The Battle Of Prague

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f there's still someone out there who thinks science and politics never mix, the story behind the Battle of Prague should change your mind.

Some have cast the debate that took place in the Czech capital during the summer of 2006 as a battle against American scientists who wanted to keep the only planet discovered by an American on an unreasonably high pedestal. On the other side of the argument, there are those who suspect that the rest of the world wanted to see Pluto demoted to punish America for its unpopular foreign policy.1

But we're not talking about that kind of politics. We're not even talking about a battle between the fans and foes of Pluto per se. Instead of thinking in terms of Republicans versus Democrats, or Plutophiles versus Plutoclasts, you have to think in terms of planetary conservatives versus liberals—or, more accurately, dynamicists versus geophysicists. The skirmishes over the definition of planethood that took place in Prague weren't so much about poor little Pluto, but about two different ways of seeing the solar system.

One way focuses on the dynamics of a planetary system: How are things moving around, and how do those things affect one another? If a celestial body doesn't have much of a gravitational effect on other bodies, that object is hard to detect and hard to track. If lots of celestial bodies are in similar orbits, they all tend to blur together.

Pluto may be the solar system's brightest object beyond Neptune, as seen from Earth.2 It may account for as much as 7 percent of the entire mass of the Kuiper Belt, a ring-shaped region that covers more real estate than the space inside Neptune's orbit.3 But because there are lots of other objects in the Kuiper Belt, dynamicists see a crowded celestial neighborhood in which Pluto doesn't stand out.

Much of what astronomers have learned about the solar system since William Herschel's day has come to light because of dynamical analysis. This is how Le Verrier and Couch found Neptune. It is how Clyde Tombaugh could figure out how far away Pluto was, even though he saw it as a mere speck of light. And seventy-five years later, it is how Mike Brown identified Xena, the dynamical blip that was farther away and bigger than Pluto. So you can't really sell the dynamicists short.

Another way of looking at a celestial body would be to look at it rather than around it. What's it made of? What kinds of geological processes are at work? Does it have a crust and a core? Is there an atmosphere, and weather? Are there volcanoes, and if so, what are they spewing out? Water? Sulfur? Methane?

Such a world doesn't have to be a planet to be of interest. In fact, some of the most interesting worlds nowadays aren't planets, but moons. The Saturnian moon Enceladus is just 300 miles wide, far smaller than Pluto's diameter of 1,430 miles, but it boasts geysers that could conceivably be spewing life-laden water.

This is the province of the planetary scientists—a breed of astronomers who focus on the way a world is put together. As a rule of thumb, if it's big enough to crush itself into a round shape due to self-gravity, it's big enough to be a planet. If it's not big enough to get round, it's a failed planet, taking on the potato or peanut shape normally associated with asteroids or comets. "These objects that we call planets have shaped themselves into spheres," said Alan Stern, the planetary scientist who worked for seventeen years to get a probe sent to Pluto.

The significance of the shape isn' t merely that a round object makes for a pretty, planetlike picture. Rather, the important thing is that such a degree of self-gravity makes it possible for a planet to have a layered composition, an active geology, perhaps even volcanic activity beneath the surface, or an atmosphere above. "It's about the physics," Stern said.

Stern likes to talk of a Star Trek test for planethood: "The Starship Enterprise shows up at a given body, they turn on the cameras on the bridge and they see it. Captain Kirk and Spock could look at it and they could say, 'That's a star, that's a planet, that's a comet.' They could tell the difference."

Roundness would provide an instant way for Mr. Spock to tell. In contrast, Stern said, having to determine whether the round thing was one object among others at the same orbital distance would force Spock to put Kirk's question on hold: "We have to make a complete census of the solar system, feed that into a computer, and do numerical integrations to determine which objects have cleared their zone."

