This issue is very convoluted and complex, but the main driving force in this issue was the discovery of
136199 Eris.
Eris (dwarf planet) - Wikipedia, the free encyclopedia You see, the trouble with Eris is, it's LARGER than Pluto, and thus the discoverer really wanted his name in the history books, as the discoverer of the 10th planet in the solar sytem. However, the catch is, if Eris is to be considered a planet, then other "small planets" of similar nature would have to be included in the list. This sparked off a year-long debate within the professional astronomy community and the IAU--in fact their first proposal was to have 12 planets in the solar system: