A planet should have a specific orbit of its own which should not intersect orbit of other planets. But Pluto's orbit intersects the orbit of Uranus. That's why it is not considered as a planet.

Another reason is: there are thousands of celestial bodies in the neighbouring region of Pluto's orbit and some of them are bigger than Pluto.
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Each and every planet should have a orbit.the planets in our solar system have the same orbit but pluto has a different its not a planet of our solar system.