The Great Planet Debate

For Alan Stern, the problem with Prague wasn't merely about what happened to Pluto; it was also about what happened to the scientific process.

For decades, the International Astronomical Union had worked by consensus, ruling on matters that already had been largely settled in the scientific community. The flap over Pluto and planethood was different, however, because the astronomical establishment had to deal with a basic question on a time scale that didn't fit the usual schedule for scientific consensus: How do you name something when the very naming will change the status quo?

As long as nothing bigger than Pluto had been found in the Kuiper Belt, astronomers could choose either to go with the decades-old tradition of having nine planets or trash it as unscientific. "It was that 'ninthness' of Pluto that bothered us as much as anything," Brian Marsden said.

But when Mike Brown discovered Xena, that forced the issue. Someone had to decide whether something bigger than Pluto, something that was unarguably a major planet if Pluto was, should be officially named after a grand Roman god or a lesser-known deity instead. The International Astronomical Union weaved one way in secret, and then a different way when its members pushed back during the Prague meeting. But it never considered putting off its ruling, even though some astronomers begged the organization's leadership to do so.

What bothered Stern was that there was no opportunity to look at all sides of an issue central to planetary scientists. And it bothered him even more that many scientists thought one series of votes would settle the matter.1 "Science does not work the way the legal system works,' he said. "We didn't vote on relativity or quantum mechanics. We don' t vote on any scientific discovery, because it just doesn't work that way. . . . The IAU can vote that the sky is green, but that doesn't mean people will follow, because it's not."

The rift over the planet-or-not question continued long after the Battle of Prague. Eris's discoverer, Mike Brown, saw that as a bad thing. As far as he was concerned, revisiting the controversy over Pluto was like picking at a wound that should have been left to heal. "There are astronomers who want it to be a planet still, and they just keep ripping those scabs off whenever possible," he said.2

But Stern thought the initial operation had been botched so badly that the subject had to be left open for discussion, at least long enough for the opposing views to get a proper airing. There was even talk about setting up an alternative to the IAU. "People are asking, 'What do we need these guys for?' " Stern said. "The IAU has no special claim. They have no police force or army. They're not the Supreme Court."

Eventually, Stern decided that the best way to counter the eight-planet view was to organize a series of teach-ins for scientists and educators as well as the general public. He wanted to demonstrate that the IAU was out of touch when it came to the detailed study of the solar system.

"The fundamental issue is that not many planetary scientists even belong to the IAU," Stern said. "The vast majority of its members work on galaxies, and stars, and black holes and cosmology. The reason most of the IAU doesn't care is because it's not their issue. The people who actually understand the physics, the chemistry, the work on planets, aren't in the IAU. It's kind of like having a bunch of French professors deciding issues regarding the German language."3

The fact that the planet debate continued at scientific meetings over the months and years that followed—at the European Geosciences Union, the American Astronomical Society, the American Geophysical Union, and the American

Annette Tombaugh-Sitze, the daughter of Pluto discoverer Clyde Tombaugh, poses with Mark Sykes, the director of the Planetary Science Institute.

Association for the Advancement of Science, to name just a few—reinforced Stern's view that the questions swirling around what to call Pluto and its kin were far from settled.

Mark Sykes had long been allied with Stern in the debate. He was on the science team for NASA's Dawn mission, which was headed to yet another dwarf planet, Ceres. He was also the director of the Planetary Science Institute in Tucson, Arizona. In addition to his Ph.D. in planetary science, he had a law degree from the University of Arizona and was admitted to the Arizona Bar. And as if that wasn't enough, he had sung professionally onstage in more than three hundred performances as a bass-baritone in the Arizona Opera Company's chorus. All this gave Sykes an appreciation of the scientific and procedural issues of the planet debate, as well as a good sense of the drama.