For dynamicists, roundness just doesn't cut it. If Kirk and Spock are looking at a point of light from tens of AU away, as Clyde Tombaugh did in 1930, they might not be able to tell if the object they're looking at is round. But by closely monitoring its motion, and the motion of other bodies, they could figure out where everything fits in a planetary system—even if it takes sixty or seventy years, as in the case of Pluto and the Kuiper Belt. "We dynamicists know all about the orbits and can say what's going on," Brian Marsden said, "but the physical people can't say a damn thing."

This back-and-forth between the dynamicists and the geophysicists was what stymied the initial efforts to resolve its planet problem. Whenever the question was considered by the nineteen members of the International Astronomical Union's Working Group on the Definition of a Planet, one faction would essentially filibuster the other. "Achieving a consensus among them was about as hard as trying to herd a group of 19 feral cats into a room with several open doors and windows," said Alan Boss, an astronomer at the Carnegie Institution of Washington who was a member of the panel.4

In addition to the scientific differences, there was a cultural split as well, having more to do with language than physics: Should the planets of the solar system be a category so special that you can count their number on two hands, or would it be okay if the category was open-ended, with the potential of adding tens or hundreds or thousands of members?

For planetary conservatives, the idea of recognizing even thirty or fifty planets in the solar system was just too much. The liberals, however, were fine with having hundreds of planets. You could break that category down into subcategories: giants like Jupiter, terrestrials like Earth, and dwarfs like Pluto. And even if you had scores of planets, you wouldn't have to force kids to memorize them all, just as you don' t force them to memorize all the world's rivers or mountains.

All these issues—the scientific as well as the cultural considerations—were dropped into the lap of a brand-new panel set up by the IAU in preparation for the Battle of Prague. This seven-member panel included five astronomers who were familiar with the issues but not counted among the leading Plutophiles or the Plutoclasts: MIT's Richard Binzel, the Université Denis Diderot's André Brahic, Junichi Watanabe from the National Astronomical Observatory of Japan, Iwan Williams from Queen Mary University of London, and the IAU's president-elect, Catherine Cesarsky. Another member was science writer Dava Sobel, the author of Longitude, Galileo's Daughter, and The Planets. The chairman was Owen Gingerich, an astronomer and historian who worked alongside Brian Marsden at the Harvard-Smithsonian Center for Astrophysics.

In April 2006, the committee was told to come up with a definition of planethood in time for the IAU's triennial general assembly that August, and to keep its deliberations secret, to avoid the kind of sniping that had stymied past efforts.

Gingerich tried to avoid dwelling on the particulars of Pluto's case. "We never asked who wanted Pluto in or out," he said. But the ground rules favored an approach that would lean more toward the geophysicists than the dynamicists. "We wanted to avoid arbitrary cutoffs simply based on distances, periods, magnitudes, or neighboring objects," he said.

After flurries of e-mails, the panel met in person to hash out their decision in June at the Paris Observatory, where Le Verrier had once worked to calculate Neptune's orbit. According to Gingerich, it didn't start out smoothly. "On the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus," he reported. "But by the end of a long day, the miracle had happened: we had reached a unanimous agreement."5

The resulting definition emphasized Stern's roundness requirement, but also distinguished between the solar system's

"classical planets"—that is, the planets identified before 1900—and the "plutons" in the Kuiper Belt. Any world that orbited the sun and had a roundish shape due to its self-gravity, a state known as hydrostatic equilibrium, would fit under the definition of a planet.

But what if the planet's shape couldn't be seen in detail? In that case, there was a rule of thumb based on estimated diameter and mass: Objects at least 800 kilometers wide with masses of at least 5 X 1020 kilograms, or about 4 percent of Pluto's mass, would be brought into the planet fold, with borderline cases decided as further observations became available. That would put Pluto as well as Xena in the pigeonhole for planets, along with the eight bigger planets and smaller Ceres, the rocky world that was hailed as a planet in 1801 but reclassified as an asteroid decades later.

