It’s actually really simple. Jupiter has the Trojans, but they make up a tiny percentage of its mass. Pluto on the other hand is a very small portion of the mass even in its region of the Kuiper belt, which includes a lot of other plutinos, not to mention that it crosses past Neptune. That’s what makes it a belt – it’s composed of a lot of little objects with no single one dominating.
It drives me crazy that this keeps being claimed as if it were confusing even though it’s completely straightforward. The eight biggest masses have nothing of comparable size near them. All the smaller masses either orbit them, occur in belts, or have been ejected somewhere. It’s all well and good to say sizes and masses are arbitrary…and yet here they picked the one place there is actually a real cut off between two categories.
That’s not something that happened by chance either. Sure, Pluto and Haumea and Quaoar and Triton and all their fellows would have formed by accretion up to the size of planetesimals or so, but then the process stopped there. Where Mercury and Mars had a phase after that where they kept growing until all the material in their region had been used up. It’s not as simple as everything forming the way it is now, and there used to be other objects like Theia that got lost, but even so there is a real reason to consider them different. To the point where if that’s not what “planet” meant I would probably ditch it for another term that did.
On the other hand…
…there is this. The idea because Pluto was popular it shouldn’t have been “demoted”. Was it an insult to mushrooms when biologists decided they didn’t belong with the plants? And yet here this keeps being taken as if it were a question of nobility instead of trying to reflect the world around us. A good description of the solar system isn’t what counts, what counts is respecting America’s favorite, which is somehow not done unless you throw it in with objects it is not like.
You mention real problems…well, I would say anti-intellectualism and hatred of science are part of those. Sure, this particular case isn’t nearly as serious as global warming denial or anti-vaccines or race pseudoscience. But when you talk about schoolkids writing hate mail because they don’t like what scientists said, maybe instead of treating that as if it meant something was dubious about the decision, you should consider what the hell went wrong that the public would react to astronomy that way.
Because to me it looks like the same entitled ignorance as those other problems. How dare anyone let facts or understanding get in the way of what I want to be true. And I’m not inclined to give that a pass just because this time it stopped with hate mail instead of actually getting people killed.
