What are the reasons why pluto became a minor planet?
Basically, discoveries of additional bodies similar to Pluto made it clear that it was far from unique. There were really three choices: (1) Recognize all the similar bodies as planets, thus creating an unwieldy and growing list. (2) Treat Pluto as a special case, while listing all the other, similar bodies differently. This makes the system inconsistent just to support habits, which is not something scientists like to do. (3) Create new categories to fix the problems with the system, and put Pluto in one of those new categories. The third choice was clearly the most consistent with scientific standards, although it means that Pluto’s status as a planet goes the way of Eohippus: a bit of abandoned scientific history.
