August 16, 2006

Pluto is safe as astronomers define planet size

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

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

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."