In a policy article published by the journal Science, Sykes revived roundness as the main criterion for defining a planet: "A planet is a round object (in hydrostatic equilibrium) orbiting a star," he wrote. Why roundness? Sykes said that when an object was massive enough to crush itself into a round shape, that object also had the potential to exhibit properties of most interest to planetary scientists: volcanoes, atmospheres, eroded valleys and uplifted mountains, and even the potential for life.4

All these arguments received an airing in August 2008 at a "Great Planet Debate" conference, organized at Johns Hopkins University's Applied Physics Laboratory, the scientific base of operations for NASA's mission to Pluto and the Kuiper Belt. Experts representing a variety of viewpoints on the subject of planethood were invited to present and discuss their different ideas. The main event at the conference was a one-on-one debate that was broadcast live over the Internet. Sykes agreed to stand up for the dwarf planets.

Sykes's opponent for the great debate was none other than Neil deGrasse Tyson, the astronomer who kept Pluto out of the big planetary parade eight years earlier. About 150 scientists, educators, journalists, and space fans attended the Tyson-versus-Sykes talkfest, moderated by public radio host Ira Flatow.

The essence of Tyson's argument was that the Kuiper Belt had emerged as a "new swath of real estate" in the solar

Neil deGrasse Tyson, director of New York's Hayden Planetarium, received "hate mail from third-graders" when he left Pluto out of the planetary parade.

system, and that Pluto was best placed alongside the other Kuiper Belt objects, big and small, rather than alongside the giants such as Jupiter. The way Tyson told the story, Pluto would be much happier there. "It's one of the kings of the comets, rather than the pipsqueak of the planets," he said.

In contrast, Sykes said the planetary pigeonhole should make room for the eight big planets as well as the biggest of the rocky asteroids and ice dwarfs. "It's good to have a very general way of categorizing things, rather than starting out with something that serves a very narrow scientific purpose of identifying just, say, dynamical giant objects rather than looking at the problem more generally," he said. "What the IAU did wasn't to expand our perspective, but rather to narrow our perspective."

Both men appealed to history. Tyson noted that Ceres was demoted from planethood once astronomers found other, similar objects with the same orbital profile. "This is déjà vu all over again," he said. "This is just like what happened in 1801." Sykes, meanwhile, noted that at one time everything that orbited around the sun—even the smallest asteroid that could be tracked—was considered a "minor planet" by the IAU's Minor Planet Center. If you had to set a dividing line between minor and major planets, roundness would be a good standard to use, he said.

In the end, the two debaters agreed to disagree. "If you want to say planets are round . . . I don't have a problem with that. But say they're round, for hydrostatic equilibrium, and put it aside and get on with the business at hand," Tyson said.

Onstage, on that one day, the outcome looked like a stalemate. But behind the scenes, scientists were indeed getting on with the business at hand—and the scientific process was actually moving forward.

Before the IAU acted in 2006, surprisingly little research had been published on the question of how a planet should be defined. Strangely enough, one of the most thoroughgoing works on the subject was a nine-page paper prepared for the

IAU by Alan Stern and a colleague of his at the Southwest Research Institute, Hal Levison.5

The paper, which was written in response to the brouhaha that erupted over Pluto in 1999, laid out many of the issues that would cause so much trouble seven years later. One of the suggested definitions went for roundness, and proposed a standard based on radius and density for judging whether a celestial body was massive enough to be crushed into a round shape. The ballpark figure was a thousandth of Earth's mass, or half the mass of Pluto. That would leave Pluto in the planetary club, admit some of its kin from the Kuiper Belt, and admit Ceres as well as Pallas and Vesta from the asteroid belt.

Then Stern and Levison sketched out another standard to classify planets further, based on orbital dynamics. They noted that a line could be drawn separating planets that had cleared out their neighboring regions from planets that had not. The former category, called "uberplanets," included the solar system's eight biggest worlds. The latter category, called "unterplanets," included Pluto, Ceres, and smaller round objects.

Stern said the distinction between unterplanets and uber-planets was meant to be "fun and playful," making light of the mania for classification. Nevertheless, the researchers laid out a serious-sounding standard that was based on the mass of the object and its distance from the sun.

Both criteria—roundness and dynamics—ended up being combined at the IAU meeting. But instead of making the more restrictive category a subset of the less restrictive category, as Stern and Levison suggested, the IAU left the unterplanets out of the planetary picture.

