Why Pluto is Not a Planet?

After being classifed as one of the nine planets in the Solar System for over a millennia, Pluto was demoted from being a planet in 2006. In 2006, the International Astronomical Union voted to define what a planet actually was. Since Pluto seems to share attributes with Kuiper belt objects and does not have enough mass to get ride of smaller things in its orbit, it did not meet the qualifications to be classified as a planet. Look here for more information: