The Space Reviewin association with SpaceNews

Pluto illustration
The current IAU definition excludes Pluto and Charon (seen in this illustration from one of their smaller moons), something that many scientists disagree with. (credit: NASA, ESA, and G. Bacon (STScI))

A planet-sized debate

Later this week the Applied Physics Laboratory of The Johns Hopkins University will host a three-day conference and educators’ workshop titled “The Great Planet Debate”. The purpose of the event, in the words of its organizers, is straightforward: “[T]o explore a basic, but controversial, question: What is a planet?” The highlight of the conference: a debate between planetary scientist Mark Sykes and astronomer Neil deGrasse Tyson over whether Pluto should be classified as a planet.

This debate, of course, got kicked into high gear two years ago this month, when astronomers attending the triennial general assembly of the International Astronomical Union (IAU) voted on the first-ever definition of the term “planet”, one that stripped Pluto of its status as a planet and created quite a fuss not just in the astronomical community, but among the general public as well (see “Demote Pluto, or demote ‘planet’?”, The Space Review, August 28, 2006, and “Inside the planet definition process”, The Space Review, September 11, 2006). While the initial reaction and controversy quickly died down, there are still some scientists arguing that the definition of planet should be revised to include not just Pluto but potentially dozens of other worlds in the outer region of the solar system.

The definition that wasn’t

What’s often forgotten from that 2006 debate is the definition that the IAU accepted was not the original definition for a planet proposed at the meeting in Prague. Earlier in the assembly a group of scientists and writers put forward an alternative definition, one that would have not only allowed Pluto to retain its status as a planet but also add three other bodies: the largest asteroid, Ceres; Pluto’s moon, Charon, which orbits around a common center of mass between the two worlds; and the outer solar system object 2003 UB313 (since named Eris), which is slightly larger than Pluto.

“There are 92 elements, why does there have to be only a certain number of planets?” asked Sobel. “If we define it and there are lots of them, so much the better.”

Dava Sobel, a science writer who was part of that seven-person planet definition committee, said the committee members reached a consensus on that definition quickly. “Although we all had different opinions at the beginning, we did come to unanimous agreement in a very short time,” she said during a panel session on “What Is A Planet?” at the annual meeting of the American Association for the Advancement of Science (AAAS) in Boston earlier this year. The small committee worked much faster than a much larger committee of scientists who did not reach an agreement after studying the issue for a longer time, she noted.

That definition, though, came under attack, and from some unlikely quarters. Mike Brown, the Caltech astronomer who discovered Eris—and would seem to have something to gain by becoming the discoverer of a planet if the committee’s definition was adopted—was against the proposal, Sobel recalled. “He thought there were so many other candidates that might be planets under the terms of our definition that the solar system would get entirely too unwieldy,” she said.

Sobel didn’t find that argument convincing. “There are 92 elements, why does there have to be only a certain number of planets? If we define it and there are lots of them, so much the better.”

Another aspect of the proposed definition that generated controversy was a proposal to create a class of planets called “plutons” that would include Pluto and other similar planets in the outer solar system. “An enormous argument was heard from geologists,” she said, because “pluton” is, in geology, a type of igneous rock formation.

“We tried to say that there could be sharing,” Sobel said. “You can have a nucleus in a cell and in an atom, and that didn’t seem to be a big problem for scientists; Mercury is an element and also a planet. But no, this was really a hot point of contention.”

The definition that the IAU adopted, she noted, was voted upon on the last day of the meeting, when only a “smattering” of people remained. “Those 400-some people got to vote, and a lot of them just didn’t like what we had done,” she said.

A flawed definition?

The definition that the IAU adopted is not that different than what Sobel’s committee initially proposed. Both require an object to orbit the Sun (thus ruling out moons), and also be large enough that its mass allows the object to assume hydrostatic equilibrium and take on a nearly-round shape. The IAU definition, though, has one additional criterion: that the object “has cleared the neighborhood around its orbit.” While somewhat vague (what does it mean to “clear the neighborhood”?) the provision allowed astronomers to take care of Pluto, Charon, Eris, and Ceres, all of which are bodies that are part of a larger population of objects.

