There are only eight planets in the solar system. Better get used to it.

The International Astronomical Union's members voted Thursday to accept a definition of the term "planet" that leaves tiny, distant Pluto out of the family.

The decision, which was also endorsed by the American Astronomical Society, means science textbooks, posters, Web sites and trivia questions will have to be updated for the first time since 1930, when Clyde Tombaugh discovered Pluto.

The IAU decision is historic because it marks the first time astronomers have a formal definition of "planet."

The word comes from the Greek planetes, meaning "wanderer," because they moved so quickly across the night sky. At first there were only six, all visible to the naked eye: Mercury, Venus, Earth, Mars, Jupiter and Saturn. But when powerful telescopes entered the equation, the solar system got more complex.

Uranus was discovered in 1781, followed by Ceres in 1801. (Originally called a planet, Ceres was later rechristened as one of thousands of chunks of rock called asteroids orbiting between Mars and Jupiter.) Neptune was found in 1846, and Pluto in 1930.

But in the early 1990s, with the discovery of the Kuiper Belt -- a ring of rocky debris beyond the orbit of Neptune -- many astronomers questioned Pluto's status. By then, though, Pluto the planet had entered the popular consciousness.

Then came 2003 UB313, nicknamed "Xena" by its discoverer, Michael Brown of the California Institute of Technology. It, like Pluto, was in the Kuiper Belt. And it posed a problem for astronomers: It was bigger than Pluto. If Pluto was a planet, Xena would have to be -- but that would open the door to a lot more.

"There are about 50 or 60 other large pieces of debris in the Kuiper Belt," said Mike Overacker, president of the Roanoke Valley Astronomical Society. If we started calling the larger objects "planets," he explained, "We'd be opening up a can of worms."

Still, there needed to be some definition. So the IAU's Planet Definition Committee came up with a proposal for its membership to vote on, defining a planet as any round object that orbited a star and not another planet.

Had it been accepted, that definition would have made planets of Pluto, its moon Charon, as well as Xena and Ceres: a 12-planet system. That would be just for starters. New, more powerful telescopes would mean we'd likely have hundreds of "planets" within a few years. Astronomers made their feelings on that clear: No way.

So the committee tried again, narrowing the definition. A planet would be any object that orbits the sun and is massive enough to become rounded by gravity.

That, too, wasn't good enough. So the committee added a key phrase; a planet also "has cleared the neighborhood around its orbit."

Thus Pluto, which shares its orbit with thousands of bits of rock, is disqualified -- relegated to the astronomical kids' table along with Ceres, Charon, Xena and other "dwarf planets."

That was what the 424 IAU astronomers in Prague, Czech Republic, chose in a yes-or-no vote.

The decision to take Pluto off the list of planets appealed to a large number of astronomers. Even Xena discoverer Brown argued that Pluto and similar bodies didn't deserve planet status, saying that would "take the magic out of the solar system."

And Clark Thomas, an amateur astronomer in Roanoke, said veteran sky watchers have always known Pluto is different.

"You can visually see Pluto with a fairly large amateur telescope, but it's virtually lost in front of a sea of stars in the Milky Way," he said.

With the decision made, what remains are the ripples it will cause. Textbook publishers are already on the job -- after all, they're used to updating their books, especially those dealing with science or current events.

Most texts are used for about five years, said David Hakensen of Pearson Education, the nation's largest textbook publisher. "If there's a state that will be adopting a new science text for the 2007-2008 school year, we will be updating ... the stuff related to Pluto," he said.

If a school district is in the middle of a five-year cycle, it probably won't spend the money to update immediately. In that case, Hakensen said, the company makes updated information available to teachers electronically so they can keep their lessons up-to-date.

How much the general public will care about Pluto remains to be seen.

"I think it would have been OK to leave well enough alone," said John Simonetti, an associate professor of physics and astronomy instructor at Virginia Tech. He pointed out that astronomy is full of arbitrary and historical definitions -- the names of constellations, for example, are based on "stick figures" the ancients saw in the sky.

Simonetti acknowledges that there was an element of looking ahead in the vote. Astronomers want a system in place for when we discover Earth-like objects around other stars. Still, he said, not a lot of people would care about that, but "When you start monkeying with our own solar system, obviously everybody's interested."

The Associated Press contributed to this report.