Demoted Pluto now a plutoid

2008-06-12 07:25

Seth Borenstein

Washington - Pluto is finally getting its day in the sun, after being stripped of planetary status by astronomers two years ago.

From now on all similar distant bodies in the solar system will be called "plutoids". That's the decision by the International Astronomical Union, which met last week in Oslo, Norway, and announced the decision on Wednesday.

The same group raised a cosmic fuss when it demoted the once-ninth planet to "dwarf" status in 2006. The new policy allows Pluto to be the standard for a whole new category of dwarf planets.

Pluto is one of only two plutoids, the other being Eris. Both are objects that circle the sun and are too small to be considered planets, but big enough to have a level of gravity that keeps them in a near spherical shape. Plutoids also must be farther from the sun than Neptune.

It was the 2003 discovery of Eris - a body bigger and farther from the sun than Pluto - that eventually led to Pluto's demotion. But the astronomers expect more plutoids to be discovered in the future.

When Pluto was demoted, the astronomical union always planned on naming the new category of objects after the former planet, but had to find the right name, said IAU president Catherine Cesarsky, a French astrophysicist. Their first choice, pluton, was already used by geologists.

The astronomers' action makes Pluto more important, Cesarsky said. Instead of being a "puny" outer planet, Pluto is now a "prototype of a new type of fascinating objects", she said.

"It doesn't really roll off the tongue very well," said Mike Brown, the California Institute of Technology astronomer who discovered Eris. "Maybe it'll make it."

It was not enough to satisfy leading Pluto-as-a-planet advocate Alan Stern, a former Nasa space sciences chief and principal investigator on a mission to Pluto. Stern said a rival group could be formed to the IAU, which he said was too secretive in its decision-making.

"It's just some people in a smoke-filled room who dreamed it up," Stern said. "Plutoids or haemorrhoids, whatever they call it. This is irrelevant."

Another Pluto supporter was at least partially pleased.

"It's going in the right direction," laughed Ralph McNutt, a planetary scientist at Johns Hopkins University. "I'd still rather have it just be known as a planet."

"I grew up with nine planets, I'm sorry," McNutt said.

For those of you keeping score at home, the solar system now stands at: Planets 8, Plutoids 2.

- SAPA