system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is pluto not considered a planet anymore?
gpt
The IAU defined a planet as something that

1) goes around the sun

2) is big enough so that it's a sphere (not like some weird potato shaped thing)

3) is the only main object in its orbit around the sun (or officially, it should have "cleared the neighborhood")

Pluto is good for the first two, but there's a bunch of stuff in the same orbit as it, so it doesn't fit the classification.