When Is a Planet Not a Planet?

Arguments for and against demoting Pluto

Does it matter what we call Pluto? Marsden insists that it does. By calling Pluto a planet, he says, astronomers perpetuate a distorted and thoroughly outdated image of a solar system that neatly ends in a ninth planet, rather than trailing off beyond Neptune into a far-reaching and richly populated field of objects. "It gives a misleading impression to the public and particularly to schoolchildren," he says. "We ought to be explaining that there are four giant planets, four terrestrial planets, two belts of minor bodies, and scattered interesting material."

At the forefront of those who reject such proposals is the observational astronomer and author David Levy. Levy is about as different from Marsden as two people who are longtime friends and equally esteemed in one field can be. Whereas the Oxford-educated Marsden spends most of his time working out the mathematics of orbits, rarely so much as peeking through a telescope, Levy has for years risen at 4:00 A.M. to nuzzle up to an eyepiece in the cold Arizona desert, hoping to spot new comets. (Among his twenty-one cometary discoveries is Shoemaker-Levy 9, which crashed spectacularly into Jupiter in 1994.) The low-key, amiable Levy is an astronomical Will Rogers to Marsden's excitable don.

Levy opens his case for Pluto by suggesting that it is too big to be called a minor planet, noting that if it rested on Kansas, it would block out the sun over more than half of the United States. He then casts his vote for the "spherical" definition of a planet, conceding that Ceres could sneak in under this rubric. However, Levy is not about to argue that Ceres should be reclassified. "Ceres has been an asteroid for a long time; it's easier to leave things as they are," he says. In any case, he adds, it's a mistake to frame the debate in terms of technical definitions, because something more important than precision is at stake. "This isn't about science, or things," he says. "It's about people."

The person it seems to be most about is Clyde Tombaugh, who died early last year at the age of ninety. Levy dates his own interest in astronomy back to an evening when he was twelve and his father described to him, over the dinner table, how Tombaugh, a twenty-four-year-old observatory assistant without a college degree and literally right off the farm, had carried out his brilliant search. Levy later became very friendly with Tombaugh, and wrote a biography of him. "I promised him I would always argue in favor of Pluto's remaining a planet,"he says. "Clyde is gone now, but his wife, Patsy, is still here, and I think changing Pluto's status would be extremely disrespectful."

It would also be disrespectful to the public, Levy argues, and to children in particular. "Kids like Pluto," he says. The story of Ceres's demotion isn't relevant, he claims, given that the public has never really thought of Ceres as a planet. Rather, he holds up the case of the brontosaurus. Early in the century paleontologists realized that the apatosaurus and the brontosaurus were actually the same creature; one of them had to go. Taxonomic convention dictates that the first-named species—in this case, the apatosaurus—subsume the second. And so it was that scientists dutifully declared the much-admired brontosaurus nonexistent. "Sure it's a rule, but every kid knows what a brontosaurus is," Levy says. "Why couldn't they have made an exception?" The brontosaurus has, of course, proved unsinkable in common usage. If astronomers ignore Pluto's place in popular culture, Levy warns, then popular culture could ignore them.

Levy further argues that the PR debacle that could result from a demotion of Pluto might well turn around and bite astronomers. One of his biggest concerns is that NASA would decide not to fund the Pluto Express, a proposed unmanned fly-by mission to Pluto, under consideration for launching around 2003. To Plutophiles, this mission is the ne plus ultra of scientific discovery; there are things we simply cannot know about an object until we go right on up to it. "I think that downgrading Pluto would kill the mission," Levy says. (For the record, NASA's director, Daniel Goldin, denies that Pluto's planethood or lack thereof would have any effect whatsoever on NASA's plans, and adds that Pluto the Kuiper Belt object is still fundamentally interesting.)

Fortunately for Levy and other supporters of planet Pluto, the Norwegian astronomer Kaare Aksnes is firmly among their number. Aksnes heads the IAU's Working Group for Planetary System Nomenclature, which has the power to force a vote on the question among astronomers worldwide—the only possible route to an official downgrading. Aksnes says he has no interest in putting the matter before the committee—like Levy, he believes that reclassifying Pluto would be a disservice to Tombaugh and to history—and that even if the subject does come up, he has already ensured that it won't garner enough votes to make it out of committee.

But there is reason for Marsden and his like-minded colleagues to take heart as well. There's nothing in the rules to stop the IAU from officially adding the designation "minor planet" to Pluto's label, so that Pluto would enjoy a sort of celestial dual citizenship. In fact, there's some precedent for this: asteroid No. 2060 also became comet 95P when it was observed to have grown a cometary tail. More important, the chairman of the IAU's committee on small bodies' names, the University of Maryland astronomer Michael A'Hearn, is amenable to the idea. Marsden himself is the committee member responsible for assigning minor planets the numbers by which they are officially known, and he has one already picked out for Pluto: asteroid No. 10,000. The committee can only make recommendations, however, and if it does recommend the added label, it can expect a fight when the vote goes before the full body of the IAU. That's because some planet-Pluto boosters regard the dual-citizenship plan the way the National Rifle Association regards proposals to ban assault rifles—as a superficially reasonable step that must be fought tooth and nail because it would give the enemy momentum.

In any case, Pluto doesn't need an official ruling to move into minor planethood. It could happen on a de facto basis, and it probably will. If textbooks begin to present Pluto as a minor planet, the teachers who use those books will almost certainly follow, along with their students. And unlike the generally conservative astronomy community, the textbook industry tends to favor new points of view—if it didn't, there wouldn't be so much need for new textbooks.

Some of the newest astronomy textbooks, in fact, are already openly questioning Pluto's status. William Hartmann, the author of one of the most popular series of astronomy textbooks in the United States, and a recent winner of the American Astronomical Society's Carl Sagan medal for communicating planetary science to the public, refers to Pluto as an "interplanetary body" in his newest books. Though he continues to include Pluto in his table of the planets, to avoid making teachers' lesson plans obsolete, he feels increasingly uncomfortable about doing so. "It's very important in the grand scheme of things for human beings to be able to picture the rest of the universe in the right conceptual terms," he says. "The way we organize things in our heads comes from the names we give those things, and that's particularly important to remember as we teach those names to the next generation."

Many teachers aren't waiting for textbooks to make the jump. I was surprised to learn recently that my twelve-year-old daughter already knows about the debate—it turns out that critically examining Pluto's planetary status is now part of the sixth-grade science curriculum in my town. Dozens of World Wide Web pages address the issue, many of them put up by teachers and students. The inevitable result is that more and more children will grow up thinking of Pluto as the biggest member of a band of icy objects beyond the outermost planet, Neptune. If enough of them become astronomers, the IAU will follow.

Marsden believes, in fact, that this would be the best way for the change to happen. "I am in favor of a more natural evolution, without imposing any edicts," he says.

That seems a reasonable compromise. Even if science can't afford to bend to sentiment, there may be no harm in letting the sentiment die a natural death.

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David H. Freedman is a contributing editor of Forbes ASAP and Discover magazine.

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