Then there were 8

The International Astronomical Union decides on a new definition of a planet, one that no longer includes Pluto

August 25, 2006|By Michael Stroh | Michael Stroh,Sun reporter

Call it a mugging in the Milky Way.

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

The International Astronomical Union, astronomy's governing body, will now officially recognize as planets only the first eight orbs circling the sun. 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 residents.

FOR THE RECORD - An article in yesterday's editions of The Sun about the reordering of the solar system contained the wrong year for the discovery of Uranus. The planet was discovered in 1781 by British astronomer Sir William Herschel.
The Sun regrets the error.

The results of 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.

"I don't think you'll see this in textbooks," he added. "It's going to go away."

The demotion of Pluto, decided yesterday during the IAU's general meeting in Prague, capped a heated, two-year effort to settle on a definition for the term "planet," a word considered more of 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 round due to gravity, and "has cleared the neighborhood around its orbit."

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 wording of the definition, which only applies to objects within the solar system and not to the growing list of planet-like 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-sized orb that holds the record as the most distant object orbiting our 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 only 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 E. Brown of the California Institute of Technology in Pasadena.

Brown, who went to Cal Tech's campus at 5 a.m. yesterday morning to watch the live IAU vote over the Internet, 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.

He wasn't the only one with a stake in the decision. Consider the scientists and engineers at Hopkins' APL, which built and manages NASA's $700 million mission to Pluto called New Horizons.

Yesterday, mission managers dismissed any notion that the ruling would put a damper on the rendezvous. Their spacecraft, which lifted off in January, is scheduled to arrive at Pluto in 2015.

"Hurrah for Pluto, first dwarf planet to be visited by a NASA mission!" said Hopkins astronomer Richard Conn Henry.

The IAU decision could pose a challenge to schools and museums. Mary Thurlow, science coordinator at the Maryland State Department of Education, said it was unclear when the "new" solar system would show up in the department's curriculum guidelines.

But Thurlow, who taught science for more than 20 years in Baltimore County, said her colleagues aren't likely to waste much time. "This is a teachable moment: We're always finding new things, discovering new things. That's the big draw of science," she said.

Jim O'Leary, director of the Davis Planetarium at the Maryland Science Center, said he met with his staff soon after yesterday's vote to figure out what to do with the proliferation of Plutobilia around the museum.

O'Leary himself had been planning a new fall planetarium program called "Beyond the Nine Planets."

"New title, I guess," he said

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