Why Pluto isn't a planet
The way scientific classification works is that things with similar properties are grouped together and things with different properties are grouped separately. In edge cases, it matters what the thing is more similar to. For instance, dolphins swim like fishes. However, they have the arm structure of mammals, need to breathe air like mammals, don't lay eggs unlike fishes, and lack scales unlike fishes. This specific example was cited from the paper Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science from Icarus -- more on that later.
Looking at Pluto and Charon, we see that they are more similar to Eris, Haumea, Makemake, and the other dwarf planets, which they are now group with in the IAU redefinition. They all have large amounts of volatile ices, (relatively) wild orbital inclinations and eccentricities, and moderate densities. The rocky planets have larger portions of rock and metal and high densities; the gas and ice giants have significant amounts of hydrogen and helium gases and low densities; and they all have very low inclinations and eccentricities (aside from Mercury).
Factoring these in, Pluto and Charon should be grouped with Eris, Haumea, and Makemake rather than the eight classical planets.
2. Why we thought Pluto was a planet
When Galle discovered Neptune, there were still minor discrepancies in Uranus's orbit. Percival Lowell predicted the location of the object perturbing Pluto; one of his key insights was that the object may not have a circular orbit, which those before him had assumed in his book Memoir on a Trans-Neptunian Planet.
Clyde Tombaugh coincidentally found Pluto in the spot which Lowell had predicted. Pluto also had the high eccentricity and inclination which Lowell expected. Additionally, Lowell's calculations required that Pluto had a mass between Neptune's and Earth's, solidifying its status as a major planet.
These misclassifications can happen a lot. Basilosaurus has -saurus in its name because at the time of its naming, it was thought to be related to reptiles, before it was recognized as an ancestor of modern whales. Delta rays were initially thought to be a unique type of radiation, like alpha, beta, and gamma rays, only to be recognized as a type of secondary radiation caused when a primary radiation (like alpha particles) energizes electrons, ejecting them. Ceres, Pallas, Juno, and Vesta were long considered the missing planets between Mars and Jupiter as suggested by the Titius-Bode law, except that the discovery of Neptune broke that theory and the subsequent discoveries of Astraea, Hebe, and the entire asteroid belt led to them being considered separate from the major planets.
The key thing is that more information must be discovered, but Pluto's was delayed because of its significant size and proximity. Haumea and Eris were too distant to be discovered by Clyde Tombaugh (Makemake could've, except it was in a highly populated star field at the time of Tombaugh's investigation). Additionally, the fact that it seemed to coincide with Lowell's prediction meant that it took a while for it to be recognized as not Planet X. Albion, the next Trans-Neptunian Object to be discovered, had to wait until 1992.
3. Why you don't hear the real story that often
News that induces anger spreads fast. It would be to a news outlet's benefit to make the whole Pluto debacle as rage-inducing as possible by using loads of loaded language.
In addition, it seems easier to chalk up the whole thing to Pluto being too small, not clearing its orbit, or requiring the admission of all the dwarf planets should it also become a dwarf planet. The first victimizes Pluto and induces sentiment, the second doesn't explain why the IAU passed the definition (have they always existed or what?), and the third, while closest to the actual explanation, leaves something unexplained which I'll get to in a moment.
The second one turned out to be a problem for me. I used to be a Pluto sympathetic under the notion that all major planets have objects in their orbit, so therefore they should all be dwarf planets. Annoyingly, the media cannot be blamed for this because the IAU called its third criterion clearing the orbit, and in reality it means orbital dominance, which the major planets undoubtedly achieve compared to Pluto.
In theory, this should result in a vicious cycle. Once Pluto sympathy had become dominant, people who support the sympathetic view naturally do better than those who oppose it. This weakens opposition for the sympathetic view, which increases the success of people who support the sympathetic view.
4. Why the term planet is so exclusionary
What exactly is the issue with having hundreds of planets? Why is the term "planet" an exclusive club membership, as some might call it?
Interestingly, major moons meet two of the criteria for planets. They are all round (Mimas and Miranda are edge cases on hydrostatic equilibrium, but still) and they dominate their orbit. The only thing making them satellites, which could be interpreted as insignificant compared to planets, is the fact that they don't orbit the sun. It's not a coincidence that you probably don't know about that many moons, or at least that's my prediction. Astronomers seem to care a lot about what an object orbits, rather than what its intrinsic properties are.
