Why isn't Pluto a planet anymore?
Pluto is no longer a planet because the definition of a planet was changed.
When Pluto was first discovered it was naturally given the status of the ninth planet. Pluto is now known the be the largest of many objects in the Kuiper Belt.
Ceres, which was actually discovered before Pluto, is the largest body in the Asteroid Belt. Although Ceres is smaller than Pluto some astronomers thought that they should have the same status.
The definition of a planet is that it is a body which orbits the Sun and it is also spheroid in shape. Both Pluto and Ceres meet these two criteria.
In 2006 the International Astronomical Union (IAU) debated the issue. They added a new definition of a dwarf planet which is also a body which meets the two criteria.
The IAU then added a third criterion to distinguish a dwarf planet from a planet. This criterion is that the body must also have cleared its orbit of other bodies.
The result of this was that Pluto was demoted from planet to dwarf planet and Ceres and other similar bodies were promoted from asteroid to dwarf planet.
The IAU 2006 decision was and still is controversial. I still think of Pluto as a planet. The photographs from the New Horizons mission proved Pluto to be a fascinating object. It is possible that the 2006 decision could be reversed.