Who has the authority to decide what is a planet or what is not?
I don’t know. I still consider Pluto to be a planet.
Some astronomy board or body. It’s just a matter of nomenclature. Pluto is still there even if it’s not called a planet.
The International Astronomical Union (IAU) is the self-appointed arbiter of what is considered a planet, and decide what name is/is not acceptable for celestial bodies.
It's sort of like the Nobel committee. It wasn't created by treaty of any legal document; it's just a collection of academics who have arrogated to themselves authority that all the other eggheads defer to.
These jackwagons decided amongst themselves that Pluto was no longer considered a planet, and the rest of the media eagerly fell into line. The majority of planetary scientists dissent, but they can't public quarrel with the IAU lest their peer reviews/grant money/telescope time dries up.