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
Jupiter and the gas giants are made of what we consider gasses, but on the planets they are mostly compressed into liquids by the intense pressure. The gas giants aren’t big clouds you could fly a submarine straight through.
Jupiter, Saturn, Uranus, and Neptune all have non-gaseous cores that are more massive than the whole of Pluto. Did you have some other planet “made entirely of gas” in mind?
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.
Because it being small isn’t itself disqualifying it from being a planet. All planets also have to have a clear orbit, where other large bodies do not regularly pass. Pluto doesn’t have that, and so in 2006, it got redesignated as a dwarf planet. It’s okay for it not to be a planet.
If we allowed it to be one, we’d also have to add Eris, Haumea, Makemake, Gonggong, and Quaoar, as well as many others. It’s much better to make a different classification for these other objects, aka, dwarf planets
They are large, and size matters. If you are big enough to clear your orbit, you are a planet. If not, you’re not.
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.
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.
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.
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.
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