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Pluto illustration
The new IAU definition reclassifies Pluto (illustrated above with its largest moon, Charon, as seen from one of its newly-discovered smaller moons) as a “dwarf planet”, a decision that has raised the hackles of many in the astronomical community and the general public. (credit: NASA, ESA, and G. Bacon (STScI))

Demote Pluto, or demote “planet”?

Who would have thought that such a small planet—er, dwarf planet—could create such a huge fuss? When the International Astronomical Union (IAU) decided in a vote Thursday at the end of its triennial general assembly in Prague on a series of definitions that, in effect, demoted Pluto from the rank of “planet” to the newly-created designation of “dwarf planet”, it set off a firestorm of discussion and controversy. From newspaper columns to the blogosphere to late-night television, it was hard to escape the news about the IAU’s decision, and the debate about whether it had made the right call.

Most of that debate has focused on whether Pluto should retain its long-standing designation of planet, an argument that is often oversimplified to one of science versus sentimentality. What’s been missing from most of the debate, though, is a more significant question: just how useful is the designation “planet” given our current understanding of the composition of the solar system? There’s a strong case that the IAU, instead of trying to come up with a rigorous definition of the term planet, might have been better off abandoning the name altogether in favor of more precise terminology that better reflects the nature of our celestial neighborhood.

Public passion

IAU meetings don’t attract a lot of media or public attention; even many astronomers skip the meetings, saving their travel money for more topical conferences. This year’s meeting in Prague, though, was different, and solely because of question of Pluto’s status. The initial proposal by an IAU working group, one that would have retained Pluto as a planet and bestowed that designation on three other objects, including Pluto’s largest moon, Charon, got a fair share of media attention when it was unveiled a week before the final vote. However, when the IAU made its final decision Thursday, the floodgates opened.

From newspaper columns to the blogosphere to late-night television, it was hard to escape the news about the IAU’s decision, and the debate about whether it had made the right call.

Since Thursday’s vote, I have found over 200 unique articles and editorials about the decision by newspapers and other publications; this includes an article about “The New Solar System” that is the cover story for this week’s issue of Newsweek. The emphasis here is on unique: most newspapers, especially smaller ones that don’t have a science writer on staff, will simply run a wire story about a major scientific event, unless there’s a major local tie-in. However, in many cases local newspapers ran their own stories about Pluto’s demotion, augmenting the basic story by talking to local students, professors, museum curators, or even that old standby, the man-on-the-street interview. A random sampling of such articles includes:

News of Pluto’s demotion swiftly spread into general popular culture, becoming a metaphor as a fall from grace or simply a convenient punch line. Even sports columnists, who would seem to be among the least likely to pick up on the latest developments in astronomy, joined the chorus: a Boston Globe columnist drew parallels between Pluto and Red Sox slugger Manny Ramirez (“Planet Manny operates in his own orbit and hits baseballs into outer space. He's certainly no dwarf planet like Pluto.”) while one for The Oregonian compared Pluto to professional women’s basketball (“Sure, you never really heard anyone gush about Pluto, but there was no reason to actively dislike it, either. It was kind of the WNBA of the solar system.”)

The Oregonian writer got at least one thing wrong, though: you have heard people gush about Pluto, at least in the last several days. The reaction to the IAU’s decision has been surprisingly strong and deep. The Bakersfield Californian reported on how students at one elementary school voted overwhelmingly to keep Pluto a planet. A spokesperson for the Chabot Space and Science Center in Oakland, California, told a reporter that there has been a “sadness” among visitors after the IAU decision. In Boston, commuters at a suburban train station that is home to a scale model of Pluto (part of a spawling solar system model based at the Museum of Science downtown) passed judgment on the decision, with one calling it “bizarre”. Regardless of whether people agreed or disagreed with the IAU’s decision, there was a level of passion rarely found among the general public when it comes to space matters.

What drives that passion? What’s remarkable about Pluto is that, even before this latest development, many people had ascribed to it human feelings, calling it a “lonely” world in the outer solar system and considering it something like the little planet that could. (The ultimate in this is a Baltimore Sun column “written” by Pluto itself in response to the IAU decision.) Now, anthropomorphizing inanimate objects is nothing new for humans: we do it all the time from items ranging from cars to computers, but less often to celestial objects. How often do people talk about Neptune’s feelings?

