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


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

Because Pluto is co orbiting a second body where all the gas giants are way more massive and are the dominant gravitational body of the masses in their orbit radius.

The center of Mass of Pluto’s dual system with Charon is outside of Pluto’s physical body. A planet orbits the sun directly and other things orbit it. Pluto isn’t massive enough to be making this happen so it lost planet status. “Other” planets, while made entirely of gas, are bigger and because they have enough mass to have independent solar orbits, they are planets.

So, 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 is in orbit around the Sun, and it is round. However there are still asteroids in Pluto’s orbit, disqualifying it from being a planet.

Because even tho those planets are made of gas, they’re still absolutely massive. Literally, full of mass, full of stuff.

If you put Uranus/Neptune on a giant hypothetical balance scale, it would take 15-20 earths to equal one of those planets, Jupiter and saturn would take 100+ earths.

They may be mostly gas, but there’s A LOT of gas, they’re just absolutely huge and massive.

Pluto on the other hand is smaller than our own moon. So, for the clarity of classifications, Pluto was reclassified as a Dwarf Planet, because is still a big object, hundreds of miles across, but it really doesn’t fit into the same group as a planet and instead is more similar to other dwarf planets like Ceres or Eris.

They are large, and size matters. If you are big enough to clear your orbit, you are a planet. If not, you’re not.