So, I recently came across these two papers by Philip Metzger et al, and if you're not aware of what's in them, I think you need to know about them.
The generally-out-there idea of the history of the word "planet" is something like this: Originally, any astronomical bodies that moved relative to the fixed stars were planets -- so, the sun and the moon were planets, but the Earth was not. Then, with the advent of heliocentrism, it came to be understood that the Earth is a planet; but the sun and the moon are not (since the moon orbits the Earth). Eventually more planets were discovered beyond the classical ones. Curiously, 4 of these planets shared approximately the same orbit between Mars and Jupiter. Eventually so many more new planets were found inbetween Mars and Jupiter that it became clear that these bodies didn't belong with the planets, and so they were reclassified as asteroids. More recently, it became clear that Pluto, which had been considered a planet, is actually just one member of the Kuiper Belt, much as Ceres, Pallas, Juno, and Vesta had turned out to just be four members of the asteroid belt; and so it became clear that Pluto shouldn't be considered a planet either, as was famously ratified in 2006 by a vote of the International Astronomical Union.
In fact, these papers contend, substantial parts of the above story are wrong, and the 2006 IAU vote was a mistake. Now to be clear, the astronomical facts are not in question -- Pluto really is part of the Kuiper Belt; it has not cleared its neighborhood. Moreover, nobody here is arguing that we should consider there to be nine planets, those being Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto; that state of affairs is clearly not tenable, and some solution to the problem was needed. But the solution to that problem that was chosen was the wrong one, and the reason it was chosen has a lot to do with the above incorrect story.
There's an assumption smuggled into the above story -- that it is generally agreed that the word "planet" must refer to something that there aren't too many of, and that if the number of planets gets too large, then the word isn't fulfilling its purpose. This assumption is core to the 2006 IAU vote -- we can't include Pluto as a planet, because then we'd have to include too much else for consistency. The problem here is that this is not the actual history of the word "planet", and that in fact, moons -- including the Moon -- were considered planets up until the 1920s, and asteroids were considered planets up until the 1950s. Not the 1850s, the 1950s! Moreover, while the demotion of asteroids from "planet" status happened (mostly) for a good scientific reason, the demotion of moons did not; it happened for another reason entirely.
The story I told you above is what many people think must have happened, but it's wrong. The Moon was not demoted from planet status with the switch to heliocentric cosmology, satellites of other planets were considered planets too, and when astronomers found hundreds of asteroids in the asteroid belt, scientists said, well, I guess there are hundreds of planets. There was no implicit cap on the number of planets that could be admitted! And when asteroids were finally demoted in the 1950s, it was for an entirely different reason.
So, what actually happened?
With the Copernican Revolution, the Earth became considered a planet, and the sun became no longer considered a planet. It was clearly different from the planets -- it shone with its own light, for instance. But the moon? The moon, people could see through their telescopes, was like the Earth. It had mountains! It was clearly the same sort of thing the Earth was. Other satellites were included as planets too, because, while their details couldn't be resolved, it was hypothesized that secondary planets (aka "moons" or "satellites") and primary planets ("planets") were all basically the same sort of thing. Which is roughly true!
A point the authors emphasize is that we have to remember here just what the Copernican Revolution meant. It wasn't just about a switch from geocentrism to heliocentrism; it was about recognizing the Copernican Principle, that we're not special, and part of that is that the cosmos can be messy. Yeah, there's planets orbiting the sun, and there's planets orbiting other planets, and maybe there's rogue planets out there wandering the universe not bound to any star! Who knows? Planets get kicked out of their orbits and captured into new ones. It happens. With this point of view, defining "planet" based on a temporary dynamical property -- remember, the Solar System is not long-term stable! -- seems a bit silly. The same body can be a "planet" for a few million years, then it gets captured by a larger body and now it's not a "planet" for the next few million years? Eh?
It's worth noting that, largely because of this, the IAU definition of planet is simply not used in many contexts -- I mean, it's only used inside our own solar system, nobody tries to apply these criteria to exoplanets or rogue planets. Consider that just as a planet's orbit can change, so can our knowledge of its orbit; and that obviously can happen on a much faster timescale. That's not going to happen much inside our solar system, but try to imagine how that would affect applying it to exoplanets! And planetary scientists don't use it either; they absolutely consider moons to be planets. (Also, having "dwarf planets" and "minor planets" not be "planets" is just awkward.)
OK. So if astronomers were perfectly happy to consider asteroids planets up until the 1950s, and moons planets up until the 1920s, what happened? How did this change, that we've all forgotten ever happened, occur?
In the case of asteroids, there's a good reason -- when we finally got a better view of asteroids, it became clear that they really weren't like planets. It had nothing to do with their being numerous or in a belt together, which had been known for a century. It was that they were small and lumpy and didn't have anything going on internally. And when this became clear, astronomers stopped including asteroids as planets pretty darn fast. Now these problems don't apply to Ceres, but I guess Ceres just got swept along (although some people continued to argue that Ceres ought to be considered a planet).