And what about Charon? Pluto's moon is nearly half as big as Pluto itself, and so, unlike every other planet, the two worlds actually orbit a common center of gravity in space, like two stars in a binary system. Some astronomers thought that would qualify Pluto and Charon as a binary-planet system, and that's what the earlier IAU panel on planethood had suggested in a footnote to their report.

"That footnote in the previous committee's report got stuck in without my quite realizing it," Gingerich said. It was one of several twists in the deliberations that he would come to regret.

Another twist had to do with the hush-hush nature of the panel's work. The IAU's Executive Committee insisted that the resolution be kept secret until the Prague meeting began. "It worked out that keeping it secret, in effect, backfired," Gingerich said. Word that Pluto would stay in the planetary fold leaked out a few days before the Prague meeting—and although the members of the panel thought their proposal would be widely accepted, others had grave doubts.

Boss recalled the tempests he and his colleagues had weathered during past discussions of the planethood issue. In an interview with the journal Nature, he predicted that a definition based on roundness would be met with "a long line of people waiting for the microphone to denounce it." And he was right.

More than twenty-four hundred astronomers converged on Prague for the XXVIth IAU General Assembly, which opened on August 14 and was due to last until August 25. Hundreds of panels and poster sessions were on the schedule, but none drew as much public interest as the deliberations over the definition of a planet.

On the third day of the gathering, the IAU finally published the resolution for all to see. The organizers put together a package of scientific documentation, including an explanation of the process that led to the definition, but it quickly became clear that not enough attention had been paid to the political spin.

Gingerich said his committee was "blindsided" when a press spokesman asked how many planets should be included in the proposed list. The number added up to twelve, including Pluto, Xena, Ceres, and Pluto's now problematic moon, Charon. "As soon as they started the planet-counting, that's when it all fell apart," Gingerich said.

Daniel Fischer, a German science writer who was working for the IAU's conference newspaper, was immediately struck by how poorly the organizers were stating their case. He recalls asking Gingerich to point out Ceres on a poster displaying the new twelve-planet lineup. "He looked and looked, and couldn't [find it]," Fischer said. "It was really an embarrassing moment when he couldn't find his own planet because there was dust on the poster."

Further embarrassments cropped up: The generic name that the panel had proposed for any dwarf planet found beyond Neptune—"pluton"—turned out to be already taken: Pluton was the name for Pluto in French, and it was also a geological term for a type of igneous rock. Also, the idea that Pluto and Charon could both be planets threw many astronomers for a loop. By that standard, our own moon, which has been slowly but steadily moving away from Earth over the course of millions of years, might someday have to be promoted to double-planethood.6

But the biggest political faux pas was that the dynamicists in the IAU felt slighted. They looked at the proposal and saw no reference to the issues they held most important: how much of an effect one object had on other objects in a planetary system, and how dominant an object was in the orbit that it traced. Some of them had devoted their entire careers to tracing those orbits.

Within a couple of days after the IAU panel's resolution was released, the opposition had written up a resolution of its own, one that reserved planethood only for celestial bodies that were by far the largest objects in their local populations. In a weird and fateful twist of language, Pluto, Xena, and Ceres could be called dwarf planets—but they wouldn't be considered "real" planets.

The opposition, led by Julio Fernandez, the Uruguayan astronomer who had played a part in figuring out the dynamics of the Kuiper Belt years earlier, rounded up some supporting signatures and presented its draft at a previously scheduled meeting of the IAU's Division III members, the astronomers who were most deeply involved in planetary science. In an informal show of hands, the opposition's draft won out handily over the resolution crafted by Gingerich's group.

Meanwhile, Gingerich and the IAU leadership scrambled to fix the most glaring problems in their own resolution. They got rid of the references to plutons. They wrote in an explicit distinction between the eight "classical planets" and the smaller "dwarf planets," such as Ceres and Pluto. They made clear that this definition applied only to our solar system, and not to the growing number of extrasolar planets. And they tried to fine-tune the criterion for a double-planet system.

