Most people think size is the factor to categorize planet as a planet. In August 2006 astronomers around the world met and finally decide formal definition of a planet. Pluto and it’s buddies didn’t included in to that cut. The three main stipulations are 1. That object should orbit a star 2. It should have enough mass to make it spherical shape 3. That object should clear it path. Pluto was not able to satisfy the third factor. So those type of objects named as Dwarf planets.