The most exclusive club in the solar system will be revising its membership rules this year

In all of history, only nine orbs in the sun’s entourage have earned the title “planet.” But that changed last summer, when an object significantly bigger than Pluto was spotted lurking in a little-explored region of the solar system known as the Kuiper Belt. Nicknamed Xena, the object-officially known as 2003 UB313-was soon being called the 10th planet by headline writers and even some astronomers.

Although other objects, similar but smaller than Pluto, have been discovered in recent years, the identification of Xena set off a wide-scale planetary identity crisis. Now the task of sorting out this mess has fallen to the International Astronomical Union, the organization that serves as the official namer of all things beyond Earth’s atmosphere. At press time, an IAU committee charged with devising qualifications for planethood was still divided, with some scientists proposing that the question be opened to public debate and voted on at the IAU’s August 2006 triennial meeting.

One leading proposal would define a planet as any object whose diameter exceeds 2,000 kilometers and that is round as a result of gravity, criteria that would encompass anything Pluto-size or larger, including Xena. But that doesn’t
sit well with some astronomers, who are irked that the scrawny iceball with the cockeyed orbit earned membership into the club in the first place. “Pluto is an impostor,” says Harvard astronomer
Brian Marsden, a member of the IAU
committee. “The simplest thing is to get rid of it and say we’ve got eight.”

Since 1992 more than 800 iceballs of all shapes and sizes have turned up in the Kuiper Belt, a bagel-shaped swath of space that extends from just beyond Neptune to a distance of five billion miles from the sun. Astronomers say that’s only a tiny fraction of the Kuiper Belt’s total population.

Michael Brown of the California Institute of Technology, who led the team that discovered Xena, says he wouldn’t be surprised if another object rivaling Pluto in size turned up this year. Others think Mars- or Earth-size orbs might be next.

Given the pace of discovery, the IAU is eager to resolve the planet dilemma. But general secretary Oddbjrn Engvold concedes that it could take time. “There are emotions involved, history involved,” the Norwegian astrophysicist says. “This has to be handled with care.”

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