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WHY PLUTO BECOME A DRAWF PLANET?

A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects. 

Pluto was known as the smallest planet in the solar system and the ninth planet from the sun.

Today, Pluto is called a "dwarf planet." A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path.