Why is Pluto no longer a planet?

Scientist thought it was to small to be a planet.

Explanation:

it's sad, it was my favorite planet. now it's my favorite dwarf planet.

The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one-it "has not cleared its neighboring region of other objects.