Planets go missing

If a chance gravitational encounter threw a large KBO (Kuiper Belt Object) into an orbit between Uranus and Neptune, would it become a planet? Why is Neptune regarded as a planet anyway, when there are whole families of asteroids and KBO’s - including Pluto - whose orbits cross its own?

And if we exclude Trojans and resonant orbiting bodies from the “cleared out orbit” rule, isn’t Pluto then a planet after all, along with Eris and others?

The IAU has rejected a clean, simple system based on purely physical elements, where you could look at a body and say, yes, that’s a planet. In its stead it has set up an artificial and unworkable one requiring ongoing arbitrary adjustments purely so that what they’ve already decided to call planets, can still be called planets.

What earthly purpose is there in the IAU taking to itself a process of “accreditation” for planets? If it was just a matter of awarding planet status to those bodies they already wanted to be planets, couldn’t they have just published a list, saying, “These are planets and nothing else is, unless we say so”.

It would have been a lot easier, and a lot less dumb. Planets are now no longer a physical class of bodies, but merely a label conferred by committee, subject to ongoing argument.

Worst of all though, is that the word will no longer have any scientific value. It has already been noted by the IAU itself that we can no longer say that the bodies we have found orbiting other stars can be called planets, because they’re too far away, and we can’t test the new criteria for planethood. So now only our own solar system has planets.

In a few years when we start to find significant numbers of terrestrial bodies around other stars, does this mean they too will have to be called something else - not because they’re strange or small or don’t look like planets, but simply because we can’t “accredit” them without first finding out if they’ve “cleared out their orbits of other bodies - except (presumably) Trojans?”

Nobody is going to go for this. The general public already see it as a joke. Certainly the scientific community will cease to use the word “planet” for any constructive purpose, and the “definition” will break down within a few years as we find bodies that don’t fit the description, or whose orbital or physical elements test the limits and the vagueness of the terms used by the IAU in its definition.

They will be obviously “planets” in any sensible sense - but we won’t be able to call them that, because a bunch of scientists didn’t like the idea of accepting the fact that, out past Neptune, planets simply look like that. They have elliptical orbits, and they’re small and icy. So what?

Ceres is a planet. Again, what’s the problem? It’s a little planet because Jupiter stole most of the stuff that would have made a decent-sized body out of it. That’s fine. At least we can look at it and say, “It’s big enough to be spheroidal, so it’s a planet”. We don’t have to investigate its “orbital neighbourhood” (however defined) to decide if it should have planethood “conferred” on it by the IAU.

Losing one planet might be regarded as a mistake, but losing four really does look like carelessness.