Why is Pluto no longer a planet? To be a planet, you have to meet three rules: 1. Revolve around the Sun (Pluto meets this rule) 2. Have enough mass to be spherical (Pluto meets this rule) And the third rule, which it didn't meet and that's why it stopped being a planet, is that its orbit needs to be cleared because it shares its orbital zone with many other objects in the Kuiper Belt. This was discovered by the International Astronomical Union in 2006. They were also somewhat forced to remove Pluto from the list of planets because Eris was also part of the Kuiper Belt, and its orbit wasn't being cleared either. If Pluto had remained a planet, Eris would have been too, along with about 12 other planets, and there would have been too many planets.