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.

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

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