New definition boots Pluto from exclusive planet club

Call it a mugging in the Milky Way.

In a stunning reversal, astronomers who were ready to expand the solar system's number of planets by three last week voted to shrink it Thursday instead, stripping Pluto of its status as a planet.

The International Astronomical Union (IAU), astronomy's governing body, will now officially recognize only eight planets in our solar system. Pluto, which was discovered in 1930 and survived several swipes at its stature over the years, has been given a new label: "dwarf planet," a separate and lesser category of solar-system resident.

The vote left some planetary scientists seething.

"It's bogus. I'm going to quote a new term: the 'Irrelevant Astronomical Union,' " said Alan Stern, a longtime Pluto researcher at the Southwest Research Institute in Boulder, Colo.

The demotion of Pluto, decided Thursday during the IAU's general meeting in Prague, Czech Republic, capped a heated, two-year effort to settle on a definition for "planet," a word considered to have more cultural than scientific importance.

Under the new rules, which passed overwhelmingly in a hand vote, a "planet" henceforth is any object that:

• Orbits the sun.

• Is big enough for its own gravity to compact it into a ball.

• Has "cleared the neighborhood around its orbit," meaning it is not surrounded by objects of similar size and characteristics.

The only objects that meet that test, according to the IAU, are the so-called classical planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The definition, which only applies to objects within the solar system and not to the growing list of planetlike orbs found circling other stars, didn't sit well with some scientists.

"What exactly is meant by a planet 'clearing its neighborhood?' " wondered astronomer Hal Weaver of Johns Hopkins University's Applied Physics Laboratory. Pluto, he notes, swings inside the path of Neptune for 20 of the 248 years it takes for the puny former planet to circle the sun.

"I'd say Neptune's neighborhood still needs some clearing!" Weaver added.

IAU officials said Pluto's odd orbit was the primary reason astronomers decided to downgrade it to dwarf-planet status.

In addition to Pluto, the members of the new "dwarf" group are Ceres, the largest asteroid in the solar system, and 2003 UB313, a recently discovered Pluto-size orb that holds the record as the most distant object orbiting the sun. (Charon, the largest of Pluto's three moons, remains a satellite and not a planet, as astronomers had initially proposed last week.)

Astronomers also voted to create a third class of objects — "small solar system bodies" — that applies to asteroids, comets and other natural satellites.

Many astronomers defended the solar system shake-up.

"The public is not going to like the fact that Pluto has just been kicked out. But scientifically, it's the right thing to do," said planetary astronomer Michael Brown of the California Institute of Technology in Pasadena, Calif.

Brown had more to lose than most of his peers: He discovered 2003 UB313, one of the new-planet candidates. Nicknamed "Xena" after the warrior princess of TV fame, the object inhabits a remote region of the solar system called the Kuiper Belt.

Brown tried to look on the bright side: "It's more fun now; finding a new planet will really mean something," he said.

Discovered in 1930 at Lowell Observatory in Arizona, Pluto was, until Thursday, only the third planet ever discovered by modern astronomers (the others were Neptune in 1846 and Uranus in 1871). It was the only planet discovered by an American, Clyde Tombaugh.

Textbook and encyclopedia publishers must now remap the solar system. Paul Kobasa, editor in chief of World Book Encyclopedia in Chicago, said the press run for the 2007 edition was shuffled because of the pending IAU decision.

Much of the story of Pluto has become folklore, including the unusual way it got its name. An 11-year-old girl from Oxford, England, Venetia Burney, suggested the name of the Roman god of the underworld because she was interested in mythology. It was a perfect fit for a planet shrouded in darkness.

Now in her late 80s, Venetia Phair said when reached by phone that she had been getting a number of calls about the IAU decision.

After more than 70 years of debate over the planet, she said she couldn't comment on it. "I'm too old," she said.

Material from the Los Angeles Times is included in this report.