When Is a Planet Not a Planet?

Arguments for and against demoting Pluto

Astronomical dispute

Pluto has been shrinking at an alarming rate for some time now. Well, not actually shrinking—rather, our awareness of how small Pluto is has been growing. Upon its discovery, in 1930, scientists trumpeted that Pluto was about as large as Earth. By the 1960s textbooks were listing it as having a diameter about half that of Earth. In 1978 astronomers discovered that Pluto has a relatively large moon, whose brightness had been mistakenly lumped in with the planet's; when this was taken into account, Pluto was left with a diameter about a sixth that of Earth, or less than half that of Mercury—long considered the runt of the solar system. Seven moons in the solar system are bigger than Pluto.

In addition to being out of place among the planets in terms of size, Pluto has always seemed conceptually lost as well. The four innermost planets are rocky and of modest size; the next four are gas giants. What was a lone, tiny ice ball doing way out at the edge of the solar system? A surprising answer has emerged over the past few years. Pluto, it turns out, is one of at least sixty, and possibly hundreds of thousands of, small, cometlike objects in a belt that extends far beyond the confines of the planets.

This discovery, while providing a scientifically satisfying answer to a long-standing mystery, has also raised a question that has proved to be painful for many astronomers: Is Pluto truly a planet? A growing number of solar-system scientists assert that Pluto's minuteness and its membership in a swarm of like objects mean that it should be classified a "minor planet," as asteroids and comets are. Others are outraged by the idea, insisting that regardless of how its identity has changed, demoting Pluto would dishonor astronomical history and confuse the public. In the end, the debate boils down to this question: What compromises in precision should scientists make in the name of tradition, sentiment, and good public relations?

Leading the assault on Pluto's planethood is Brian Marsden, who for thirty years has been the director of the Central Bureau for Astronomical Telegrams of the International Astronomical Union (IAU), the worldwide nexus for comet and asteroid sightings. Marsden appears to be an unlikely agent of change. In his small office in Cambridge, Massachusetts, he is surrounded by so many teetering piles of paper that he seems to be not sitting in the office so much as embedded in it. Marsden is short and ruddy-faced; his chin and neck are lost to rotundity, and his wispy white hair is arranged in the style of an explosion. But as he makes his case against planet Pluto, his initially soft, stammering, English-accented rumble becomes the resonant boom of an exasperated lecturer. "Pluto has been a long-standing myth that's difficult to kill," he says.

Marsden doesn't have anything against Pluto itself. Quite the contrary: a life spent trying to calculate the complex orbits of the tiniest celestial objects has left him with a fondness for the one planet whose location a hundred years from now cannot be precisely predicted. However, people wouldn't find Pluto's history of planethood quite so deserving of celebration, Marsden says, if they were aware of the circumstances under which that planethood was obtained.

The famous "search for Planet X," which culminated in Pluto's discovery, was the pet project of Percival Lowell, a Boston Brahmin and amateur astronomer who around the turn of the century became obsessed with two notions: that Martians had constructed canals on the surface of their planet, and that tiny, gravity-induced wiggles in the orbits of Uranus and Neptune indicated that a planet with a mass some six times that of Earth lay farther out. Lowell built and endowed an impressive observatory in Flagstaff, Arizona, to prove himself right, but he died in 1916 without having succeeded on either count. The observatory's directors, aware that their institution was something of a laughingstock because of the Martian search, were determined to salvage its reputation by finding the at least marginally less improbable Planet X. They hired a young amateur astronomer by the name of Clyde Tombaugh to do the grunt work involved. Tombaugh proved to be resourceful and diligent beyond all reasonable expectations, and more or less single-handedly picked dim Pluto out of a thick field of stars—a feat that is still considered one of the most impressive in the history of observational astronomy.

Though some leading astronomers of the day, along with Tombaugh himself, suspected from the beginning that the newly discovered object was not the massive Planet X of Lowell's fancy, the observatory directors launched a public-relations blitz designed to link the two inextricably in the public's mind, and also minimized the opportunities for other astronomers to gather contradictory evidence. The observatory withheld news of the discovery for nearly a month, until the seventy-fifth anniversary of Lowell's birth—even while its orbit was carrying Pluto away from clear observability. After the announcement the observatory refused for six weeks more to release the details needed to find Pluto. Lowell's name and prediction were plastered all over the observatory's press releases, though Tombaugh was barely mentioned, and only in later releases. After briefly considering naming the new object for Lowell or his widow, the observatory chose the name Pluto, largely because a symbol for it—PL—could be fashioned out of Percival Lowell's initials.

