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Pluto Now Called a Plutoid

2008-06-12 06:28:30

Robert Roy Britt

Senior Science Writer

SPACE.comWed Jun 11, 11:49 AM ET

Updated 11:36 a.m. ET

The International Astronomical Union has decided on the term "plutoid" as a name for dwarf planets like Pluto.

Sidestepping concerns of many astronomers worldwide, the IAU's decision, at a meeting of its Executive Committee in Oslo, comes almost two years after it stripped Pluto of its planethood and introduced the term "dwarf planets" for Pluto and other small round objects that often travel highly elliptical paths around the sun in the far reaches of the solar system.

The name plutoid was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III and by the IAU Working Group for Planetary System Nomenclature (WGPSN), and approved by the IAU Executive Committee at its recent meeting in Oslo, according to a statement released today.

Here's the official new definition:

"Plutoids are celestial bodies in orbit around the sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit."

In short: small round things beyond Neptune that orbit the sun and have lots of rocky neighbors.

The two known and named plutoids are Pluto and Eris, the IAU stated. The organization expects more plutoids will be found.

Controversy continues

Already the IAU recognizes it is adding to an ongoing controversy.

The IAU has been responsible for naming planetary bodies and their satellites since the early 1900s. Its decision in 2006 to demote Pluto was highly controversial, with some astronomers saying simply that they would not heed it and questioning the IAU's validity as a governing body.

"The IAU is a democratic organization, thus open to comments and criticism of any kind," IAU General Secretary Karel A. van der Hucht told SPACE.com by email today. "Given the history of the issue, we will probably never reach a complete consensus."

It remains to be seen whether astronomers will use the new term.

"My guess is that no one is going to much use this term, though perhaps I'm wrong," said Caltech astronomer Mike Brown, who has led the discovery of several objects in the outer solar system, including Eris. "But I don't think that this will be because it is controversial, just not particularly necessary."

Brown was unaware of the new definition until the IAU announced it today.

"Back when the term 'pluton' was nixed they said they would come up with another one," Brown said. "So I guess they finally did."

More debate coming

The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt between Mars and Jupiter, according to the IAU. Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind, the IAU stated. Therefore, a separate category of Ceres-like dwarf planets will not be proposed at this time, the reasoning goes.

A meeting, planned earlier this year for Aug. 14-16 at Johns Hopkins University Applied Physics Laboratory, aims to bring astronomers of varying viewpoints together to discuss the controversy. "No votes will be taken at this conference to put specific objects in or out of the family of planets," APL's Dr. Hal Weaver, a conference organizer, said in a statement in May. "But we will have advocates of the IAU definition and proponents of alternative definitions presenting their cases."

The term plutoid joins a host of other odd words -- plutinos, centaurs, cubewanos and EKOs -- that astronomers use to define objects in the outer solar system.

Why Planets Will Never Be Defined The History of the Pluto Controversy Gallery: Our New Solar System Original Story: Pluto Now Called a Plutoid