However, perhaps the biggest reason why the IAU’s decision got so much attention was that it changed one of the fundamental facts that everyone knows. The vast majority of people alive today have only known a solar system with nine planets, even if they were hard-pressed to name those nine. Yet suddenly, with little more than a stroke of a pen, that basic fact was suddenly changed. It would be like waking up to find out there were only 49 states in the Union, or that Snow White cavorted with only six dwarfs.

Making matters worse

One can understand how the IAU got itself into this mess. Before Thursday’s vote, there was, remarkably, no official definition of the term “planet”. Instead, astronomers applied what might be called a “Potter Stewart” philosophy for identifying planets: they knew a planet when they saw one. And, for most of human history, that approach worked pretty well, from the naked-eye planets known since antiquity to the planets in the outer solar system discovered since the late 18th century using telescopes.

Perhaps the biggest reason why the IAU’s decision got so much attention was that it changed one of the fundamental facts that everyone knows. It would be like waking up to find out there were only 49 states in the Union, or that Snow White cavorted with only six dwarfs.

However, that approach has broken down within the last decade or so as astronomers have discovered hundreds of small bodies—and a few not-so-small bodies—in the outer solar system. Some of these Kuiper Belt Objects appeared to be a significant fraction of the size of Pluto, raising the question of whether Pluto should remain classified as a planet, or if these new objects should also be considered planets. In one infamous move, the Hayden Planetarium in New York City unveiled a new exhibit about the solar system that excluded Pluto from the planets, a decision that generated considerable press at the time. In early 1999, in response to reports that the IAU was considering demoting Pluto, or at least giving it “dual citizenship” as a planet and a minor planet, the organization issued a statement denying such an effort was underway.

What was clear, though, was that it was time for a more formal, scientific definition of the term planet. The breaking point came with the discovery last year of object 2003 UB313, informally but popularly known as Xena. Observations of Xena have shown that it is at least slightly larger than Pluto. Thus, if Pluto is a planet, Xena should be one too; else, if Xena is not considered a planet, then neither should Pluto.

So how should one define the term planet? The committee that drafted the original proposal introduced at the IAU general assembly based it, in effect, on orbit and size. As long as the object was orbiting a star, wasn’t itself a star or orbiting another planet, and was massive enough that its internal self-gravity had brought it into hydrostatic equilibrium—that is, it had a “nearly round” shape—then it qualified to be a planet. A simple enough definition, one might argue, and one inclusive enough to encompass both Pluto and Xena, as well as the largest asteroid, Ceres, and Pluto’s moon, Charon (considered eligible for planet status since the center of mass of the Pluto-Charon system is between the two bodies).

What the IAU ultimately approved as a definition of a planet, though, added another criterion after days of reportedly contentious internal debate. In addition to the size and orbit characteristics of the original proposal, the final definition requires that a planet to have “cleared the neighborhood around its orbit”. If an object meets the size and orbit requirements, but has not been deemed to have cleared its neighborhood, it is instead a “dwarf planet”. (The IAU also tweaked the definitions to refer to only our solar system; extrasolar planets are instead judged by a separate definition that primarily considers whether the object is big enough to sustain fusion.) Under that definition Pluto, Ceres, and Xena are all classified as dwarf planets (the IAU statement makes no specific mention of Charon), leaving eight qualifier-free planets in the solar system.

So, case closed? A triumph for science? That’s what some have argued. In an editorial congratulating the IAU on its decision, the New Hampshire Union Leader said that “that there was some pressure on the astronomers to keep Pluto a planet for sentimental reasons. But the astronomers understood how silly that was. Science is science, not art or politics.” Or, as Richard McCray, a professor emeritus of astrophysics, told the Denver Post, “Scientists don't give edicts. They're in the business of trying to discover.”

Yet what the IAU decided Thursday qualifies very much as an edict, a decision made by only a few hundred of its members (those who had stayed to the final day of the meeting, when the vote on the resolution with the proposed definition took place). It’s also hard to argue that what the IAU decided qualified as “science” or “discovery”: we know no more about Pluto now than we did Wednesday. Instead, what the IAU performed was simple nomenclature, assigning names to objects.

The neighborhood-clearing requirement now makes a planet’s location as important as its size, if not more so: would Pluto or Xena qualify as planets if they were as large as, say, Mars?

