it's an arbitrary definition anyways.
but pluto's not on the ecliptic plane, and its orbit intersects another orbit. it's not part of the traditional planetary system, kind of an odd man out. it's much more like an object in the kuiper belt (consider it spends most of its time there). the only thing is that it's large and round. so now that we have a few more large round objects in the kuiper belt... we either demote pluto, or include 4 or 5 other planets.
pluto was probably originally a moon anyways.
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