this weekHeavenly Bodies

August 11, 2004

The Pluto Controversy

Defining and counting planets

Everything I needed to know about astronomy I learned from folk music. Well, not quite…but isn’t that how people normally keep their fingers on the pulse of scientific progress? Several years ago I went to hear Christine Lavin in concert. This New York-based folk singer is perhaps best known as the composer of “Sensitive New Age Guys,” but she’s been recording funny and clever songs for over 20 years and is a delightfully entertaining performer. At this concert, she asked if there were any mathematicians in the audience. A few people raised their hands. Then she asked those with their hands up if by any chance one of them happened to be an astromathematician. One woman said that was indeed her profession. Christine was thrilled, because she had a song about an astromathematician and wanted to make sure someone in the audience could truly appreciate it. The song was called “Planet X,” and it recounted in great detail the story of Pluto—its discovery, naming, and the ongoing controversy over whether or not it should be considered a planet. It was brilliant, and also educational: I learned more about Pluto from that one song than I’d ever known before.

What most people know about Pluto is that it’s a tiny, cold planet on the outer edge of our solar system that shares its name with a Disney character and the Roman god of the underworld. Less well-known is the strange history of its discovery and its central role in a controversy over the very definition of a planet, the outcome of which could mean that there are not nine, but as few as eight or as many as dozens of planets orbiting the Sun.

Finding Mystery Planets
Let’s go back to the first half of the 19th century. At that time, there were seven known planets—Uranus (which actually didn’t have that official name until 1850) had been recognized as a planet since 1781. But astronomers studying the orbit of Uranus noticed something odd: it wasn’t quite the right shape. It was almost as though its orbit were being influenced by another mass of matter somewhere. After making detailed mathematical calculations, astronomers deduced that the presence of another, more distant planet could account for the irregularities, and they set about searching for it. In 1846, they finally located Neptune, whose mass, position, and orbit correctly explained what was happening to Uranus.

In the early 1900s, astromathematician Percival Lowell was studying Neptune’s orbit and realized that it, too, exhibited some strange anomalies. He suspected that a ninth planet, which he dubbed “Planet X,” was to blame, and he spent the last eight years of his life searching diligently for it. When he died in 1916, however, Planet X was still a mystery. Building on Lowell’s research, astronomer Clyde Tombaugh finally located the elusive planet in 1930. In keeping with the tradition of naming planets after the gods of Roman mythology, the name Pluto was chosen—but it was no coincidence that the planet’s abbreviation, PL, also formed Percival Lowell’s initials.

Revelation Number Nine
But in the decades that followed, astronomers began to notice some disturbing things about Pluto. For one thing, they determined that it was much too small to have produced the aberration in Neptune’s orbit that Lowell had found. It turns out that his calculations had been wrong—that there were no problems with Neptune’s orbit—so in a way the discovery of Pluto was a lucky mistake. But Pluto was in fact very small—smaller than our Moon. In addition, its orbit was significantly skewed from the plane of the other eight planets, and was also so elliptical in shape that it actually comes closer to the Sun than Neptune for part of each orbit. These facts in themselves needn’t have been a problem, except that a lot of other largish objects began to turn up in Pluto’s vicinity—and with similar orbital characteristics. Astronomers started to wonder whether Pluto should have been considered a planet at all; perhaps it was merely the largest of a bunch of rocks known variously as Kuiper Belt Objects (KBOs), Trans-Neptunian Objects (TNOs), or Plutinos.

The International Astronomical Union (IAU), the official body charged with naming and categorizing objects in space, had never bothered to come up with a definition for “planet,” since to that point it had seemed obvious. So astronomers debated the issue strenuously. Some wanted to define a planet as an object that orbits the sun independently and has a mass large enough that gravitational forces will shape it into a sphere. By this definition, Pluto is a planet—but then, so are several other objects, including Pluto’s moon, Charon, and the asteroid Ceres that orbits the sun between Mars and Jupiter. Others wanted to tighten the definition to exclude Pluto by saying that no object that’s part of a larger cluster or swarm of objects can be considered a planet.