In the ensuing euphoria over the apparent discovery of a new planet (elation was especially pronounced in the United States, where the public was happy to welcome the first "American" planet to the solar system), those voices that questioned Pluto's size were drowned out, and the IAU awarded Pluto official planet status. It wasn't an entirely unreasoned decision. As the observatory argued, Pluto had been found close to where Lowell had predicted Planet X ought to be if it was causing those orbital wiggles (though Tombaugh, skeptical of Lowell's predictions, hadn't focused his search on that area), so it would be quite a coincidence if this new object wasn't the enormous Planet X. Besides, if the object was small, it shouldn't have been visible at all so far away from Earth.

Unless, that is, this new object happened to have a highly reflective icy surface, like that of a comet—which eventually proved to be the case. Pluto is much too small to account for the wiggles on which Lowell had based his predictions. Not that that matters, for there were no wiggles—the observations that had implied them were erroneous. And even if there had been wiggles, they probably wouldn't have led astronomers to Planet X, because Lowell's calculations were dubious at best. It was sheer coincidence that Pluto happened to be at the predicted spot. And so it was on a staircase of mistakes, hubris, and hype that Pluto was elevated to planethood.

Marsden has publicly questioned Pluto's planethood for nearly two decades. His case picked up steam in the early 1990s, when the astronomers David Jewitt, of the University of Hawaii, and Jane Luu, of Harvard, began spotting cometlike objects just beyond Neptune's orbit. Some followed paths almost exactly like Pluto's highly elliptical and oddly angled loop—an orbit vastly different from those of the eight other planets. Marsden was convinced that the new objects finally explained Pluto's niche in the solar system. "It all fell into place," he says. "Pluto has more in common with comets than it does with planets."

Jewitt and Luu have now discovered sixty objects in what has come to be known as the Kuiper Belt, named after the astronomer Gerard Kuiper, who suggested the existence of such a belt in the 1950s. About a third of them are in Pluto-like orbits, and all of them appear to be, like Pluto, amalgams of ice and rock. As a result, few astronomers now question that Pluto should be regarded as a member of the Kuiper Belt. But does that mean it shouldn't be considered a planet? After all, though Pluto's diameter of approximately 1,400 miles makes it tiny for a planet, it is huge for a Kuiper Belt object; the next largest known member is only about 300 miles across.

Actually, Marsden points out, astronomers have had to ask themselves this sort of question before, and under circumstances that could be seen as clearly establishing a precedent for Pluto. When the small, rocky body later named Ceres was discovered between the orbits of Mars and Jupiter, in 1801, it was proclaimed a planet. A year later a second rocky body was found in a similar orbit; several other discoveries along the same lines soon followed. Even though the approximately 600-mile-wide Ceres is nearly twice the size of the next largest asteroid, it was evident that it was merely the largest member of what we now call the asteroid belt. In 1802 Ceres's planethood was summarily revoked.

Marsden sees little ambiguity in the situation: Pluto should follow Ceres's trail into nonplanethood. "If you are going to call Pluto a planet, there is no reason why you cannot call Ceres a planet," he says. And no one, he points out, is campaigning to have Ceres's planethood restored.

It would help if "planet" had a formal definition against which Pluto could be measured, but none exists. Astronomy got by quite nicely for thousands of years on a we-know-one-when-we-see-one basis. But now that questions about Pluto are forcing the issue, many astronomers find themselves gravitating toward one or the other of two proposed definitions. The first is "a non-moon, sun-orbiting body large enough to have gravitationally 'swept out' almost everything else near its orbit." Among the nine planets Pluto alone fails this test, and it does so spectacularly, owing to the Kuiper Belt. The second is "a non-moon, sun-orbiting body large enough to have gravitationally pulled itself into a roughly spherical shape." Pluto passes this test—but so do Ceres, a half dozen or so other asteroids, and possibly some other members of the Kuiper Belt. "It's very difficult to come up with a physically meaningful definition under which we'd have nine planets," says Hal Levison, an astronomer at the Southwest Research Institute, in Boulder, Colorado.

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.