The revised resolution looked as if it was stitched together by Dr. Frankenstein, but it served as the starting point for an open forum set for August 22, just three days before the end of the conference. The biggest practical difference between Gingerich's language and the opposition's was linguistic: Would dwarf planets be considered a type of planet or not? The stage was set for the IAU's showdown over that very issue.

When the IAU's outgoing president, Australian astronomer Ronald Ekers, called the lunchtime session to order on August 22, he reminded his colleagues that the planethood question was too big to decide by science alone. "This is not just a scientific issue of what is correct," he said. "There is no correct answer to this question. The question is, what is a sensible compromise that will work? And not just work for the professionals in the field, but work for everybody who is interested in the skies, the planets, is curious, is educating, and so on."7

Gingerich explained that his panel decided to "use a physical definition without an arbitrary cutoff"—that is, roundness. "It would be letting nature make the decision about what was a planet or not," he said.

When it was Junichi Watanabe's turn at the podium, he acknowledged the "extreme opinion that planets should be restricted to just eight bodies," and said he had heard the criticism that having an open-ended list of planets would be too complex for teachers to explain.

"Our solar system is already scientifically complex," he said in response. "This is a very good example to show how science is making rapid progress."

Then it was Richard Binzel's turn. He pointed out that the dividing line between planets and stars was drawn on the basis of an object's self-gravity: If the object's gravity was powerful enough to start a nuclear fusion dynamo going in its core, then it was considered a brown dwarf or a star rather than a planet. That dividing line came when the object was thirteen times as massive as Jupiter.

"Because the upper end of planets should be defined by gravity, so should the lower end," Binzel said. That's why the panel decided to draw the dividing line where an object was massive enough to crush itself into a rough sphere.

To demonstrate, Binzel used a couple of visual aids. In one hand, he held a lump of clay molded into a sphere. In the other, he held a squishy squeeze of clay. "At the lowest end, you can teach young students that this is round, it's a planet—and this is not," he said. "That's the most simple, teachable example of what we mean by the new definition of 'planet.'"

He acknowledged that there'd be a gray area where astronomers couldn't be sure how the roundness standard might apply. In that case, Division III could set the standard, perhaps based on the object's brightness.

" In summary," he said, " you can vote based on physical principle, that physics is a good way to define a planet. Or maybe you have some preconceived notion of what a planet should be. This is exactly the thing that we've been wrestling with in Division III and in our committee. Our recommendation is that you decide based on physics."

Those were fighting words for the opposition—and Italian dynamicist Andrea Milani threw the first verbal punch when Ekers opened the floor for discussion. Milani complained that the initial resolution didn' t give any consideration to planetary dynamics, which was "the historically most important contribution of astronomy to modern science." He said the resolution would wrongly bring Ceres back into the planetary fold, more than a century after astronomers took it out of the solar system's top lineup. Most of all, he took offense at the way the IAU's resolution was being presented as the most scientifically sensible option.

"I would like to note that the two speakers who have spoken so far have both done the same extremely insulting gaffe," he said. "They have used the expression 'a physical definition of a planet' —by implication, suggesting that a dynamical definition is not physics!"

He said he felt he had to teach the panel "something you should know": that dynamics was indeed physics, and in fact was addressed before solid-state physics in every textbook ever written.

Milani acknowledged that the revised resolution did include more of a reference to the dynamics of the solar system but said it was too little too late. "You are perpetuating a kind of, let's say, offense to the entire dynamical astronomy community," he said.

Fernandez felt similarly hurt. He noted that he was about to become the president of the IAU's Commission 20, which focuses on the positions and motions of minor planets, comets and satellites.

"It's a pity that, occupying so seemingly high a position in the IAU, I only learned about the proposal by the Executive Committee when I arrived here, not before," he said. Then he noted that "there is an important group of people that disagree with the Executive Committee [and] have been working on an alternative proposal." He wanted assurances that the alternative would get equal treatment when it came time for a vote.

