Our solar system is suffering an identity crisis.
For decades, it has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.
Should this newly found icy rock known as "2003 UB313" become the 10th planet? Should Pluto be demoted? And what exactly is a planet, anyway?
Ancient cultures regularly revised their answer to the last question and present-day scientists aren't much better off: There still is no universal definition of "planet."
That all could soon change, and with it science textbooks around this planet.
At a 12-day conference that began yesterday, scientists will conduct a galactic census of sorts. Among the possibilities at the meeting of the International Astronomical Union in Prague: Subtract Pluto or christen one more planet, and possibly dozens more.
"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."
The debate intensified last summer when astronomer Michael Brown of the California Institute of Technology announced the discovery of a celestial object larger than Pluto.
Like Pluto, it is a member of the Kuiper Belt, a mysterious disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. (Brown nicknamed his find "Xena" after the warrior heroine in a cheesy TV series; pending a formal name, it remains 2003 UB313.)
The Hubble Space Telescope measured the bright, rocky object at about 2,400km in diameter, roughly 113km longer than Pluto. At 14.5 billion kilometers from the sun, it is the farthest known object in the solar system.
The discovery stoked the planet debate that had been simmering since Pluto was spotted in 1930.
Some argue that if Pluto kept its crown, Xena should be the 10th planet by default -- it is, after all, bigger. Purists maintain that there are only eight traditional planets, and insist Pluto and Xena are poseurs.
"Life would be simpler if we went back to eight planets," according to Brian Marsden, director of the astronomical union's Minor Planet Center in Cambridge, Massachusetts.
Still others suggest a compromise that would divide planets into categories based on composition. Jupiter could be labeled a "gas giant planet," while Pluto and Xena could be "ice dwarf planets."
"Pluto is not worthy of being called just a plain planet," said Alan Boss, an astrophysicist at the Carnegie Institution in Washington. "But it's perfectly fine as an ice dwarf planet or a historical planet."