The only reason Pluto was called a planet is because it was the first type of that object we ever found. But for centuries we didn't actually have a scientific definition of a planet. So they made a definition with multiple requirements. This search for a definition meant one of two things. Either: A. Pluto would be a planet, but so would dozens to hundreds of other 'planets' exactly like Pluto (and many larger) in our Solar System, or B. Pluto is not a planet. The latter was chosen. So we can either have 8 planets, or hundreds. Pluto is not a planet because it did not meet the criteria of being large enough to have a gravitational pull sufficient to clear away its orbit.