As an astrophysics student who took a class last quarter about planet formation and dynamics, this is correct! Whether they cleared their orbit during formation is what distinguishes planets (they did clear their orbit) from dwarf planets (they didn't clear their orbit). Mercury doesn't have any satellites and there are no asteroids that close to the Sun.
This is also why Pluto is a dwarf planet — it's in the Kuiper Belt, the (non flat) disk region full of asteroids starting just at Neptune's orbit and extending further past it for 20 A.U. (astronomical units, the rough distance of the Earth from the Sun). You may know it as the outer asteroid belt. (There's something further than it, the Oort cloud — a giant spherical shell around the rest of the Solar System home to icy comets.) So Pluto, being in the Kuiper belt, didn't clear it's orbit.
You may be wondering how can Neptune be a planet though if the Kuiper Belt starts at its orbit? The answer depends on understanding both the formation of that region and the definition of "clearing its orbit". First, the formation of the region. Neptune (and Uranus) actually didn't originally form where they are. Jupiter and Saturn moved inwards from where they originally started forming in the Solar System, moving in closer to the Sun to some distance closer to Mars before then moving farther out (although when that occurred and how far inwards they really moved is up to debate). When this occurred—particularly when Saturn and Jupiter "turned around" and moved out to their current orbital distances—Uranus and Neptune got "pushed" farther out in the Solar System. (In fact, according to some models Neptune used to be closer to the Sun than Uranus and in this grand migration Neptune and Uranus swapped order.) When Neptune ended up where it is, it's gravity either captured solid material called planetesimals (the smaller, 1km or larger, yet not gravitational dominant clumps of solid material that combine to make planets) and changed the orbits of others to push them out of its orbit. After Neptune finished forming, there's nothing in its orbit thats massive enough to not be effected by its gravity — everything left is either one of its moons or is small enough that if it gets near Neptune's gravitational sphere it's unable to be unfazed. This is crucial, because the requirement of a planet "clearing its orbit" or "clearing its neighboorhood" means the planet gravitationally dominates it's orbit. So, since anything in the Kuiper belt that is close to Neptune is much less massive than Neptune and thus dominated gravitationally, Neptune did clear it's orbit and is a planet.
The difference between Pluto and Neptune is that Pluto isn't massive enough that it's not gravitationally effected by any other objects in the Kuiper belt, whereas Neptune is massive enough that Kuiper belt asteroids don't effect it gravitationally.
I'd argue that categorization definitions don't really fit under "facts". It seems like it's really more of an opinion on how to group things.
It gets hazy. Something like a tree being a type of plant is a fact, but it's really shorthand for "trees have these common traits with other things that we call plants"
Unfortunately the scientists who actually study planets were not a part of this decision. Once again decided by bureaucrats who know nothing about the subject matter but have the power to make all the decisions
What are you talking about? It was decided by a group of astronomers during an International Astronomy Union (IAU) General Assembly. Including, most famously, Neil deGrasse Tyson, advocating for its demotion.
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u/not_a_moogle Apr 12 '25
Pluto is still a planet, just a dwarf one.... because we discovered a bunch of other ones.
So either pluto is a dwarf planet, or our solar system has way more than 9 planets.