Vote Makes It Official: Pluto Isn’t What It Used to Be
By DENNIS OVERBYE
Published: August 25, 2006
Throw away the place mats. Redraw the classroom charts. Take a pair of scissors to the solar system mobile.
After years of wrangling and a week of debate, astronomers voted for a sweeping reclassification of the solar system. In what many of them described as a triumph of science over sentiment, Pluto was demoted to the status of a “dwarf planet.”
In the new solar system as defined by the International Astronomical Union, meeting in Prague, there are eight planets instead of nine, at least three dwarf planets and tens of thousands of so-called smaller solar system bodies, like comets and most asteroids.
For now, the other dwarf planets are Ceres, the largest asteroid, and an object known as 2003 UB 313, nicknamed Xena, that is larger than Pluto and, like it, orbits beyond Neptune in a zone of icy debris known as the Kuiper Belt. But there are dozens more potential dwarf planets known in that zone, planetary scientists say, and so the number in the category could quickly swell.
In a nod to Pluto’s fans, the astronomers declared it to be the prototype for a new category of such “trans-Neptunian” objects, but declined in a close vote to approve the name “plutonians” for them.
The outcome yesterday completed a stunning turnaround from only a week ago, when the assembled astronomers were presented a proposal that would have increased the number of planets in the solar system to 12, retaining Pluto and adding Ceres, Xena and even Pluto’s moon Charon.
The reversal, said Dr. Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington, speaks to the integrity of the planet defining process.
“The officers were willing to change their resolution,” Dr. Boss said, “and find something that would stand up under the highest scientific scrutiny and be approved.”
Jay M. Pasachoff, a Williams College astronomer who attended the Prague congress and favored somehow keeping Pluto a planet, said, “The spirit of the meeting was of future discovery and activity in science rather than any respect for the past.”
Mike Brown of the California Institute of Technology, who discovered UB 313 three years ago and so had the most to lose personally from the downgrading of Pluto and Xena, said he was relieved.
“Through this whole crazy circuslike procedure, somehow the right answer was stumbled on,” Dr. Brown said. “It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved.”
It had long been clear that Pluto, discovered in 1930, stood apart from the previously discovered planets. Not only is it much smaller — only about 1,600 miles in diameter, smaller than the Moon — but its elongated orbit is tilted with respect to the other planets, and it goes inside the orbit of Neptune on part of its 248-year journey around the Sun.
Pluto, some astronomers had argued, made a better match with the other ice balls that have since been discovered in the dark realms beyond Neptune. In 2000, when the Rose Center for Earth and Space opened at the American Museum of Natural History, Pluto was denoted in a display as a Kuiper Belt object and not a planet.
In the decision yesterday as to what constitutes a planet, astronomers voted by standing and holding up yellow cards. In the crucial vote, the result was sufficiently one-sided that no formal count was taken.
Under the new rules, a planet must meet three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The last of these criteria knocks out Pluto and Xena, which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.
Dwarf planets, on the other hand, need only orbit the Sun and be round.
“I think this is something we can all get used to as we find more Pluto-like objects in the outer solar system,” Dr. Pasachoff said.
The final voting was by some 400 to 500 of the 2,400 astronomers who registered for the congress; many others had already left.
Pointing to the very small fraction of the world’s astronomers who had been in Prague and thus eligible to vote, Alan Stern, lead investigator for New Horizons, NASA’s mission to Pluto, called the resolution “laughable.” Dr. Stern, of the Southwest Research Institute in Boulder, Colo., pointed out that both Earth and Jupiter have asteroids in their neighborhoods.
“This is so scientifically sloppy and internally inconsistent,” he wrote in an e-mail message, “that it is embarrassing.”
This is not the first time that astronomers have rethought a planet. The asteroid Ceres was hailed as the eighth planet when it was discovered in 1801 by Giovanni Piazzi, floating between Mars and Jupiter. But the subsequent discovery of more and more things like it in the same part of space led astronomers to dub them asteroids.
Although many astronomers watched the vote on the Internet, Neil deGrasse Tyson of the Rose Center said he had not bothered.
“Counting planets is not an interesting exercise to me,” Dr. Tyson said. “I’m happy however they choose to define it. It doesn’t really make any difference to me.”
Far more compelling, he added, are aspects of planets like weather, ring systems and magnetic fields.
Dr. Tyson said a continuing preoccupation with what the public and schoolchildren would think about this was a concern and a troubling precedent.
“I don’t know any other science that says about its frontier, ‘I wonder what the public thinks,’ ” he said. “The frontier should move in whatever way it needs to move.”
- Copyright 2006 New York Times
