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Sedna, Xena and Buffy

The familiar solar system that you learned about in school - nine well-behaved planets, from Mercury to Pluto, circling sedately in tidy paths around the sun - isn't what it used to be.

Astronomers recently have discovered a flock of at least eight other planetlike objects in far-out, sometimes wildly eccentric orbits. Four new "ice dwarfs," plus two more probable moons around Pluto, were announced publicly in the last six months.

The latest mini-world, temporarily nicknamed "Buffy" and located more than 5 billion miles from the sun, was announced on Dec. 13. The object, about half the size of Pluto, was spotted roaming through the so-called Kuiper Belt, a vast junkyard of icy, rocky bodies stretching for billions of miles beyond the orbit of Neptune.

The first scientific mission to explore Pluto and the Kuiper Belt, a nine-year voyage to the outer solar system, is scheduled to be launched in January. "Next month we set sail for Pluto," Alan Stern, the project's chief scientist at the Southwest Research Institute in Boulder, Colo., exulted.

Unlike the rest of the planetary family, Pluto resides in the Kuiper Belt. But it's not alone there. More than 1,000 frozen chunks of debris left over from the formation of the solar system have been found since 1992. Astronomers expect that there are at least half a million more pieces out there.

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

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

Even the inner solar system doesn't look the way it used to. Astronomers no longer think that the four biggest planets have always been spinning along in their present...

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