About 2,500 scientists meeting in Prague have adopted historic new guidelines that see the small, distant world demoted to a secondary category.
The researchers said Pluto failed to dominate its orbit around the Sun in the same way as the other planets.
The International Astronomical Union's (IAU) decision means textbooks will now have to describe a Solar System with just eight major planetary bodies.
Pluto, which was discovered in 1930 by the American Clyde Tombaugh, will be referred to as a "dwarf planet".
There is a recognition that the demotion is likely to upset the public, who have become accustomed to a particular view of the Solar System."