Question

Why Pluto is not considered as a planet?

Answer 1

A planet is an astronomical body orbiting a star (here the Sun) that -
1) is massive enough to be rounded by its own gravity,
2) is not massive enough to cause thermonuclear fusion, and
3) has cleared its neighboring region of planetesimals.
Pluto follows the first two rules. 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.