And the nomenclature the IAU did come up with leaves something to be desired. What does “cleared the neighborhood around its orbit” really mean? Evidently it’s something Pluto hasn’t done but that the eight remaining planets have, despite the fact that, for example, the Earth’s orbit takes it through a population of small asteroids. Even the “nearly round” criterion that dates back to the original resolution leaves something to be desired: how round does an object have to be to qualify as “nearly round”? Never fear: according to the IAU resolution, “An IAU process will be established to assign borderline objects into either dwarf planet and other categories.” That sounds a little too much like science by committee.

It’s also hard to understand why, from a scientific standpoint, that a neighborhood-clearing criterion is needed. That requirement now makes a planet’s location as important as its size, if not more so: would Pluto or Xena qualify as planets if they were as large as, say, Mars? (Speaking of Mars, it’s awfully close to the asteroid belt. Hmmmm…) It may also mean that, early in the history of the solar system, there were no planets, despite the formation of a number of large bodies: with protoplanetary debris still floating around, it may have taken many millions of years for an object like Jupiter to have cleared the neighborhood of its orbit. In other words, for a time, Jupiter would have been classified as a dwarf planet.

A better solution?

Why the IAU added the neighborhood-clearing criterion to its definition of planet isn’t stated in the press releases accompanying the resolution. (This is in contrast to the press release used to unveil the original proposed definition, which explains in some detail why the specific criteria were chosen.) However, some people opposed the original proposal because it would have increased the number of planets to at least 12, and perhaps dozens more over time, something that seemed, if noting else, inelegant. The new definition not only reduces the number of planets to eight, it makes it difficult for that number to increase: virtually any future candidate in the Kuiper Belt would likely be disqualified on the same grounds as Pluto and Xena.

However, one can argue that the term “planet” itself has become something of an anachronism. It dates back to an era where, thanks to our ignorance, we had a much simpler model of the solar system: the Sun, the Moon, and some bodies wandering across the night sky—“planets”. Over time that model has been reshaped to some degree, but has largely held in place even as new objects, large and small, were discovered in the solar system.

Now, though, that approach, and that nomenclature, is straining under the weight of all the new objects that have been discovered in recent decades. What has become clear is that, rather than a discrete handful of large objects (planets) and a much larger number of smaller bodies (asteroids, comets, KBOs), we see something approaching a continuum of sizes, with the largest asteroids and KBOs having many of the same physical characteristics as the smallest “planets”. Thanks to the discovery of well over 100 extrasolar planets, we’re seeing a similar continuum of sizes on the large end as well, all the way up to the point where bodies start to support fusion and become classified as brown dwarfs rather than planets.

This begs for a reconsideration of the generic term “planet”. With the large population of objects found both in this solar system and elsewhere, it seems that the term itself has lost much of its original meaning, forcing astronomers to bend over backwards to craft definitions that result in roughly the number of such objects that we have come to expect historically. It’s convenient, but is it really that scientific?

If we’re going to raise the ire of the public by reclassifying the solar system, we might as well do it for a good cause, and with a strong scientific basis.

What if, though, the IAU had taken a different approach? Perhaps astronomers could have decided not to make a formal definition of the term “planet”, finding it to be too general and vague. Instead, the IAU could have adopted specific definitions for classes of planets, based on their size (using the hydrostatic equilibrium criterion, for example) and other key characteristics. One might imagine three broad classes of planets: “gas giant planets” for gaseous worlds like Jupiter, “terrestrial planets” for rocky worlds like the Earth, and “ice planets” for worlds like Pluto. Under such a system we would not have an eight- or nine-planet solar system, since “planet” alone would have no official meaning: instead we would have a solar system with four gas giant planets, four (or five, depending on how Ceres was classified) terrestrial planets, and several ice planets, including Pluto. (One could add up the number of three different types of planets to determine the total number of “planets” in the solar system, but such a figure would be greater than nine, and would lead right back to the issues surrounding the original IAU proposal for the definition of the term planet.)

The IAU chose not to take that route, at least this time around, leaving us with the mess we have today, the result of the collision of scientific discoveries about our solar system with the expectation from popular culture about the number of planets the solar system “should” have. Abandoning the generic term “planet” for several more specific classes might not seem very simple or elegant, but has the advantage of offering a better description of the nature of our solar system, and potentially other solar systems as well. Winning the general public over to such a system might be difficult, but as the last few days have shown, even a modest change like demoting Pluto will generate a strong reaction. If we’re going to raise the ire of the public by reclassifying the solar system, we might as well do it for a good cause, and with a strong scientific basis.