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Pluto illustration
Pluto’s reclassification as a “dwarf planet” by the IAU six years ago still generates debate today, although scientists don’t spend a lot of time on it. (credit: NASA, ESA, and G. Bacon (STScI))

Dwarf planet, dwarf controversy


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This week, astronomers from around the world are gathering in Beijing for the 28th General Assembly of the International Astronomical Union (IAU), the major triennial meeting of that organization. Attendees will “discuss and debate the most recent discoveries about the universe,” as the meeting’s web site states, making it “an important part of the vitality of our science.”

“There are those of us who are interested in the planetary system as a whole,” said Levison. “And then there are people who go out any study the individual objects.”

It was six years ago this month that, at the IAU General Assembly in Prague, attending astronomers discussed and debated a central and seemingly commonsense question: what is a planet? That debate led to a vote on an official IAU definition of the term, one that famously—or infamously, depending on your point of view—excluded Pluto (see “Inside the planet definition process”, The Space Review, September 11, 2006). That world, which had been considered a planet since its discovery in 1930 by Clyde Tombaugh, was reclassified—or demoted, again depending on your point of view—as a “dwarf planet”, a new category that, despite the name, was not a type of planet but instead something that fell just short of being one.

The IAU’s decision triggered a strong reaction from some planetary scientists, who thought the IAU’s definitions were flawed, and even an emotional reaction from people who believed the distant, icy world had somehow been slighted. The controversy eventually died down in the public’s view, but the debate has never gone away. That was on display last month at a workshop for journalists organized by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado in Boulder. While the topic of the event was the “Future Exploration of the Moon and Small Bodies”, one morning turned into an impromptu debate about Pluto’s status, with two scientists explaining their differing views on the subject but also putting that debate into perspective.

Size distribution versus physical characteristics

The contrasting presentations and ensuring discussion at the LASP workshop boiled down to a central issue: do you use a body’s physical characteristics, or how it compares to other bodies in the same system, as the primary means of determining if it’s a planet or not? “I think this debate really hinges on this difference,” said Hal Levison of the Southwest Research Institute (SwRI) in Boulder. “There are those of us who are interested in the planetary system as a whole: we look at the whole thing as a population, and are interested in how they interact with one another. And then there are people who go out any study the individual objects.”

Levison considers himself firmly in the first camp. To that end, he displayed a chart that plotted, on a log-log scale, the diameters of solar system objects versus their average distance from the Sun. “The answer to what is a planet and what is not is obvious” in that chart, he said. The eight worlds now defined as planets by the IAU stand out in the upper-left-center portion of the chart: large diameters, and relatively close to the Sun. The distinction between the four smaller, closer-in terrestrial planets and the four larger, more distant giant planets is also clear in the plot. Pluto, though, is part of a distinctly different group of bodies with smaller diameters at greater distances from the Sun. “To me, this is how we should answer the question” of what is a planet, he said.

Alan Stern, also of SwRI and principal investigator on the New Horizons mission to Pluto, falls in the second camp. “There is a common misconception in some of the press—the less scientifically literate press, at least—that Pluto and these other planet-sized objects are really tiny things,” he said, referring to other large Kuiper Belt Objects (KBOs) discovered in recent years. “They are not.”

Stern is particularly critical of the provision in the IAU’s definition of a planet that requires an object to have “cleared the neighborhood around its orbit”. He notes that if the Earth was placed in the same orbit as Pluto, it too, would fail that test and would not be considered a planet. “Any definition which precludes an Earth-mass object from being a planet is inherently flawed.”

“We used to think of the solar system as the four terrestrial planets, the four giant planets, and misfit Pluto,” Stern said. “Who’s the misfit now?”

He prefers what he called the Geophysical Planet Definition (GPD), which is based on the characteristics of the object and not its location in the solar system. This definition confers planet status on objects massive enough to be in hydrostatic equilibrium—that is, be nearly round—but not so large as to sustain fusion in their cores. Such a definition, based on the object’s physical attributes and not its orbit, is consistent with other definitions in astronomy. “Stars are not classified by whether they are in a galaxy or not in a galaxy,” he said.

The GPD has its flaws, though, argued Levison, particularly with determining whether an object is in hydrostatic equilibrium or not. One issue, he said, is simply “how round is round?”—that is, how round must an object be in order to be considered in hydrostatic equilibrium? With the large number of objects in the Kuiper Belt with a smooth size distribution, wherever you draw the line there will be two nearly-identical objects, only one of which will meet the definition for being a planet. Moreover, he said, in order to determine if an object is in hydrostatic equilibrium, you need to know its mass and radius, information that’s very challenging to obtain for distant, dim bodies with a wide range of albedos and densities.

Another problem with the geophysical definition of the planet, Levison claimed, was the number of planets that would result. “If you look at the Kuiper Belt, there’s going to be something like a thousand planets in the solar system, which seems a little awkward to me,” he said, based on estimates of the size distribution of objects in the Kuiper Belt.

Stern, though, has no problem with a large number of planets. Under the GPD, there are today about 20 known solar-orbiting objects that would qualify as planets, two-thirds of which are dwarfs, plus several moons of other planets. “We used to think of the solar system as the four terrestrial planets, the four giant planets, and misfit Pluto,” he said. Now, with a potential population of 1,000 planets, most of which are worlds like Pluto, “who’s the misfit now?”

Dealing with a flawed definition

While Levison and Stern may disagree on how to define a planet, the LASP seminar revealed common ground on one issue: neither likes the IAU definition of planet that the General Assembly approved six years ago.

“I agree with Alan: I hate this definition,” Levison said, saying it was not “aesthetically pleasing.” He said later that if he was asked to rewrite the definition, he would use the size distribution he previously described rather than the “cleared the neighborhood” provision in the current definition.

Stern blamed the fact that most members of the IAU are not planetary scientists. “Most people in the IAU are as close to planetary science as a divorce lawyer is to a tax lawyer,” he said. “They don’t really know the subject.”

“Eventually it’ll shake out,” said Stern. “I don’t worry about this being a controversy in 2050 or 2060. But it sure is a controversy now.”

Stern added that the vote itself damaged the field in the eyes of some observers. “I have been told by people in other scientific fields, I have been told by people in Washington, that this has done great damage pedagogically to science,” he claimed. “This image, of science voting, gives the impression that it’s arbitrary, it’s political, as opposed to being based on fact.”

Don’t expect, though, another vote at either this month’s IAU General Assembly or a future one to rectify this flawed planet definition and, perhaps, restore planet status to Pluto. For one thing, it’s not a high priority for people like Levison. “I haven’t thought about this issue in a couple years,” he said, adding that he uses the issue primarily as a “hook” to get people interested in other topics, such as the formation of the solar system. “We don’t sit in our offices and yell at each other very often about this.”

Stern thinks the change in how a planet is defined will come gradually. “It’s not going to be by voting in committees, but it’s going to be individual by individual, scientist by scientist, coming to their own conclusion about which is the better scheme,” he said.

“I think we’re in the middle of a multi-decade process of sorting this out, where the data is going to keep informing us: all these discoveries we’ve made in our solar system and farther away,” Stern added. “Eventually it’ll shake out. I don’t worry about this being a controversy in 2050 or 2060. But it sure is a controversy now.”


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