A Planet by Any Other Name
The notion that Pluto could be “demoted” from planethood to a KBO, a “minor planet,” or even a comet, provoked a great deal of public outcry in the late 1990s. The essence of the argument was that everyone knows Pluto is a planet—this is what we’ve all been taught since we were kids—so it would be wrong for astronomers to go changing their minds on us. In response to such complaints, the IAU issued a press release reaffirming that they had no intention of referring to Pluto as anything other than a planet. This did not entirely put the issue to rest, however, as there are still astronomers pushing for the looser definition of “planet” to be adopted, thus causing a different problem for the schoolchildren of the world: memorizing more names.

Then there are the party-poopers—like me, for example—who say this whole debate is rather silly. Everyone agrees that there’s a round object out in space that we call “Pluto.” Nothing whatsoever about Pluto will change if it gets put on a “KBO” list instead of a “planet” list, just as nothing about tomatoes changed when we realized they should be on the “fruit” list instead of the “vegetable” list. Nor would it matter if we called it “Cindy.” It is what it is, and it will be just fine no matter what we think of it. There may be other large objects out there that are equally worthy of our attention and a catchy name, but none of them is likely to have the remotest impact on anyone’s life, whether literally or astrologically speaking. We can let this one go. But probably we won’t; NASA is working on a space probe called New Horizons, which is supposed to be launched in 2006 and arrive in the vicinity of Pluto in 2015. It will take pictures and tell us, at least, about the composition of Pluto’s surface and atmosphere. Regardless of the results, they’re likely to provoke another round of debate as to whether Pluto is a planet. That’s too bad, because what we should be debating is whether Pluto is really a dog. —Joe Kissell

UPDATE: New Horizons did indeed launch in January 2006, and it carried with it some ashes of Clyde Tombaugh, who died in 1997. The spacecraft’s fuel is, of course, plutonium dioxide. Meanwhile, an astronomical body named Xena has been found orbiting the Sun even farther away than Pluto, and since Xena is larger than Pluto, it would have to be considered a planet if Pluto is. The debate continues.

More Information about The Pluto Controversy…

A terrific (and frequently updated) overview of the Pluto Controversy is Pluto the Accidental Planet at JohnShepler.com. See also Pluto: The Planet That Used to Be by Jeffrey Terraciano at Wired News (Jan. 23, 2001).

Two articles by Robert Roy Britt at Space.com (both posted on February 27, 2003) involve Pluto. Race Against Time: Long Road to Pluto and Why We’re Going discusses the New Horizons mission to Pluto, while Controversial Proposal Would Boost Solar System’s Planet Tally to 12 is about the proposal to reduce the requirements for admission into the Planetary Club.

New Horizons is on its way; see New Horizons blasts off for Pluto in New Scientist.

To get detailed information on the definition of a planet, check out A Good Definition of the Word “Planet”: Mission Impossible? by Gibor Basri, UC Berkeley, at The Universe in a Classroom (see page 2 in particular). Also see Earth awaits verdict on new definition of ‘planet’ in New Scientist.

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You can learn much, much more about Pluto and its neighbors in Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System by Alan Stern and Jacqueline Mitton, Beyond Pluto by John Davies, or for kids, Clyde Tombaugh and the Search for Planet X by Margaret K. Wetterer.

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Christine Lavin’s song “Planet X” recounts the whole tale of the Pluto controversy in rhyme, and is also the only song I can think of that contains a complete URL in the lyrics. The song is on her Shining My Flashlight on the Moon CD. And the URL in question is http://dosxx.colorado.edu/plutohome.html—the Pluto Home Page at the University of Colorado. For more info on Christine Lavin, visit her home page.

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The B-52’s have a song called “Hallucinating Pluto” that does not have anything whatsoever to do with Pluto. It’s on their Time Capsule: Songs for a Future Generation album as well as a CD single (along with several remixes).

Is Pluto really a dog? Read a contrarian article in the Wikipedia.