Michael Guillen weighs in on planets. He is right, saying concepts like 'planet' should't be voted on. The *point* of science is to evolve our conceptualization of things. There is deep, interesting history how 'planet' evolved in the tug-of-war between science and culture. /1
2/ There were FOUR main events in the evolution of 'planet'. The 1st was the Copernican Revolution, where scientists rejected the dynamical concept (wandering stars orbiting Earth) & replaced it with a geophysical concept: planets are bodies like Earth regardless what they orbit.
3/ That concept was embraced by astronomers for over 300 years until sometime in the 1900s. Around 1920, evidence shows that scientists simply forgot that moons are also a type of planet. That's when the textbook definition of 'planet' stopped including moons as planets.
4/ I say they "simply forgot" because we can see there was nothing happening in planetary science that drove a change in the definition. Instead, we see usage of terminology slowly drifted as a pure Gaussian (non-driven diffusive process) until moons were rarely called planets.
5/ That's when textbooks stopped giving the historic definition that existed for over 300 years since Galileo. The idea that planets have to directly orbit a star was introduced by default, no scientific reason. Dynamics had made a sneaky comeback into the concept of a planet!
6/ (That laid some of the groundwork for the bad vote that happened in 2006.) When we ask, *why* did astronomers default to putting 'orbits' back into the concept of a planet, there's no known answer except it must've been cultural bias. Scientists are affected by culture, too.
7/ And *why* does culture default to making moons not planets, holding a concept that planets must orbit a star directly? I can't see any answer in the record other than it is similar to the older geocentric view, where planets are orderly in a tidy sequence.
8/ OTOH, in the 1960s planetary scientists started calling moons "planet" again. By then, spacecraft missions were visiting those planets & we rediscovered that the historic concept we had from Galileo, purely geophysical with no reference to what a body orbits, is still useful.
9/ But sadly the community was fractured by then, and a most astronomers weren't dealing with the geology in comparative planetary, so they never went back to the historically useful concept of 'planet'. Most astronomers kept orbital dynamics as part of their concept.
10/ Historically, planets that orbit a star directly are called "primary planet" (or just "planet" for short), while "secondary planets" are also called "satellite" or "moon". "2ndary planet" is too verbose so it fell out of favor. Thus, astronomers forgot that moons are planets.
11/ After the Copernican Revolution that was the 2nd change that happened in the concept of a planet. A generation of astronomers were not explicitly taught that dynamics is irrelevant to planethood, that was all it took to get screwed up☹️ It was 1920 to 1960 when that occurred.
12/ The 3rd major change will have to wait for a moment while I drive my daughter to the skate shop :)
14/ Recap: Galileo found lunar mountains, so he forged a new concept of ‘planets’: they are geophysical objects like Earth (including moons). This new concept was born from alignment with new theory, and so it replaced the old dynamical concept. He had to fight culture over this! 
15/ Then, over a period of 300 years, astronomers kept including moons as actual planets, because there was no change in the theory and thus no motive to forge a new concept of ‘planet’. Every textbook for 300 years explicitly said that moons are planets. But, slowly...
16/...astronomers changed terminology and forgot the term “secondary planet.” There was no science driving that change. Nobody was arguing that moons are different than planets. To the contrary, the *commonality* of moons and primary planets was often used in scientific papers.
17/ So the 2nd major change in the ‘planet’ concept appears to have been primarily a cultural influence rather than scientific. It will take more space to fully make this argument, and a paper is forthcoming. This brings us to the 3rd major change in the concept of ‘planet’.
18/ The 3rd major change in the concept of ‘planet’ involves the *size* of planets. There was a long trend through the 1800s of finding ever-smaller asteroids, and they were all included as primary planets. They were abruptly made non-planets following a paper by Kuiper in 1953.
19/ Laplace’s old planet formation theory was believed to be proven by the asteroids, so a concept of ‘planet’ that included asteroids was aligned with that theory. Kuiper’s 1953 paper on accretion theory split the ‘planet’ concept into 2 groups, so asteroids became non-planets.
20/ The bibliometrics shows that astronomers quickly stopped calling asteroids “planet” in the late 1950s. (Papers on theory called them planets right up to Kuiper’s 1953 paper then *immediately* stopped calling them planets.) So what was the essence of this 3rd concept change?
21/ Galileo gave us the concept that a planet is a geophysically complex body like Earth. All astronomers began arguing the complexity of all the planets, even the idea they all possess life. By including the ever-smaller asteroids, we were undermining that concept, in effect.
22/ By removing asteroids from the concept of ‘planet’, scientists were restoring Galileo’s concept that they’re Earth-like complex geophysical bodies. This was motivated by science, aligning the planet concept with theory. This was good! That’s how science is supposed to work!
23/ Strangely, in this 3rd case scientists didn’t have to fight culture, and they weren’t submitting to culture either. Culture had *never* accepted the asteroids as planets. Science happily moved to a concept that culture already held. Nobody got tried, like Galileo did.
24/ The essence of this 3rd concept change was that a planet is geophysically complex, but small asteroids are not. (Papers argued that asteroid Ceres *is* a planet for precisely this reason.) This concept aligned usefully to support new *theory*, which is how science works!
25/ So now, the 4th major event in the concept of a planet: the vote in 2006. They said a planet is “an object that clears an orbit”. That is a *completely* different concept than we *ever* had before. It was designed to select roughly the same set of 8 or 9 objects, but...
26/ to do so they invented an orbit-clearing idea, never part of the concept ever before. It was motivated in the interplay of science and culture, again, but this time the astronomers unwittingly rejected science for culture. Look at the arguments on *both* sides:
27/ AFAICT, the underlying motive was to preserve the idea that planets are “special”, reflecting an orderliness of nature that culture desires. It was easy to fall into that: the idea that moons aren’t planets already injected dynamics into the concept held by most astronomers.
28/ It wasn’t motivated by newly developed theory like the case with asteroids. But presentism (wrongly interpreting past events by projecting current beliefs onto ppl who lived in the past) was used to misinterpret the history of how asteroids became non-planets.
29/ Presentism is also what keeps astronomers nowadays from grasping that moons have always been true planets, since the Copernican Revolution and even before, and that until very recently “orbiting a star” was never part of the concept of a planet (except in culture).
30/30 This interplay of culture and science is fascinating. We see 4 different ways that it played out in the 4 different events affecting the concept of ‘planet’. It makes you wonder, why are planets so darned important to culture? Culture doesn’t care about the defn of beetles!
