Answer :
Pluto is the smallest planet.So it is defined as "dwarf planet". And it is far away from other planets.Moreover the main reason is,when other planets revolves it clears all particles on his path due to gravity but Pluto does not do this.
Pluto was known as the ninth planet of the solar system but now it is called a dwarf planet . It is too far away from sun and other planets and is very small. It is so small that it cannot clear other objects out of its path.