OK, but what about moons? Why did astronomers stop considering those to be planets back in the 1920s? Well, this wasn't a sudden change like with asteroids, but rather seems to have happened more slowly via a process of diffusion. OK, but where did it start? Well, there's something important I've left out of the above -- above I talked about what scientists considered to be planets. But there's a parallel story of what the public considered to be planets, and that goes a little differently.
Whenever an existing word becomes a technical term, there's likely to be some disagreement between the existing usage and the technical usage. Sometimes, those using the term technically are able to convince the public to go along -- e.g., just about everyone these days appreciates that whales are not fish; the technical usage has overtaken the original common usage. And sometimes they coexist harmlessly. But sometimes it's a bit more troublesome than that.
See, knowledge of heliocentric astronomy didn't filter down to the public so quickly. It became generally accepted among astronomers in the early 1600s, but it didn't filter down to educational materials until the early 1700s, and it wasn't until the second half of the century that the general public started to really accept it. And when they did -- well, the general public may finally have been ready for heliocentrism, but they weren't really ready for the full implications of the Copernican Revolution. While the scientific notion of "planet" included moons, and later asteroids, the popular notion would exclude both of these.
Why? Well, there were two main reasons. The first was astrology. Having too many planets isn't a problem for astronomy... but it is for astrology! Astrologers went along with Uranus, and some of them went along with the initial 4 asteroids for a while, but a whole asteroid belt? Ridiculous. These had to be excluded from the planets; astrologers couldn't possibly come up with meanings for so many bodies.
Similarly, moons, in addition to being potentially numerous, had the problem that they were astrologically pointless -- they wouldn't get far in the sky from their primary, so there was no point in considering them separately. Earth's Moon is the exception there, of course, but that had to be excluded for consistency. It was still plenty significant astrologically, of course, but it wouldn't get counted under the planets anymore, just as the Sun no longer was.
(Actually, the authors show there was a transitional period; astrological almanacs went from including the sun and the moon as planets, to including the sun and the moon and the Earth as planets; to including the Earth as a planet and classifying the Sun and the Moon as non-planetary bodies.)
The second was theology -- the cosmos had to be orderly! People just weren't ready for a messy solar system where planets could be kicked out or captured; there had to be a few main planets to maintain the same orderliness that the geocentric cosmos had had. Thus, planets were the big things that orbited the sun; moons were not planets, that was too messy.
So what we seem to have here is a case where, instead of the scientific notion pushing out the folk notion as with "fish", instead the folk notion pushed out the scientific one. Starting in the 1860s, educational materials seem to start using the folk notion of planets over the scientific one. Then from about 1900-1950, astronomy (and planetary science in particular) seems to go into a bit of a lull, and in the 1920s, the folk notion of "planet" starts to take over there (with the exception that only moons are getting excluded, not asteroids). And then in the 1950s asteroids get excluded when it becomes clear they don't belong.
(...and then in the 1960s planetary scientists apparently go "hey, why aren't we considering moons to be planets? Let's include them", but nobody else listens.)
So this brings us to 2006, and the IAU vote. What happened here? And by that I mean two things -- why did the vote go the way it did, but also, why did the vote happen at all?
As I said above, it was clear that a 9-planet list was not tenable. The question, then, was the appropriate solution -- redefine "planet" with an orbit-clearing criterion, to exclude Pluto? Or -- alternatively -- expand the notion of planet to include Ceres, moons, etc, much like it had used to; and instead come up with a new term -- say, "major planet" -- to denote the 8 big boys? (Apparently Alan Stern suggested "uberplanet", which, uh, I dunno, I think "major planet" is better.)
The former was chosen, and the authors contend that reasons for this include the whole thing being rushed leading to things getting heated with people not having proper time to consider things, and also the fact that planetary scientists mostly aren't members of the IAU! But wait -- why settle this via vote in the first place? That's not how science works, right?
The authors spend a fair bit of time on this point, pointing out that taxonomical disputes should be settled via the normal process of argument and slow consensus over time, i.e. the normal progress of science, just as it happens in biology with its many taxonomical disputes; there isn't any need to get some sort of formal resolution via a vote, and to do so is contrary to the process of discerning good categories via study and argument. When the authors say that the vote was a mistake, they don't just mean that it went the wrong way, but that it shouldn't have happened at all. Moreover they say that the IAU should rescind its definition, and not put a new one in its place; how to form taxa of astronomical bodies is a question for the normal process of science, not for the IAU.
So why did the vote happen, then? There isn't any other similar case of a vote happening, and it doesn't seem like there was some urgency forcing a decision. Well, according to the authors of this paper, the real reason for the vote was that there was urgency of sorts... to get a name put on the body now known as Eris! Why does naming a Kuiper Belt object require deciding what's a planet and what isn't? Because planets and minor bodies have different naming conventions.
If the authors are correct and that really is the real reason for the vote, that's, uh, obviously a pretty silly reason. The authors then discuss how this illustrates that having naming conventions tied to taxa like this is quite a bad idea, because taxa often change quite a bit as we learn more, and doing this sort of thing requires making decisions prematurely, and also helps fix them in place. So... huh. That isn't somewhere I expected this to go.
