this post was submitted on 28 Jun 2024
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The issue is, as I understand it, we either have 8 planets (or 9, if there is an exoplanet), or a whole bunch of planets, depending on how narrowly we define them.
Yeah this is the correct take. Either Pluto (and by extension, any object of similar size) is a planet, which would mean there's thousands of Pluto-sized planets in the solar system; or pluto is 'too small' to be a planet. Which is the answer they (Sci community) settled on, because if every comet/asteroid is within the threshold definition of 'planet' then there would be no point in distinguishing planets at all.
Kinda like how we have dwarf-stars and supermassive stars 1000x bigger than our sun. If they were all the same size there would be no point defining them beyond 'star'.
Pluto being too small isn't actually the grounds on which it got demoted. The size requirement is just being massive enough to reach hydrostatic equilibrium - that is, be heavy enough that it's round. Pluto does meet this one
The one it fails is clearing its orbit. This basically means being much heavier than everything else in the same orbit. Be gravitationally in charge of your orbit. The other eight are all hundreds if not thousands of times heavier than everything else in their orbit (not including moons, since they're gravitationally bound to the planet anyway), whereas Pluto is less than a tenth of the total mass in its own orbit. Ceres is actually more gravitationally dominant over its orbit than that, although still nowhere near the eight planets.
This one sounds a bit weird at first, but I kinda like how it has such a massive delineation between the things we instinctively think of as planets and everything else.
It's also the fact that Pluto doesn't have its own orbital slot. It is clearly something that escaped Uranus at some point, that's why their orbits intersect. A planet doesn't just have to have a certain size, it also has to have its own distinct orbital path.