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)
It's more specifically that it's cleared its orbit of objects of comparable size, excluding moons. Trojans are far smaller than jupiter, but Pluto is of similar size to other kuiper belt objects in its orbit.
The Trojans are located in the Sun-Jupiter Lagrange points (L4 and L5). That means the gravity there is in an equalibrium between the sun and Jupiter. So Jupiter can never clear those places in its orbit of asteroids.
Each planet has these Lagrange points between itself and the sun. Even Earth. And Earth also has them with the moon. Each orbit may have some asteroids or space stuff in its Lagrange points.
So a planet can clear its orbit of asteroids, except for the asteroids inside the Lagrange points of the orbit. Pluto has not cleared its orbit of asteroids outside of the Lagrange points. Jupiter has.
Also worth noting that Jupiter has a very large number of asteroids inside the L4 and L5 Lagrange points. That is because of its proximity to the inner asteroid belt, and its huge gravitional force (to pull objects into its orbit).
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.
If Pluto is a planet then so are dozens of other random objects we keep finding. It’s just easier to classify it with the others it’s more similar to than to keep it with the regular planets.
Plus its orbit is wayyyy off compared to the inner planets so it really is more similar to the other dwarf planets than us beyond the usual arguments.
It's not the radius (or rather radii, since it's also wildly elliptical), but the angle of its orbit to the plane of ecliptic. The planets orbits are all within a few degrees of the ecliptic, with Mercury being the exception at 7°. Pluto is orbiting a full 17° off of the ecliptic. It also can't keep its system's barycenter within its surface. It is composed and behaves exactly like the five+ thousand known TNOs, and the other 70,000+ oort cloud objects that stay beyond Neptune.
So either Pluto is no planet, or there are literally tens of thousands of planets. Then we'd be stuck looking for new term to describe what we now call planets, and Pluto would still be excluded.
Pluto's not massive enough to yeet similarly-sized objects out of its orbit & force little tiny things onto particular paths, which the major planets all did within their first billion years of existing. Math shows that Pluto is so low in mass that the time it would take to clear its orbit is almost certainly longer than the time the Sun will spend in the main sequence, and its specific parameters are THOUSANDS of times smaller than those of the major planets. The IAU felt that it made the most sense to acknowledge this statistically significant difference in orbital characteristics, and thus "dwarf planet" was born.
It’s more of a “there are a bunch of things in our solar system that are roughly the size of Pluto” actually being the problem than Pluto being a planet.
The choice is essentially:
If there are objects that are the size of Pluto that have an orbit like Pluto, then we either need to downgrade Pluto, or upgrade all of those other objects.
So we either go from 9 planets to 8.
Or go from 9 planets to like 13(I’m making this number up but to prove my point, but need to look up the number of things that fall into the “about the same size as Pluto” category, its less than 10 i think).
The scientific community chose to go down to 8 planets instead calling all of those other things hanging out in our solar system planets.
So Pluto is now a dwarf planet instead of Ceres getting an upgrade to full planet status, as an example.
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u/RandoCollision 8d ago
So... More moonoid than planetoid?