The word planet has never had an official definition. As this Space.com article explains, the International Astronomical Union's proposed new definition of planet could mean the solar system has at least 12 planets.

"The proposal, which sources tell SPACE.com is gaining broad support, tries to plug a big gap in astronomy textbooks, which have never had a definition for the word "planet." It addresses discoveries of Pluto-sized worlds that have in recent years pitched astronomers into heated debates over terminology.

The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme. Pluto would remain a planet and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets. A far-out Pluto-sized object known as 2003 UB313 would also be called a pluton."