How can you tell when a world is round? And how do you decide when an orbit has been cleared out? After all, there are thousands of asteroids that travel at roughly the same distance from the sun as Jupiter, lining up ahead and behind of the giant planet. And if you wanted to get technical about it, Neptune could hardly be said to have cleared out its orbit when Pluto regularly came closer to the sun. Almost no one was happy with this idea of "clearing out the neighborhood"—and so that was one phrase that cried out to be fixed.

"'Cleared' was a poor choice of terminology," said astronomer Steven Soter, a colleague of Tyson's at the American Museum of Natural History. "It confused the public, and it gave ammunition to astronomers who didn' t like that definition."

Soter preferred the term "dynamical dominance"—that is, how much power a celestial object can exert on the objects around it. Even before the meeting in Prague, he wrote up a detailed paper that built on the formula laid out by Stern and Levison. By his calculation, the numbers for the eight most dominant planets came out at least a thousand times bigger than the numbers for anything else in the solar system. That power gap showed up as well in the way planets pushed around smaller objects gravitationally.6 "You have this gap that nature is providing for us, which we can quantify both observationally and theoretically," Soter said.

By calculating the dynamical power of bodies orbiting the sun, a clear line could be drawn between the solar system's eight biggest planets and everything else—something that Stern and Levison noted years earlier.

"If it turned out that we had a thousand objects on the heavy side of the gap, they would all be planets," Soter said. "But that's not how nature did it. The solar system has room enough for only a few dynamically dominant objects—planets."

Soter's analysis showed that mass wasn't the sole determining factor for the IAU's planet definition. When it came to the dynamical effect of a celestial body, something that was far out didn't matter as much as something that was close in, even if it was as big as Mars. The analysis demonstrated that the definition depended on where a planet was and how it formed, not merely how big it was.

"If you moved Mars to Pluto's distance, it would clearly not be a planet by this definition," Soter said. " But here's the problem: You can't form Mars at that distance."

Soter and the other astronomers who were fleshing out the IAU definition thought of planets in a particular way—the way that Herschel and Le Verrier thought of them, as ruling over a region of the solar system and having an effect on the cosmic clockwork.

That wasn't the way planetary scientists such as Stern and Sykes thought of them. In fact, the claim that Mars or even Earth wouldn't fit the formula for a planet if they were farther out struck them as one of the best reasons to throw out the definition. "Any definition of a planet would be laughed out of the house unless Earth is a planet," Stern said. "Anytime you take a picture of an object, and the picture is of Earth, that has to be a planet. We live on a planet."

But how can you take a picture of a planet (or a nonplanet) billions of miles away? Some astronomers saw this as the drawback to a definition based on roundness. Soter said there would be no clear dividing line between something that was round and something that was not quite round enough. "Nature is not providing us with a gap," he said.

Stern, however, said the boundary line between dwarf planets and the smaller bodies of the solar system could be drawn based on the underlying physics—that is, the measured mass and density of an object—rather than its surface appearance. "The point is that you never have to see the object or measure its roundness," he insisted. "This is a mass criterion that gives a size boundary. It's not that the object is round; it's that it is large enough to be rounded, by dint of hydrostatic equilibrium. This is very different from needing to measure roundness, or having to decide how round is 'round,' or worrying about whether an object has been hit and misshapen."

One of the astronomers who worked for Plutois demotion in Prague, Uruguayan astronomer Gonzalo Tancredi, proposed a detailed formula for dwarf planets in a paper he cowrote with a colleague, Sofia Favre. Their analysis drew the line at a diameter of about 280 miles (450 kilometers) for icy objects and 500 miles (800 kilometers) for rocky objects. Anything smaller would be denied dwarf-planet status.

By that measure, Pluto, Ceres, and Eris were definitely dwarf planets, and another ten to sixteen objects beyond Neptune 's orbit were on the candidate list.7 Experts could quibble over the list, of course. The important thing was that there was something concrete to quibble over. "At last we have something to talk about, whereas at the time of the Prague meeting, there was not anything quantitative on these issues," said Lowell Observatory astronomer Ted Bowell, the president of the IAU's Division III.

Along with the IAU's planetary and small-body working groups, Division III's board played a lead role in fleshing out the system for naming dwarf planets—the issue that touched off the showdown over Pluto. It was this gathering of experts that came up with the name "plutoid" for all the dwarf planets that roamed beyond Neptune, as well as the naming system.

