Earlier, Pluto was also regarded as one of the nine planets of the solar system but on 24 August, 2006, it was removed from the list of planets by IAU (International Astronomical Union) because Pluto failed in fulfilling all criteria set for being a planet. As per Resolution 5A a planet must orbit the sun, must have enough gravity to become a ball-like structure and needs to have cleared its orbit.
First two criteria were fulfilled but Pluto failed in satisfying the third one because a new object was discovered beyond its orbit which was larger than Pluto. That object was Eris and Pluto failed in clearing this from its orbit which was the third requirement of being a planet. Here Pluto failed.
Now both Eris and Pluto are taken as dwarf planets and now we have only 8 planets in our solar system. The earth has also failed in clearing asteroids from its orbits.
