Astronomers had just voted to strip Pluto of its status as a planet - instead, it is now classified as a dwarf planet.
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".
Related: The Woman Who Named Pluto, previously on Neatorama, about Venetia Phair, who at 11 years old suggested the name "Pluto" for the newly discovered (then) planet.