Ekers said he wanted to avoid having dueling definitions. "We would certainly rather find a compromise, rather than vote on two resolutions," he said.

As speaker after speaker came to the microphone, it was clear that a compromise would have to be struck. Some said that the resolution didn't address extrasolar planets. Some said it would be impossible to draw the line between dwarf planets and smaller objects. One complained that "inflation in the number of planets deflates, in some sense, the value of our major planets"—as if designating planets were like giving out high school grades.

Even Mike Brown, who had discovered Xena and at one point said he'd be okay with calling it a new planet, sent in an e-mail siding with the opposition—a message that was read out from the audience. "I don't know who is leading the charge, but tell them for me: I will be very happy to have my name attached to the list of supporters of the only reasonable proposal I have seen so far," Brown wrote.

The irritation in Ekers's voice rose as the objections piled up. But in the end, he was resigned to the fact that the IAU resolution was not going to fly, even in its revised form. A show of hands at the end of the meeting confirmed that impression. Ekers scheduled another meeting with the opposition, later in the day, to iron out the new wording.

"They are in control of things," Gingerich said when the session was over.8

Like many of the astronomers at the general assembly, Gingerich wasn't due to stay until the very end. He had airplane tickets to leave Prague on the day after the tide turned, so he missed out on the battle's final skirmishes. "Had I been there, I would have worked out a compromise," he said.

The revised definition, known as Resolution 5A, set aside any pretentions of proposing a universal definition for planethood. Instead, its scope was explicitly limited to our own solar system. Three conditions were laid out: (1) A planet had to orbit the sun; (2) it had to be big enough to crush itself into a roundish shape—that is, it had to be in a state of hydrostatic equilibrium; and (3) it had to have cleared the neighborhood around its orbit of other objects.

That third condition was the key one for the dynamicists. If a celestial body satisfied the first two conditions, but not the third, it would be considered a "dwarf-planet," complete with quotes and a hyphen. Some astronomers thought that would make clear that Pluto and the other dwarf planets were not, in fact, planets—even though the quotes and hyphen instantly fell by the wayside in popular usage.

The resolution's backers also thought the phrase "clearing the neighborhood" would be more easily understandable than talking about dominant orbital objects. But that was the part of the definition that caused the most trouble for the general public, as well as for some of the astronomers left in Prague. Could Neptune really be said to have cleared out its orbit if Pluto and the other plutinos were still buzzing back and forth? Some were even willing to argue that Pluto had done a fine job of clearing out its orbit—as evidenced by the way it survived its smash-up with the protoplanet that spawned Charon, and by the way it settled into an orbit that kept it so far away from Neptune.

To close off that argument, a footnote was added declaring that there were eight planets in the solar system, and another resolution—Resolution 6A—was drawn up stating specifically that Pluto was a "dwarf-planet" (again with the quotes and the hyphen).

Even though Gingerich had left the meeting, he made one last stab at getting some more respect for dwarf planets, by suggesting a follow-up Resolution 5B that would revive the term "classical planets" for the eight roundish things circling the sun that managed to clear out their neighborhoods. If 5B passed, that resolution could have been interpreted as recognizing that dwarf planets are really planets, too—just as our sun and other dwarf stars are really stars.

In contrast with the raucous session two days earlier, the meeting to vote on the final resolutions was surprisingly sedate. One reason for that was that the number of attendees had dwindled dramatically, to about four hundred. Another was that the clash earlier in the week had made clear who had the upper hand. As is the case with most showdowns, the outcome in Prague was determined well before the final tally was taken.

