Throw away the place mats. Redraw the classroom charts. Take a pair of scissors to the solar system mobile.

After years of wrangling and a week of debate, astronomers voted for a sweeping reclassification of the solar system. In what many of them described as a triumph of science over sentiment, Pluto was demoted to the status of a “dwarf planet.”

In the new solar system as defined by the International Astronomical Union, meeting in Prague, there are eight planets instead of nine, at least three dwarf planets and tens of thousands of so-called smaller solar system bodies, like comets and most asteroids.

For now, the other dwarf planets are Ceres, the largest asteroid, and an object known as 2003 UB 313, nicknamed Xena, that is larger than Pluto and, like it, orbits beyond Neptune in a zone of icy debris known as the Kuiper Belt. But there are dozens more potential dwarf planets known in that zone, planetary scientists say, and so the number in the category could quickly swell.

In a nod to Pluto’s fans, the astronomers declared it to be the prototype for a new category of such “trans-Neptunian” objects, but declined in a close vote to approve the name “plutonians” for them.

The outcome yesterday completed a stunning turnaround from only a week ago, when the assembled astronomers were presented a proposal that would have increased the number of planets in the solar system to 12, retaining Pluto and adding Ceres, Xena and even Pluto’s moon Charon.

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Under the new rules, a planet must meet three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The last of these criteria knocks out Pluto and Xena, which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.

Dwarf planets, on the other hand, need only orbit the Sun and be round. (...)

in: The New York Times