Why did Pluto becopme a dwarf planet?

Answer 1

Although Pluto hasn't changed in size, the 2006 (re)definition of a planet by the IAU includes three criteria - 1) it is in orbit around the Sun, 2) it has achieved hydrostatic equilibrium (characterized by sufficient mass to have a rounded shape), 3) it has cleared the area around it orbit. Under this last criteria, Pluto, formerly considered a planet, is now classified as a dwarf planet as it has only cleared a tiny fraction of its orbit.

Answer 2

The category of "dwarf planet" is fairly new; at the time Pluto was discovered, this category hadn't been invented yet. Also, Pluto was initially believed to be larger; it was later discovered that it's really smaller than initially believed.

Answer 3

When Pluto was discovered:

* The category "dwarf planet" didn't exist yet.

* Pluto was believed to be far larger than it actually is.