Science: Is Pluto truly a planet?

By David H. Freedman

THE planet 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, comet like 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 dishonour astronomical history and confuse the public.

Leading the assault on Pluto’s planet hood is Brian Marsden, the director of the Central Bureau for Astronomical Telegrams of the International Astronomical Union. 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.

The famous “search for Planet X,” which culminated in Pluto’s discovery, was the pet project of Percival Lowell, an 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 an impressive observatory to prove himself right, but he died in 1916 without having succeeded on either count. The observatory’s directors were determined to salvage its reputation by finding the at least marginally less improbable Planet X. They hired a young amateur astronomer, Clyde Tombaugh, to do the grunt work. 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.

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, In the ensuing euphoria over the apparent discovery of a new planet the voices that questioned Pluto’s size were drowned out, and the IAU awarded Pluto official planet status.

But Marsden has publicly questioned Pluto’s planet hood for nearly two decades. His case picked up steam in the early 1990s, when astronomers began spotting comet like objects some of which 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.”

Surely 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. Right now there are 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, 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 a half dozen or so other asteroids and possibly some other members of the Kuiper Belt.

Does it matter what we call Pluto? By calling Pluto a planet, Marsden 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 opens his case for Pluto by suggesting that it is too big to be called a minor planet. He then casts his vote for the “spherical” definition of a planet, conceding that some asteroids could sneak in under this rubric.

The person it seems to be most about is Clyde Tombaugh, who died early last year at the age of ninety. Levy owes his interest in astronomy to him. Later he became friendly with him and wrote his biography. “I promised him I would always argue in favour 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. 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.

But some of the newest astronomy textbooks 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, refers to Pluto as an “interplanetary body” in his newest books. “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

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.

Marsden believes, in fact, that this would be the best way for the change to happen. “I am in favour 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. —Atlantic

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