When I was growing up, I had this cool poster on my bedroom wall that showed the Sun and nine planets in their orbits around her, along with the mysterious asteroid belt. All neat and orderly.

Too bad it's not quite that simple any more:

Scientists no longer are sure what a planet is and how many reside in our system.

The International Astronomical Union, a worldwide alliance of astronomers, has been struggling for about two years to agree on a definition for planets. Three proposed definitions are being studied, but a decision isn't likely until spring, according to Robert Williams, an astronomer at the Space Telescope Science Institute in Baltimore.

So Pluto, which most of us were taught as the ninth planet, may lose that status. Then again, maybe not.

"The discovery of the Kuiper Belt in the 1990s has given Pluto a place to call home, with icy brethren to call its own," said Neil deGrasse Tyson, the director of the Hayden Planetarium at the American Museum of Natural History in New York, in an e-mail.

"The Kuiper Belt is the largest structure in the solar system," Stern said recently. "We used to think Pluto was a misfit," he added. Now Earth and the other inner planets are the oddballs.

Depending on what definition of "planet" is chosen, our solar system may have as few as eight (demoting Pluto) or as many as seventeen (!!!) planets. Astronomers have already discovered a body larger than Pluto in the Kupier Belt.

The largest and most distant of the ice dwarfs is nicknamed Xena after the television warrior princess. Discovered in 2003, it's 1,600 miles across and 20 percent bigger than Pluto is. Xena has a moon of its own, named Gabrielle after the TV Xena's sidekick.

These bodies haven't been assigned official names yet, which is why you see whimsical designations like Santa (which has a moonlet named Rudolph), Easter Bunny, Orcus, Quaoar, Ixion, Buffy and Sedna.

Even asteroids have been discovered with their own moonlets, and at least four moons in our system are geologically active.

What a wonderfully messy and chaotic neighborhood we live in.

Thanks to Chris Hall for the pointer.

Posted by: Ted at 12:10 PM | category: Space Program
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