Here's where the IAU's diplomatic skills were put to the test: The names for dwarf planets had to be approved jointly by the planetary and small-body working groups, while the small-body committee alone dealt with the names for smaller asteroids and Kuiper Belt objects. This meant the IAU had to decide whether something was round without taking a picture.

The experts could go with the complicated formula suggested by the dynamicists—but that hadn't yet stood the test of time. They could go with a standard based on an estimate of mass—but for many of the Kuiper Belt objects, it was impossible to figure out the mass. Or they could base it on diameter—but here again, the size of a faraway object couldn't always be calculated to the required accuracy.

Instead, the astronomers finessed the issue by saying that the determining factor would be an object's inherent brightness, just for the purposes of naming it. If the object had an absolute magnitude of 1 or brighter, it would go through the dwarf-planet naming process. If it was dimmer, it would be considered a lesser Kuiper Belt object. All this may seem like a ridiculously complicated criterion. But it was actually a clever dodge that helped the IAU avoid—or at least defer—a fresh controversy over dwarf planethood.

The naming standard pared the IAU's list of dwarf planets down to five:

• Ceres, which was clearly round on the basis of Hubble imagery, even though it didn't meet the brightness standard.

• Eris and Pluto, which satisfied the brightness standard and looked round in the Hubble imagery.

• The two remaining members of Mike Brown's Kuiper Belt Triumvirate, which were nicknamed Santa and Easterbunny. These objects couldn't be seen well enough to get a sense of how round they were, but they satisfied the brightness standard.

All that remained, then, was to give Santa and Easterbunny their official dwarf-planet names. Brown had to cast about for more than a year to find an apt name for Easterbunny, the bright Kuiper Belt object that was found in 2005, just after Easter.

Finally, he found a reference to Makemake, the fertility god of the Polynesian people who settled on Easter Island.8 That name made sense to Brown on more than one level, because his wife was pregnant when Easterbunny was discovered, and the astronomer said he had the "distinct memory of feeling this fertile abundance pouring out of the entire universe." So Makemake it was.9

Santa was a much harder case, but not because there was any difficulty in finding a name. In fact, there were too many names to choose from, due to the flap surrounding the announcement of its discovery in 2005. Spanish astronomers were the first to go public with its location. Brown, however, wondered whether they had peeked at his own observations of the curious Kuiper Belt object, which were stored in a publicly accessible online database.

Some additional server-l og sleuthing indicated that the Spanish astronomers had indeed checked the database just before making their announcement—but they contended that this was done after they had discovered the object, merely with the aim of finding out whether Brown was on the same trail. Their protestations did little to ease Brown's suspicions of scientific dishonesty or fraud.

The Spanish wanted to name Santa—or, more properly, the object provisionally named 2003 EL61—after an Iberian fertility goddess called Ataecina. A member of Brown's team, Yale astronomer David Rabinowitz, suggested a Hawaiian fertility goddess, Haumea. Would the IAU go with the Spanish suggestion, which came from the team initially credited with the discovery? Or would it throw out the Spanish claim as illegitimate and go with the suggestion from Brown's team?

In the end, the IAU' s working groups steered a middle course. They approved Haumea as the object's name, but left Spain's Sierra Nevada Observatory as the site of discovery. The name of the discoverer, however, is left blank in Haumea's official listing.10

Brian Marsden said all that ambivalence was intentional. He and his colleagues tried to strike a compromise between the Spanish astronomers and Mike Brown—one of the most clever compromises devised since Solomon suggested splitting a baby in half. The Spanish said they were unhappy with the outcome, while Brown thought the dispute was resolved about as well as it could have been.11

" They 're the discoverers, but it's his name that's being used," Marsden said. "Posterity will realize what the situation really is."

How many more dwarf planets will be named in the decades ahead? Will further evidence strengthen or weaken their claim on planethood? On these questions, too, posterity will have the final word—but the findings of the past couple of years already have focused a brighter spotlight on the solar system's far frontiers.

Was this article helpful?

0 0

Post a comment