WASHINGTON—Sorry, kids. You'll no longer have to memorize the names of just nine planets. Apparently, there are now going to be 12.
A committee of the International Astronomical Union, which decides such matters, voted unanimously Tuesday to add three worlds to our solar system's planetary population. More planets will be added later, astronomers said.
Little Pluto, which had been in peril of losing its place among the planets, keeps its status, but only in a new category of "plutons," distant oddballs wandering outside Neptune in weirdly shaped orbits.
The IAU committee recognized two other plutons—Pluto's smaller companion, Charon, and Xena, an icy body bigger than Pluto that was discovered in 2003.
In addition, Ceres, the biggest asteroid between Mars and Jupiter, will regain the planetary status it enjoyed in the 19th century.
Thus the new roll call of planets, starting closest to the sun, would be: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and Xena. (Xena is officially known as UB313 until the IAU gets around to formally naming it.)
The lineup was recommended unanimously by a seven-person Planet Definition Committee created by the IAU after two years of heated controversy. The recommendation must be approved by the 3,000 astronomers attending an IAU conference in Prague, Czech Republic, on Aug. 24.
For years, some astronomers have argued that Pluto should be demoted because it's too small and peculiar to belong with the eight major planets.
Others wanted to keep Pluto as a planet and add a flock of "trans-Neptunian objects," like Xena, which are located in the so-called Kuiper Belt, a horde of thousands of icy lumps left over from the formation of the solar system.
The proposed new category of plutons is a compromise.
"In July, we had vigorous discussions of both the scientific and culture-historical issues," said Owen Gingerich, a professor of astronomy and the history of science at Harvard University and chairman of the IAU Definition Committee.
"On the second morning, several members admitted that they had not slept well, worrying that we would not be able to reach a consensus. But by the end of a long day, the miracle had happened. We had reached a unanimous agreement."
Stripped of technical jargon, the proposed definition sets two conditions for a body to be named a planet: First, it must orbit a star, but not be a star. Second, it must be round, unlike most asteroids, comets and meteoroids.
To meet those conditions, a planetary candidate has to be big enough so that its gravity pulls it into a nearly spherical shape. That means it must be at least 500 miles wide, the committee said.
At 580 miles, Ceres makes the grade. So do the plutons: Charon at 780 miles, Pluto at 1,430 miles and Xena at about 1,864 miles. By comparison, our moon is 2,160 miles across.
Plutons are distinguished from the other major planets by their distance—they take longer than 200 years to circle the sun—and by their eccentric, tilted orbits.
The IAU maintains a watchlist of a dozen "candidate planets" that could be added as plutons in coming years.
The wrangling over Pluto, which was discovered only 76 years ago, may not be settled by its designation as a pluton, however.
Michael Brown, a planetary astronomer at the California Institute of Technology in Pasadena and the discoverer of Xena, doesn't like the revised definition of a planet.
There are already 53 round objects larger than 500 miles across that would qualify as plutons, he said. The number is growing rapidly as he and other scientists detect more objects in the Kuiper Belt.
"Instead of nine or 12, there will be a couple hundred `planets' in the next few years," Brown said. "The simplest solution is the one that makes most people cringe: Admit that we made a mistake in 1930 by calling Pluto a planet."
But Alan Stern, director of the NASA mission that will reach Pluto in 2015, agreed that Pluto, Charon and Xena should be called planets. "A Chihuahua may be small, but it's still a dog," he said.
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(c) 2006, McClatchy-Tribune Information Services.
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