Question

Why isn't pluto considered as a planet anymore? In which year did this happen? Who decided it ?

Answer 1

According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:

1.A planet must be round.
2.A planet must orbit the sun.
3.A planet must have “cleared the neighborhood" of its orbit. 

Pluto follows the first two rules: It is round, and it orbits the sun.

It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this rule, Pluto is no longer considered a planet.

in 2006, it was relegated to the status of dwarf planet by the International Astronomical Union (IAU).