Why Planets Will Never Be Defined

by Robert Roy Britt, Senior Science Writer
Date: 21 November 2006 Time: 06:07 AM ET

Before the dust even settled after the Great Pluto War at the International Astronomical Union (IAU)'s General Assembly in Prague, one thing became clear: There will never be an accepted scientific definition for the term "planet."

Rather than crafting an acceptable definition, the IAU alienated members, put the group's authority in jeopardy and fueled schisms among astronomers on theoretical grounds and even nationality.

And the whole affair was scientifically pointless, many astronomers say.

The controversial planet-definition resolution, passed Aug. 24 in a vote of just 424 IAU members, will not stand as worded. Some 300 astronomers have pledged not to use it, and many others say it must be redone to eliminate contradictions. It will be reworked, at the least, and possibly overturned at the 2009 IAU General Assembly in Rio de Janeiro, Brazil.

Meanwhile, the debate-which the IAU limited to defining round things in our solar system-was a neighborhood nomenclature brawl amid a universal war of words. Any terminology that might be relevant to our little solar system will be laughably inadequate if applied across the galaxy.

Shortly after the Prague vote, I posed a series of questions about the new definition's merits and shortcomings to several astronomers, among them Geoff Marcy at the University of California, Berkeley. Marcy and his colleagues have found more planets beyond our solar system than any other team.

"Your questions imply that a definition of the word 'planet' is useful scientifically. That is a view not shared by many professional planetary scientists," Marcy replied. "The astrophysics of planetary bodies is so rich and complex that defining 'planet' has never been an issue under discussion among professionals. So, some of your questions read to me like the old phrase 'When did you stop beating your wife?' The taxonomy of asteroids, comets, moons, planets and brown dwarfs is far too limited to capture the diversity of their origins and internal constitutions."

Diverse indeed. During 2006, the tally of known extrasolar planets surpassed 200, and the range of sizes and setups illustrates why a universal definition is impossible in light of the fact that scientists are sharply divided on what to call Pluto.

Arguing since 1990

The debate over what constitutes a planet flared up after the 1990 discovery of the first round objects orbiting another star. The three so-called pulsar planets are about the same size as Earth. They are often forgotten in discussions about exoplanets. Some astronomers don't see them as planets at all, because they orbit a fast-spinning, dead star that cannot support life.

Other worlds several times the mass of Jupiter float freely in space; they have no host star. Are they planets? Other oddities abound.

"It is a little-known fact that nearly 25 percent of the known extrasolar planets are in binary- or multiple-star systems," said Stephen Kortenkamp , a research associate at the University of Maryland. "That further complicates the notion of creating a universal definition of planet."

One day during what Kortenkamp calls the Great Pluto War, he browsed his dictionary. "I see lots of words that have multiple definitions, depending on the context in which they are used," he told me back then. "I don't see why the word 'planet' can't be treated the same way."

Kortenkamp figures "planet" means one thing in our solar system and something else around other stars, and also has varying meanings for geologists or planet-hunters or the public. "The IAU would have been better off with this approach rather than trying to dictate a single definition for what is really a cultural term that means different things to different groups of people," he said.

The known setups are a tiny sample of what's out there. There are perhaps 250 billion planets in our galaxy, says Gregory Laughlin, an exoplanet hunter and planetary system theorist at the University of California, Santa Cruz.

Eventually, astronomers could find two Earth-size objects orbiting each other around a center of gravity in the space between them, Laughlin said. Other worlds might be accompanied by planet-size "Trojans" that move with them in a horseshoe-shaped pattern. The present IAU definition, requiring a planet to clear out the path of its orbit, is not set up to handle such offbeat configurations.

It's also possible two planet-mass objects could be found orbiting each other with no star involved.

Complicating the idea of planet-hood are the very massive objects that have been the easiest to find with current technology. There are dozens of them, each several times the heft of Jupiter, and many bump up against the mass range of brown dwarfs to create yet another fuzzy area of definition.

Brown dwarfs are big balls of gas that can be up to 70 times as massive as Jupiter but not massive enough to carry out the thermonuclear fusion of hydrogen that powers real stars. Generally, the lower cutoff is thought to be at 13 times the mass of Jupiter, a level that triggers the fusion of deuterium, which gives brown dwarfs a warm glow that Jupiter can't muster. Thing is, astronomers don't know how gas-giant planets are born nor what conditions create a planetary mass object versus a brown dwarf.

In many astronomers' minds, formation scenarios must play a role in any useful planet definition. The current IAU definition does not even address formation.

'Major rifts'

The Great Pluto War alienated many of the roughly 10,000 professional astronomers around the world who did not have a chance to cast a vote. It also created "two major rifts" among astronomers, said David Morrison, an astronomer at NASA's Ames Research Center who was among the few who did vote.

"Most important was a rift between astronomers who study physical properties of objects and those who study orbits (dynamics)," Morrison told me. "The dynamicists dominated at the IAU, and many of them would not accept any definition that was based solely on physical properties such as size."

"The second division was along national lines," Morrison explained. "Some astronomers seemed irritated by perceived American domination of the process. Some felt, with considerable justification in my opinion, that some Americans astronomers defended Pluto as a planet in large part because an American had discovered it. As in so many other international contexts, there can be reaction against perceived American arrogance."

In an interview with SPACE.com published in September, IAU president Catherine Cesarsky said there is no reason to question the governing body's authority. But when asked if that authority had been weakened, she also said: "It is too early to tell."

In the broadest terms, a planet could be thought of as anything from an 800-kilometer-wide (500-mile-wide) round rock orbiting a dead star to a colossal gas ball floating alone in space. No accepted definition will be possible unless the IAU democratizes the decision-making process by allowing all members to vote.

Even then, defining and categorizing all these different worlds is seen as impossible by some astronomers. Many think it is simply irrelevant, or, as Geoff Marcy puts it: "Categorizing them does not magically add insight."

This article appeared in the Winter issue of Ad Astra magazine.

Full Coverage: The Debate and the IAU Vote

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