Exclusion of Pluto as a planet was based on an unvalid definition
Pluto should be back as a planet - short science articles and news

Pluto lost its planet status in 2006, since a group of astronomy experts, defined a planet as a body, which clears its orbit i.e. be the largest gravitational force in its orbit. However, since Neptune’s gravitational pull influences Pluto and the fact that Pluto shares it orbit with objects in the Kuiper belt, it lost it planet status. However, now researchers state that this criterion to define a planet is not supported in literature. Interestingly, there is only one publication in the last 200 years, back in 1802, which used this definition, and it has been since disproven. They further state that a planet should be defined based on its intrinsic properties and not on external dynamics like the planet’s orbit.

Read the full story: University of Central Florida
Scientific publication: Icarus