Pluto was demoted because in the Kuiper Belt there are so many objects almost exactly like Pluto such as Eris and Makemake, like Ceres in the asteroid bely

Also since it's smaller it's called a dwarf planet

A planet needs to 1. Have cleared its orbit of anything else, 2. Be round due to its own gravity (hydrostatic equillibreum) and 3. It must orbit the Sun

Pluto fulfills 2 and 3 but not 1 and so its a dwarf

Yes I know I was using the simple version so people would actually understand what I was saying