Another is that the reason why the IAU decided to change the definition appears to have been for cultural reasons as well. Astronomers discovered other Kuiper belt objects. This included Eris, which was more massive than Pluto. They'd either have to tell people "There are more than 9 planets, and we don't know how many there are," or kick Pluto out and continue the myth of the simple solar system (with one fewer planet now). So they went with the more conservative option, by creating a new definition of planet that still isn't formally defined, and by explicitly saying dwarf planets aren't planets. Eris was discovered in 2005, the new IAU definition of planet was agreed upon in 2006.
It's worth noting that prior to this, the IAU didn't even define the word "planet," it was a popularly defined term. So there's an element of asserting control over something outside of their purview simply because they objected to the cultural connotations.
Here's an interesting article from the time that shows some of the thought that went into this[1].
Old definition of 'Planet' is still in play. Because Pluto doesn't meet the definition of 'Planet', it is now called a "Dwarf Planet".
Ultimately, it comes down to the scientific issue of Pluto, Charon, and Eris. Its hard to make Pluto a planet without calling Eris a planet. Pluto also interacts with Neptune and Charon in unfortunate ways unlike other "proper planets", like Earth, Mars, or Mercury.
The IAU definition of “planet” explicitly applies only to the solar system, and the only definition even attempted for exoplanets IIRC is to try to define a dividing line between them and brown dwarfs, effectively “exoplanets” are extrasolar planemos.
> It would be much more practical - that is, useful for actually communicating empirical information - to define a planet as a planemo, Luna, Ceres and Eris included.
I don't see how that follows, since having cleared an orbit is addition empirical information, and information that we are able to develop on solar system objects, if not extrasolar ones, in reasonable time. As is, e.g., whether or not an object is a natural satellite of something orbitting the sun or does so itself.
Right, which I think proves my point - surely we'd like planet and exoplanet to mean equivalent things.
> I don't see how that follows, since having cleared an orbit is addition empirical information, and information that we are able to develop on solar system objects, if not extrasolar ones, in reasonable time.
Well, I'd argue that Pluto (and Ceres before it) shows that it may take us an unreasonably long time. And the current definition awkwardly mixes two things that are really separate concerns: the characteristics of the object itself and what it's orbiting.
I remember actual excitement among the layperson upon the announcement of discovering a 10th planet.
Now with "dwarf planets" no one cares.
Tempus fugit!
It’s so much more exciting to know that the solar system is an enormous continuous spectrum of debris. The debris is named and catalogued, it’s that tangible.
I love seeing children go wide eyed at the mention of Ceres. It’s like the Azores of the planets — there’s another place/planet in the middle of nowhere that no one talks about?!
The final step in this is to recognize that, out of the full spectrum of Trojans to TNOs to moons to planets, the planets really are in a class of their own in terms of long term effect on other bodies in their orbit.
Pluto isn’t a planet. But that doesn’t mean we exclude it. It means we include Ceres and a whole lot more whenever we talk about our local system.
(Or at least the inner parts of our local system that we’ve found, so far.)
If this line of argument doesn't work, I point out that in a phylogenetic/cladistic nomenclature, all tetrapods are fish and dolphins are tetrapods, therefore dolphins are fish (and so are we.) Of course in the traditional nomenclature 'fish' is a paraphyletic group that specifically excludes tetrapods.
To add a more scientific example, "The Biology and Ecology of Giant Kelp Forests" (2015) similarly begins by explaining that kelp are brown macroalgae belonging in the kingdom Chromista rather than Plantae, and are thus is not technically plants. They then explain that they will call them plants anyway for the rest of the book.
It is, by the way, a surprisingly interesting book. Kelp was vastly more important between 150-100 years ago, and the science occasionally dips back to the scientific age of wonder of the late 1800s. (I always enjoy reading the classic papers from those times. They're written completely differently from modern scientific papers. Much less formal.)
Very common in biology: tomatoes and coconuts are fruits, bugs are a specific order of insects, electric eels are not eels, etc...
The people who named Pluto conflated the scientific discovery with an ancient mythology and tradition of wandering stars, as if it were a continuation of it, and now certain people affiliated with the scientific community go around on the Internet telling members of the general public that they're speaking imprecisely about their horoscopes, as if that behavior will have no impact on whether the public will listen to the scientific community on matters like how not to spread covid, vaccines, and climate change.
Edit: To explain my position better, modern Science has been using an old word - which belongs to everyone - for its own purposes, which is of course fine! However, members of the general public get to do that, too. Outside Science classrooms, I've seen a lot of annoying linguistic prescriptivism explaining that a committee of scientists got into a room and made a judgement call that the person being addressed is now wrong and must change their behavior because the committee said so, which is a great, if small, example of why there is an ongoing wave of anti-intellectualism.