The International Astronomical Union has officially voted to change the number of planets in the solar system from nine to eight.

The removal of Pluto from the list of classical planets was suggested after a proposed resolution offered to add three new objects to the planetary roster. Instead of including the new objects, Pluto was demoted Thursday after heated debate at the IAU General Assembly in Prague, Czech Republic.

The “Planet Definition” resolution, had it been approved by the voting astronomers attending the IAU General Assembly, would have changed the definition of a planet to include Ceres, a member of the asteroid belt; Charon, Pluto’s moon; and an object called 2003 UB313, unofficially dubbed “Xena” by its discoverer, Mike Brown of Caltech.

The controversy over the definition of a planet arose when it was discovered that Xena was larger than Pluto. Xena orbits the sun beyond Neptune in a collection of icy fragments called the Kuiper Belt; questions had been raised as to whether Pluto is one of these Kuiper Belt objects.

“If Pluto had not been discovered way back in 1930 by accident and prematurely named as a planet, we would not be having this discussion,” said Charles Kerton, assistant professor of physics and astronomy. “We now know that it is very different from the other planets in terms of its composition and orbit.”

Pluto is much smaller that the other planets and has a tilted, more irregular orbit compared to the other eight planets. Also, it is mainly comprised of ice and some rock, which supports the theory that it is more closely related to the nearby Kuiper Belt objects than to the other planets.

“It’s by no means a purely scientific question; it’s cultural and historical too,” said Curtis Struck, professor of physics and astronomy. “People are used to thinking that we have nine planets and that Pluto is one of them. The mnemonics kids learn in elementary school make up the basis of many people’s astronomy knowledge.”

The originally proposed definition of a planet would dictate that a planet must be massive enough for gravity to make it round [usually about 500 miles in diameter] and must be in orbit around a star while not being a star itself. These characteristics would have not only included the three objects recently considered for planet status but would also apply to at least a dozen more objects discovered throughout the solar system, potentially more.

“I’d feel sorry for any third-grade kid who had to memorize the names of the dozens of possible planets out there, according to these criteria,” Kerton said of the original resolution draft.

The newly approved definition of a planet according to the IAU is “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighborhood around its orbit.” Pluto fails to fit the new definition due to its unusual orbit, which is oddly oblong and intersects with the orbit of Neptune.

Pluto is now classified as a “dwarf planet,” a category similar to what have long been called “minor planets.” A third class of lesser objects that orbit the sun will now simply be called “solar system bodies” and includes asteroids, comets and other natural satellites.