Who decides if a planet is a planet?

Answer 1

The IAU decides on what is a planet under new guidelines

The International Astronomical Union decided that upon further discovery of smaller body's of planet like material in our solar system but far from the sun or Neptune's orbit of the sun that all new planets found would probably be small and are to be named as dwarf planets.

This was prompted recently by the discovery of the then named Xena now Eris our 5th dwarf planet

Eris is larger and rounder in shape than Pluto which prompted the Question When is a Planet not a planet? in 2006 they issued new guidelines where Pluto lost it's planetary status and was demoted to a dwarf planet.

Answer 2

The International Astronomical Union (IAU) decides the criteria for classifying an object as a planet. In 2006, the IAU defined a planet as a celestial body that orbits the Sun, is round due to its own gravity, and has cleared its orbit of other debris. Any object meeting these criteria is considered a planet.

Answer 3

Well the definition of a planet would be any celestial body (other than comets or satellites) that revolves around a star according to wordnetweb.com

So just base it to that.

Answer 4

The definition of a planet:

It orbits around the sun.

It has to have a great mass to make hydrostatic equilibrium. (a round shape)

it cleared the neighborhood.

Answer 5

The definition of a planet was set by the International Astronomical Union.