That provision grates among those who preferred the committee’s version, noting that where an object is in the solar system should not make a difference about whether or not it’s called a planet. “For example, if you put the Earth in Pluto’s orbit today, the IAU definition would say the Earth is not a planet,” said Alan Stern, who at the time of the AAAS meeting was the NASA associate administrator for science (a position Stern has since left). “The definition relies upon kind of a bogus dynamical criterion specifically designed to achieve one objective: to make sure the number of planets is small.”

“There’s broad agreement that it’s a flawed definition and it needs to be reworked,” Stern said.

Size should also not be a major factor in determining what should be a planet, Stern argued. He noted that both the Sun and the red supergiant Antares are considered stars, although the difference in sizes between the two is much greater than the range of sizes of planets. “Why do we call these objects stars? Because they share some fundamental—genetic, if you will—property: they fuse hydrogen,” he said. “In the case of planets, I believe the hallmark of planethood is the ability to become round due to self-gravity.”

There’s also the issue of “dwarf planets”, a separate category of objects designed to accommodate Pluto, as well as Eris and Ceres: objects that would have met the committee’s original definition of a planet but have not cleared out their neighborhood. (Charon was not included in the list of dwarf planets because it is considered a satellite of Pluto.) However, confusingly, “dwarf planet” is not a subcategory of “planet”, although elsewhere in astronomy a “dwarf star” is a type of star and a “dwarf galaxy” is a type of galaxy.

“We ended up with this extremely awkward situation where Pluto is now a dwarf planet, and a dwarf planet is not a planet,” said Sobel. “This is something a lot of people feel very uncomfortable about, and we probably have not heard the end of it.”

“There’s broad agreement that it’s a flawed definition and it needs to be reworked,” Stern said. Many scientists, he claimed, aren’t using the official IAU definition, and there’s nothing the organization can do to enforce that particular definition. “The IAU has no police force,” he quipped.

Towards a more complex classification

Many astronomers haven’t taken a stand on the question of what is a planet, finding it to be of either little interest (especially since the IAU definition is constrained to the solar system and ignores the growing number of extrasolar planets outside our solar system) or too complex to weigh in on. “I don’t know what a planet is and, perhaps more importantly, I’m not that interested in a precise definition,” said Michael Meyer, an astronomer at the University of Arizona who studies extrasolar planets.

“We may be in need of some kind of Linnaean kind of organization of planetary systems, so that we can intelligently communicate with each other,” said Tyson.

The IAU definition would seem to be acceptable to those who have argued that Pluto is not a planet, chief among them Tyson, who made waves nearly a decade ago when the Rose Center for Earth and Space, which Tyson runs, excluded Pluto from the list of the solar system’s planets in a display. “For six years, not a week would go by where I would not get email, mostly hate mail, from elementary school children” who liked Pluto, he said in the AAAs session. (Tyson is writing a book, The Pluto Files: The Rise and Fall of America’s Favorite Planet, scheduled for publication early next year.)

However, Tyson argued that the term planet itself—or at least the quest to develop a single, straightforward definition for it—has become a bit anachronistic. “What we need is not an exercise in trying to define the word ‘planet’, because I feel the word ‘planet’ is a little outdated,” he said. “We can talk about so much more than that word could ever give us.”

Tyson suggested that, as an alternative, astronomy borrow from biology, which has a complex, hierarchical system for classifying organisms. “We may be in need of some kind of Linnaean kind of organization of planetary systems, so that we can intelligently communicate with each other,” he said. He described a grid containing various attributes of planets—whether or not they have rings, how fast they rotate, and so on—that would be used to classify and organize objects. Scientists could then take “slices” through the grid to identify those planets that have certain attributes or meet certain criteria.

“I feel that’s what we really need, rather than distract ourselves with the definition of that one word,” Tyson said.