Billions and billions of definitions?
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 at 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.

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Complicating the idea of planethood 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.

ALL ABOUT PLUTO
Pluto's day: 6.4 Earth days.
Pluto's year: 248 Earth years.
Average distance from sun: 3.7 billion miles, or 5.9 billion kilometers.
What's in a name? Pluto is also the name of the Roman god of the underworld. It was suggested by many people, but credit was given to an 11-year-old girl from England.
Graphic: Where the planets are
MSNBC / AP
"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 American 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."

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