It’s no secret Pluto has been out of the privileged circle of celestial bodies to be considered planets for over a decade now. But many don’t realize what events led to this ousting, and the reasonings for doing so.
Pluto had long since been considered a planet since its discovery in the 1930’s. Yet starting in 1992, discoveries were made which put this status in jeopardy. Several celestial objects were found in the Kuiper belt which were as large, if not larger, than Pluto. Naturally this lead to the question: should these be planets? Or should Pluto not be a planet?
This debate was fueled even more by the 2005 discovery of Eris, a dwarf planet in the scattered disc which was found to be 27% more massive than Pluto. This was the final nail in the coffin for Pluto’s membership to the planet club, with the International Astronomical Union establishing guidelines on what is considered a planet that explicitly dropped Pluto from the roster in 2006 at their General Assembly. Although Pluto may not be batting in the big leagues anymore, it will always be a planet in my heart.
Danny's Solar System Blog 