Classifications impose structure on a collection of celestial objects, and we can derive relationships, commonalities, understand their development, the physics governing these objects, and build useful physical and mathematical models to widen our understanding of the universe.
Most reasonable scientists understand that some of these objects will blur the edges—like many moons being more massive and larger than Mercury. Or like the largest super-Earth exoplanets being nearly as massive as Neptune, or the largest gas giant exoplanets being 30-80 times as massive as Jupiter and hence having a surface temperature of ~2000 K. But that is part of science, and possibly even where the most interesting work is.
I wasn't arguing against classification, just that I remain unconvinced that the classification which IAU adopted is superior to the alternatives.
One problem with the IAU definition: it only works for our planetary system, not for exoplanets – with the end result that we have two different definitions of "planet", one for our system, one for everywhere else.
Now, of course, that in part reflects that we have vastly more information about our own system than any other – but, a few centuries or millennia from now, I expect we'll know a lot more about some of those other systems – we will probably have sent probes to explore the nearer ones by then – at which point people (or AIs?) will probably push to unify the two definitions. My own bet is they'll be more likely to unify them by dropping "clear the neighbourhood" from the our-system definition than extending it to apply to other systems. Splitting the common category of "planet" into two subcategories based on "does it clear its neighbourhood" works much better than insisting on two disjoint categories based on that criterion when you have very limited information on where that criterion is met.
It very much seems like the definition was just shoe-horned to stem any more planets needing to be added. It would be interesting to know if astronomers walk the walk when it comes the definition. If I research when planets were formed would I get the date when their neighborhoods were cleared? Do discussions around large events that send debris into the neighborhood of planets correctly reflect that they cease to be planets until it is cleared?
Alan Stern said it best
>"We're going to call it a cow, except when it's in a herd." A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.
>And the IAU definition is even worse, because it produces different categorizations for identical objects, depending on where they are. Get this — Earth at the same distance from the sun [as Pluto] would not be a planet by the IAU’s measure, because Earth can’t clear that zone either. I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth!
>That's crazy. No wonder teachers and students are frustrated, confused, unhappy. It reflects poorly on astronomy, to have something so illogical.
https://www.space.com/12710-pluto-defender-alan-stern-dwarf-...
Now, Stern's broader argument in that interview is that we should not take into account the dynamical behaviour of planets. I disagree, especially since dynamics determine the static future state of any given object. Take chthonian planets—former gas giants whose gaseous envelopes have been stripped away by the primary. Do we insist that they are still gas giants? No, we even came up with a name for the transitory period when these planets are losing their gas envelopes.
Personally, I think Stern was being a little more animated to drum up support for and shine a beacon on New Horizons, a then-ongoing mission that he was principal investigator of.
To directly answer your question:
> If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet.
There actually are (multiple) mathematical formulations for 'clearing the neighbourhood'[2]; it's not purely empirical or based on vibes. Stern proposed one in the above-mentioned paper. Jean-Luc Margot suggests several more[3]. I suspect a more universal model will be probabilistic but grounded in celestial mechanics.
> Does it realistically outweigh decades or centuries of peoples intuition of the existing definition?
People's intuition for 'planet' in the centuries before telescopes were invented was 'something that moves against the fixed background of stars'—that is literally the Greek etymon for the English 'planet'. The Sun and Moon used to be a part of the 'classical planets'. We didn't even find the last two (or three) planets in our own Solar System until around two centuries ago. We didn't know about nuclear fusion until 1925, and we didn't know other galaxies existed until similarly recently. We didn't understand the inflationary model of the universe until the 1960s. The first exoplanet was discovered only in 1992.
We have repeatedly and retroactively renamed, retconned, reworked, revamped and rewound scientific definitions and models over and over again. I think planets will be OK.
[1]: https://doi.org/10.1017/S1539299600013289
[2]: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood
It's clear that they worked backwards from the eight planets to come up with a classification system that fit, but in realizing the problems with the approach, just gave up and created the carve-out.
Also, just as another example of the absurdity of the IAU definition, there's a pretty strong argument to be made that Earth's moon would be a planet under the criteria: it orbits the sun, it's round, and it's massive enough / positioned such that it's capable of clearing its neighborhood.
Is this classification useful for everyday life? Not at all, but looking at things from different perspectives for a change can be quite amusing.
Even something as simple as "planets" contains "major planets" and "minor planets" would have avoided 20 years of people arguing about the names instead of caring to learn what's different about them. But you never hear people explaining why the IAU insists "dwarf planets" aren't "planets" even when "dwarf planets" has several subcategories which don't share the "planet" moniker, you just hear people argue about missing Pluto or call other people ignorant of the technical details of the categories.
I find people do this a lot with space stuff because they're interested in it but don't really know much about it.
It's ultimately irrelevant to the subject at hand and detracts from the conversation.
Just expect to be corrected and "well, achually"-ed by people who follow the current naming system, especially in science circles.
With such “well, ackshually”s as “you didn’t really do any work at all when you were pushing on that tree trunk” and “you specified watts, but you really wanted to use lumens”.
My Very Elegant Mother Just Served Us Nachos
By some extreme definitions, Earth might not be a planet. Some say the moon is too big to qualify the earth "cleared out its orbit" criteria, making Earth-moon some sort of double planet... like Pluto-Charon.
If they really didn't want people to continue to refer to them as planets, then they should have chosen a completely different noun.