Resolution 5A, which set the solar system's planet count at eight in perpetuity, as far as the IAU was concerned, was approved overwhelmingly. To illustrate the potential impact of the follow-up Resolution 5B, one of the IAU's officers, Jocelyn Bell Burnell, brought out her own batch of visual aids: a blue balloon that represented the eight planets, a box of cereal for Ceres and the asteroid belt, and a plush Pluto toy to stand for Pluto and the Kuiper Belt. Then she placed an umbrella labeled "PLANETS" over the three props. Voting for 5B would keep the box of cereal and the Pluto toy under the planetary umbrella, while voting it down would leave them uncovered.

This vote was engineered to proceed without discussion, except for a short statement by Binzel in favor and by the Armagh Observatory's Mark Bailey against. Ninety-one voted in favor of keeping Pluto and the box of Crunchy Crisp cereal under the umbrella, but far more raised their hands and their yellow voting cards to keep Pluto out. Ekers didn't even bother to have the "no" votes counted. One of the session's biggest rounds of applause came when he ruled that Resolution 5B had failed, meaning that the IAU would no longer count Pluto as a planet.

Jocelyn Bell Burnell uses props to explain the potential impact of the vote on one of the resolutions considered at the IAU General Assembly in 2006. The balloon represents the eight biggest planets in the solar system, the stuffed doll stands for Pluto and its celestial kin, and the cereal box represents Ceres. Should all three classes of objects be covered by the umbrella provided by the word "planet"?

Jocelyn Bell Burnell uses props to explain the potential impact of the vote on one of the resolutions considered at the IAU General Assembly in 2006. The balloon represents the eight biggest planets in the solar system, the stuffed doll stands for Pluto and its celestial kin, and the cereal box represents Ceres. Should all three classes of objects be covered by the umbrella provided by the word "planet"?

One loose end was tied up when the astronomers voted overwhelmingly to affirm Pluto's status as a dwarf planet and the "prototype for a class of trans-Neptunian objects." But the IAU's members couldn't agree on what to call that class: The planet definition panel's suggestion, "plutonian objects," went down to defeat by the narrow margin of 183 to 186.

Brian Marsden said he voted against the suggestion only because there was no recognition of a similar class of bodies in the main asteroid belt, which he thought could be called "cerean objects," in honor of the newly designated dwarf planet Ceres. If that change had been made, he would have voted for the resolution—and he might have persuaded one other voter to do so as well, reversing the outcome.

Nearly everyone who was there, even those on the winning side, would agree that the Battle of Prague was not the IAU 's finest hour. "I think things were rather badly handled," Marsden admitted. But at least he could take comfort in the way things turned out. After twenty-six years of lobbying, he finally saw Pluto taken off the list of planets. "All things considered, we did the right thing," he said.

Astronomers vote on the resolution defining the word "planet" at the lAU's General Assembly in Prague in 2006.

His office mate, Owen Gingerich, wasn't so sure. "I realize in retrospect," he wrote later, "that the IAU should never have attempted to define the word planet. It is too culturally bound, with elastic definitions that have evolved throughout the ages. What the IAU could legitimately have done in its role of naming things was to have defined some subclasses, such as 'classical planets,' leaving the planetary door open not only for plutonians and cereans but for the exoplanets as well. These terms would be eminently teachable and would help students understand the complexity and richness of the solar system that modern science is revealing. And astronomers could have left Prague without muddle on their faces."9

Most of the astronomers in Prague felt they had to approve something, even if the process or the result was flawed. "It would be disastrous for astronomy if we come away from the General Assembly with nothing," the Royal Astronomical Society's Michael Rowan-Robinson told his colleagues just before the final votes. "We will be regarded as complete idiots."

So, once the votes were taken, sighs of relief could be heard in scientific circles around the world. "Now we can move on and get on with life, not argue over what is really not the major issue it's been blown up to be," said Fran Bagenal, a planetary scientist at the University of Colorado.10

Whether they agreed with the outcome or not, many in the scientific and educational community thought the decades-long debate over Pluto was finally over. How wrong they were.

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