Who defined that planet Pluto is not a planet anymore?

Answer 1

The definition of a 'planet' is not as simple as it sounds. The discovery of the new planet about 80 years ago produced a lot of excitement in the public and in scientific circles. As more information has come to light regarding Pluto, it is clear that Pluto is not completely like the bigger planets. Pluto's mass is far smaller than any other planet (1/400 of earth's mass).

In 2006, the International Astronomical Union published guidelines for planets, minor planets, and Kuiper Belt objects. It was this definition that excluded Pluto. If Pluto were included as a planet, it would be necessary to add 2 or 3 other objects (Eris, Ceres, possibly others) also as planets.

In 2008 and 2009, scientists and astronomers met to discuss Pluto's classification. No consensus could be reached, but the attendees seem to have acquiesced to the IAU by describing Pluto as a Plutoid Trans-Neptunian Kuiper Belt object.