Why is Pluto not considered a planet because it’s too small, but some planets are made entirely of gas and considered planets?

Question

Edit: I know I sound like an idiot in the replies, but I’m really asking genuine questions so thank you all for being patient and answering

In: Physics

Answer 1

The three criteria of the IAU for a full-sized planet are:

1. It is in orbit around the Sun.
2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
3. It has “cleared the neighborhood” around its orbit.

Pluto meets 2 out of 3 criteria. It has not cleared it’s neighbourhood. It is in the Kuiper Belt (an asteroid field). Therefore it has been downgraded to “Dwarf Planet” status.

Also, there are no planets that are entirely made up of gas. Jupiter, Saturn Neptune and Uranus all have solid cores, and all of those cores are larger than Earth.