Pluto - not the dog - isn't a planet anymore. Know why?

 

No - not that one. This one:

 

Pluto hasn’t completed a single orbit around the sun since it was discovered 'as a planet' in 1930 and downgraded to a dwarf planet in 2006. It takes Pluto 248 years to orbit the sun once.

Here's a great article describing it in detail:

https://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html