When Clyde Tombaugh discovered Pluto in 1930, he made a mess of the solar system.
Pluto was too small, too distant, and of the wrong makeup to qualify as a planet, many astronomers said.
They were overruled. Any 6-year-old can tell you there are nine planets.
Maybe there ought not to be. Nearly 2,500 astronomers -- from 75 countries on planet Earth -- are in Prague, Czechoslovakia, for a meeting of the International Astronomical Union.
During their 12-day meeting, they plan to debate a question that has long divided them: Just what qualifies as a planet?
This is more than an academic exercise.
Science looks for order, and until Pluto was found, the solar system had been very orderly.
The inner planets -- Mercury, Venus, Earth and Mars -- are small and rocky.
The outer planets -- Jupiter, Saturn, Uranus and Neptune -- are gas giants.
A noisy little movement in the 1990s said Pluto, at best, qualified as "other." It's only 1,400 miles across, is smaller than Earth's moon, and travels in a lopsided orbit.
So what makes a planet? Anything larger than Pluto? Anything spherical that orbits the sun?
In that case, there are numerous asteroids that qualify -- Neil deGrasse Tyson, director of New York's Hayden Planetarium, smiles about the discovery of "the planet Ceres" between Mars and Jupiter in 1801.
Astronomers have a problem of cosmic proportions on their hands.
The situation was complicated last summer when Michael Brown, an astrophysicist at the California Institute of Technology, reported finding a new object, well beyond Pluto's orbit.
He gave it the playful nickname "Xena," after the warrior princess of syndicated TV fame, but for now, the astronomers' union is sticking to the astronomical designation 2003 UB313.
Brown used to argue that Pluto didn't qualify as a planet -- too small, too unlike the other planets -- but last year he said he'd changed his mind.
Frankly, he said, because of all those preschoolers who think of Pluto as a planet.
Later, when follow-up observations suggested that 2003 UB313 was 70 miles larger in diameter than Pluto, Brown said the case was clear.
It also deserved to be a planet, for which he'd submitted a serious -- as yet unannounced -- name.
"Suddenly, there's an object the size of Pluto or bigger that's three times farther out there," he said. "That's sort of an amazing thought."
If Pluto qualifies as a planet, then Xena certainly must.
Some astronomers, though, worry that such a ruling will open floodgates, because Pluto and Xena also qualify as "Kuiper Belt Objects" -- icy, rocky worlds, currently beyond counting, that orbit in a vast band more than three billion miles from the sun.
Perhaps Pluto ought to be demoted, and counted as one of these.
Some astronomers have suggested a new term -- "planemo" -- for spherical worlds too small and too different from the original planets to qualify for planetary status.
However the debate ends, astronomers say, it will change the number of planets.
Either there will be eight -- with Pluto demoted -- or there will be many more, with Pluto, Xena, and other Kuiper Belt Objects included.