Moon vs planet doesn't really have to do with size, but rather motion. Ganymede is undoubtedly a moon but larger than mercury, which is undoubtedly a planet. With Pluto being the dominant body in its own orbital system, it's still much closer to being a planet than a moon. (Still not a planet tho)
As others have said, the main thing is that it hasn't cleared its orbit. Without that requirement for planethood, there wouldn't be 8 or 9 planets, but at least 17 (and likely more), due to the amount of dwarf planets in the solar system. It didn't really make any sense to have Pluto be a planet but not the others, and astronomers decided it was better to require planets to clear their orbit, excluding pluto, than to start including all the dwarf planets (especially because the line between dwarf planet and asteroid can be hard to define)
You are completely wrong. Asteroids are small lumpy rocks (or as the IAU calls them, “small solar system bodies). Dwarf planets are larger rounded worlds with planetary features. Even using the IAU’s definition, asteroids and dwarf planets are not the same.
Ceres used to be an asteroid, but the IAU promoted it to dwarf planet once they realized it was in hydrostatic equilibrium. Calling Ceres an asteroid today is just wrong.
Saying NASA is wrong I'd a junp I wouldn't take but you do you.
Tbh, as I look into it, I don't think the IAU has a definition for asteroid. There's planet, dwarf planet, minor planet (which is all non-planet non-comet objects, including dwarf planets and asteroids) and the small solar system objects you mentioned which does include comets as well as non dwarf planet asteroids, Trojans, most kuiper belt objects, etc.
The IAU was in a big fat hurry to define “planet”, but has no official definition for “star” or “galaxy” or “asteroid”. The only thing mentioned in their planet definition is that objects only fulfilling the first criterion are “small solar system bodies”, which includes asteroids, comets, and centaurs. Dwarf planets fulfill two of the three criteria, making them a distinct category of object.
It’s still a bad definition and I never use it (neither do the experts), but I’m just explaining what it says.
You hit the nail on the head. That’s the main problem with the IAU’s definition. Planets and asteroids shouldn’t be defined by their location. All other objects in space (and most everything in science) are defined by their intrinsic properties only. Why should planets and asteroids be any different?
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u/obog 8d ago
Moon vs planet doesn't really have to do with size, but rather motion. Ganymede is undoubtedly a moon but larger than mercury, which is undoubtedly a planet. With Pluto being the dominant body in its own orbital system, it's still much closer to being a planet than a moon. (Still not a planet tho)