Originally created 08/25/06

Pluto demoted as astronomers approve new definition for planets

PRAGUE, Czech Republic - For decades, it's been confused with a cartoon dog and ridiculed as a puny poser. Now Pluto, the solar system's consummate cling-on, has suffered its worst humiliation: It's not even a planet anymore.

After a tumultuous week of clashing over the essence of the cosmos, leading astronomers Thursday stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is - and isn't - a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.

The historic vote by the International Astronomical Union officially shrinks Earth's neighborhood from the traditional nine planets to eight.

But the scientists made clear they're as sentimental as anyone else about the ninth rock from the sun.

Jocelyn Bell Burnell - a specialist in neutron stars from Northern Ireland who oversaw the proceedings in Prague - urged those who might be "quite disappointed" to look on the bright side.

"It could be argued that we are creating an umbrella called 'planet' under which the dwarf planets exist," she said, drawing laughter by waving a stuffed Pluto of Walt Disney fame beneath a real umbrella. Later, she hugged the doll as she stood at the dais.

"Many more Plutos wait to be discovered," added Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology.

The decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.

For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Much-maligned Pluto - named for the God of the underworld - doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets." The definition also lays out a third class of lesser objects that orbit the sun - "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.

Experts said there could be dozens of dwarf planets catalogued across the solar system in the next few years - handing the world's school teachers a challenge.

Neil Crumpton, a science teacher at Mountfitchet High School in Stansted Mountfitchet, north of London, called the announcement "very exciting."

"To be honest, this has been brewing for a while. Pluto has always been a bone of contention among astronomers because of the odd way it orbits the sun," Crumpton said. "For a start, we'll have to change all the mnemonics we use to teach children the lineup of the planets. But Pluto has not disappeared and it doesn't hurt children to know about it."

NASA said Thursday that Pluto's demotion would not affect its $700 million New Horizons spacecraft mission, which earlier this year began a 9½-year journey to the oddball object to unearth more of its secrets.

"We will continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized," Paul Hertz, chief scientist for the science mission directorate, said in a statement.

The decision at a conference of 2,500 astronomers from 75 countries was a dramatic shift from just a week ago, when the group's leaders floated a proposal that would have reaffirmed Pluto's planetary status and made planets of its largest moon and two other objects.

That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto's undoing. In the end, only about 300 astronomers cast ballots.

Now, two of the objects that at one point were cruising toward possible full-fledged planethood will join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto whose discoverer, Michael Brown of the California Institute of Technology, has nicknamed "Xena."

Charon, the largest of Pluto's three moons, is no longer under consideration for any special designation.

Brown was pleased by the decision. He had argued that Pluto and similar bodies didn't deserve planet status, saying that would "take the magic out of the solar system."

"UB313 is the largest dwarf planet. That's kind of cool," he said.

But as it all sank in, he added: "Deep down inside, I know this is the right thing to do. It's sad. As of today, I have no longer discovered a planet."


AP Science Writer Alicia Chang in Los Angeles contributed to this story.

Text of resolutions that define a planet - and demote Pluto

Text of the resolutions approved Thursday by the International Astronomical Union, which define planets and downgrade Pluto:


Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets." The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

The IAU therefore resolves that "planets" and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A "planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as "Small Solar-System Bodies."

The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

On the Net:

International Astronomical Union, www.iau.org


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