The historic vote is expected to send teachers and publishers scrambling to alter textbooks and classroom displays to adhere to the new guidelines. The eight major planets are now Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Pluto, discovered in 1930 by American Clyde Tombaugh, has long been considered the ninth planet, located the farthest from the sun.
However, the frozen object generated heated debate because its orbit is tilted the furthest from the plane on which the Earth and other planets travel. The debate gained steam in 2003 when Mike Brown of the California Institute of Technology discovered another planet-like object, nicknamed Xena, circling the sun that was even larger than Pluto.
Under the new definition, Xena is not a planet either.
The scientists decided that a planet must be in orbit around a star, but itself not be a star. It must be large enough for its own gravity to pull it into a nearly spherical shape and have "cleared the neighborhood around its orbit."
Pluto is disqualified because its oblong orbit overlaps with Neptune and instead is placed into the "dwarf planet" category.
The IAU also created a third category of "small solar system bodies," which applies to asteroids, comets and other natural satellites.
The sentimentality surrounding Pluto showed through when the scientists made their announcement.
Jocelyn Bell Burnell, a specialist on neutron stars from Northern Ireland who oversaw the IAU proceedings in Prague, held up a stuffed Walt Disney character of Pluto the dog beneath a red umbrella.
"It could be argued that we are creating an umbrella called 'planet' under which dwarf planets exist," she said, reported the Associated Press.
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