Cue that song from Frozen for all Defenders of Pluto:
Can anyone give me a decent reason for the Plutoid category, other than to not have to classify any Trans-Neptunian Object as a planet? Because it seems more reactionary to me to not have bodies with moons classified as planets simply by being outside the orbit of Neptune,
Unless I’m misremembering my Sagan, appeal to authority has no place in science, so telling me the IAU decided it doesn’t count.
(I’m a grumpy old shit, and I feel like we wouldn’t be having this discussion if someone decided all Shakespeare plays were “Shakespheroids” and not plays. If there is an actual semantic or semiotic purpose to “Plutoid”, or if it can be applied to bodies outside our solar system, or hell to anything beyond Pluto, Makemake or Haumea, I’m on board with it. )
Plutoid is a sub-category of Dwarf Planet. It’s like saying some Shakespeare plays are tragedies and some are comedies. And ‘dwarf planet’ exists for the same reason we call Shakespeare plays SHAKESPEARE Plays - to distinguish them from other plays that have many similarities, but differ in significant ways.
So, look, a planet is round, orbits a sun, and ‘clears’ its neighborhood of any other crap that might have been in that same orbit. Pluto passes through the Kuiper Belt as part of its orbit, in fact, it spends most of its time out there, and has clearly NOT cleared the Kuiper Belt of material, even within its own orbit. Since Pluto is round and orbits a sun, it still qualifies as a dwarf planet.
Even if Pluto *were* a planet, it would still be a Trans-Neptunian Object - because it’s BEYOND NEPTUNE. If we ever find a TNO that has cleared its orbit within the Kuiper, and is round, we’d be obligated to call it a planet.
A Plutoid/Pluton is a dwarf planet with a highly inclined, highly eccentric orbit. Dwarf planets with this kind of orbit are more than likely either captured, or are just extremely large Kuiper Belt objects. It’s at least as useful a distinction as distinguishing between ducks and penguins, who are both just “swimming birds”, really.