The IAU defined a planet as a celestial body that:
1. Is in orbit around the Sun.
2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. Has cleared the neighborhood around its orbit.
Pluto failed to meet the third criterion, as its orbit is shared with other icy bodies in the Kuiper Belt.
So, while Pluto is located in the outer solar system, it's not considered one of the eight planets.