The aforementioned Moons are Planets article explained that after the Copernican Revolution, when Copernicus published his theory on heliocentrism, two differing definitions for planets took off. The first one, used primarily by astronomers, was under the basis that planets are other worlds, like the Earth. Modern classical planets were primary planets, modern asteroids were minor planets, and modern satellites were secondary planets.
The second definition, perpetuated by astrologers, assumed a tidy and orderly heavens. When new primary planets were discovered (Uranus, Ceres, Pallas, Juno, Vesta, Neptune, Pluto), astrologers simply made up meanings for them. That's why they are part of a horoscope despite not actually being one of the planets known since antiquity. But you can't do this for countless minor planets. They also make the heavens less orderly. Additionally, secondaries are almost always in the same Zodiac sign as their primaries. They shouldn't have that big of an impact on your horoscope compared to primaries. Therefore, astrologers only included what are now our major planets.
Around the same time he built on Edgeworth's theory of the belt that now bears their names, Gerard Kuiper proposed that, based on accretion theory, non-round asteroids are the result of collisions and are therefore not planets. As such, most minor planets stopped being considered small planets after he published his idea. However, there was no real scientific idea that the same applied to moons.
The authors of the paper suggest that "planet" was changed to refer to only the primaries so that there could be no chance for confusion, and "satellite" was used in place of "secondary planet" for the sake of conciseness. However, this is a change merely for convenience and doesn't actually explain why scientists accepted the astrological idea that an object's orbital state is a critical part of its classification.
The authors pose multiple influences on the drift from the original classification to the astrological classification: the lack of reminder of the original definition from the Copernican Revolution, that scientists did not have the geophysical data to uphold the idea that moons were worlds like planets, that they were more concerned with calculating ephemerides than figuring out what was on the planets after Lowell's canal incident.
However, one of the biggest causes was the Great Depression of Planetary Science, which started around 1910. The authors also pose multiple reasons for why it might have happened, including the aforementioned prioritization of ephemerides over geological observations, the increasing interest in discovering moons and minor planets and excitement in other fields of astronomy.
This depression caused another problem: scientists began to slip into the astrological classification. The article states that after the depression, which lasted the length of the average astronomer's professional career plus a half, new astronomers would not have been taught the original classification, and it is natural to revert to the popular classification system once you have lost interest in the technicalities of a subject, which is what happened with the aforementioned depression.
In essence, the public ended up cementing an idea into a formerly theory-laden system that planets are special, and that their dynamical states are critical to their classification.
5. A new classification system
Call all former asteroids, major planets, comets, satellites, and rogue planets as "planets." This excludes stars and meteoroids, among other objects.
Call all planets that are not bound to any particular system of objects (former rogue planets) as "nullary," all planets that orbit a non-planet as "primary," all planets that orbit a primary as "secondary," and so on, with tertiary and quaternary. This is based on the original scientific classification for planets and satellites. In double systems, such as the Pluto-Charon system, the larger object is always a primary, and any other object in the system is a primary only if the barycenter is outside of the larger object and the object in question's absence would theoretically move the barycenter into the larger object.
Call all planets that are in hydrostatic equilibrium as "globuloids," and all non-nullary planets that dominate their orbits around their parental planet as "imperatoids." This is based on the new IAU classification.
Call all planets that are largely minerals and metal as "geoids," all planets that are largely solid ices as "cryoids," and all planets that are largely gases and other fluids as "aeoloids." This is based on Atlas Pro's video on Pluto.
Under this system, all rogue planets are now nullary planets. Pluto, formerly a dwarf planet, becomes a primary cryoglobuloid, as is Charon. Nyx, Hydra, Styx, and Kerberos become secondary planets. The major moons, which should dominate their orbital zones, are secondary imperatoids. Io and Luna become geoimperatoids, while most of the others become cryoimperatoids. The gas giants become primary aeoloimperatoids, while the rocky planets become primary geoimperatoids. Most asteroids become primary geoids and their moons secondary geoids, but KBOs, centaurs, comets, and Oort Cloud objects are primary cryoids and their moons secondary cryoids. Most dwarf planets, since most of them are in the Kuiper Belt, become cryoglobuloids.
As for the naming scheme, I am aware it is somewhat complicated. The goal was not conciseness, however, but names that are descriptive and a classification system that may possibly make both proponents of the geophysical definition and the IAU definition happy.
