wednesday that will expand the number from nine to 12, and create different categories of planets in a nod to technological advances that allow astronomers to look deeper into space.

In defining for the first time what exactly a planet is, a seven member committee of the IAU has offered some 2,500 astronomers and scientists gathered from around the world a chance to answer the key question: Does celestial size matter?

"Had astronomers realized in 1930 that Pluto was smaller than our Moon and with a mass well under 1 percent that of the Earth, perhaps some special designation would have been devised for it," said Owen Gingerich, head of the committee.

Debate has raged within the scientific community over the status of Pluto for decades.

It intensified in 2003 when astronomers at the Cal Tech discovered UB 313. Nicknamed Xena after the character 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.

Under the definition put forward on Wednesday, and subject to ratification by a vote of IAU members on August 24, an object must be in orbit around a star, while not being itself a star.

Secondly, the object must be large enough in mass for its own gravity to pull it into a nearly spherical shape.

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

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.

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

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