why, the pluto is not one of the eight planets of solar system .
We called celestial body a planet if:
-It orbits around the Sun;
-It has sufficient mass for its own gravity to achieve hydrostatic equilibrium (nearly round) shape;
-It is dominant in its orbit.
Pluto does not meet one of the criteria of the this definition of a planet: the mass of the objects that has the same orbit that are not his satellites is too large compared to the mass of Pluto, to assume that it dominates