Anyway, yeah -- up till now I'd always taken the point of view that the 2006 IAU vote was pretty sensible, because, well, I hadn't really ever seen the good arguments for the other side. Now that I have... I'm pretty convinced. The notion of "planet" should be broadened; instead of saying there are only 8 planets, we should say there are only 8 major planets.
...I mean, unless there turn out to be more, which if you're not aware is fairly likely! But 8 that are known at present...
-Harry
The generally-out-there idea of the history of the word "planet" is something like this: Originally, any astronomical bodies that moved relative to the fixed stars were planets -- so, the sun and the moon were planets, but the Earth was not. Then, with the advent of heliocentrism, it came to be understood that the Earth is a planet; but the sun and the moon are not (since the moon orbits the Earth). Eventually more planets were discovered beyond the classical ones. Curiously, 4 of these planets shared approximately the same orbit between Mars and Jupiter. Eventually so many more new planets were found inbetween Mars and Jupiter that it became clear that these bodies didn't belong with the planets, and so they were reclassified as asteroids. More recently, it became clear that Pluto, which had been considered a planet, is actually just one member of the Kuiper Belt, much as Ceres, Pallas, Juno, and Vesta had turned out to just be four members of the asteroid belt; and so it became clear that Pluto shouldn't be considered a planet either, as was famously ratified in 2006 by a vote of the International Astronomical Union.
In fact, these papers contend, substantial parts of the above story are wrong, and the 2006 IAU vote was a mistake. Now to be clear, the astronomical facts are not in question -- Pluto really is part of the Kuiper Belt; it has not cleared its neighborhood. Moreover, nobody here is arguing that we should consider there to be nine planets, those being Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto; that state of affairs is clearly not tenable, and some solution to the problem was needed. But the solution to that problem that was chosen was the wrong one, and the reason it was chosen has a lot to do with the above incorrect story.
There's an assumption smuggled into the above story -- that it is generally agreed that the word "planet" must refer to something that there aren't too many of, and that if the number of planets gets too large, then the word isn't fulfilling its purpose. This assumption is core to the 2006 IAU vote -- we can't include Pluto as a planet, because then we'd have to include too much else for consistency. The problem here is that this is not the actual history of the word "planet", and that in fact, moons -- including the Moon -- were considered planets up until the 1920s, and asteroids were considered planets up until the 1950s. Not the 1850s, the 1950s! Moreover, while the demotion of asteroids from "planet" status happened (mostly) for a good scientific reason, the demotion of moons did not; it happened for another reason entirely.
The story I told you above is what many people think must have happened, but it's wrong. The Moon was not demoted from planet status with the switch to heliocentric cosmology, satellites of other planets were considered planets too, and when astronomers found hundreds of asteroids in the asteroid belt, scientists said, well, I guess there are hundreds of planets. There was no implicit cap on the number of planets that could be admitted! And when asteroids were finally demoted in the 1950s, it was for an entirely different reason.
So, what actually happened?
With the Copernican Revolution, the Earth became considered a planet, and the sun became no longer considered a planet. It was clearly different from the planets -- it shone with its own light, for instance. But the moon? The moon, people could see through their telescopes, was like the Earth. It had mountains! It was clearly the same sort of thing the Earth was. Other satellites were included as planets too, because, while their details couldn't be resolved, it was hypothesized that secondary planets (aka "moons" or "satellites") and primary planets ("planets") were all basically the same sort of thing. Which is roughly true!
A point the authors emphasize is that we have to remember here just what the Copernican Revolution meant. It wasn't just about a switch from geocentrism to heliocentrism; it was about recognizing the Copernican Principle, that we're not special, and part of that is that the cosmos can be messy. Yeah, there's planets orbiting the sun, and there's planets orbiting other planets, and maybe there's rogue planets out there wandering the universe not bound to any star! Who knows? Planets get kicked out of their orbits and captured into new ones. It happens. With this point of view, defining "planet" based on a temporary dynamical property -- remember, the Solar System is not long-term stable! -- seems a bit silly. The same body can be a "planet" for a few million years, then it gets captured by a larger body and now it's not a "planet" for the next few million years? Eh?
It's worth noting that, largely because of this, the IAU definition of planet is simply not used in many contexts -- I mean, it's only used inside our own solar system, nobody tries to apply these criteria to exoplanets or rogue planets. Consider that just as a planet's orbit can change, so can our knowledge of its orbit; and that obviously can happen on a much faster timescale. That's not going to happen much inside our solar system, but try to imagine how that would affect applying it to exoplanets! And planetary scientists don't use it either; they absolutely consider moons to be planets. (Also, having "dwarf planets" and "minor planets" not be "planets" is just awkward.)
OK. So if astronomers were perfectly happy to consider asteroids planets up until the 1950s, and moons planets up until the 1920s, what happened? How did this change, that we've all forgotten ever happened, occur?
In the case of asteroids, there's a good reason -- when we finally got a better view of asteroids, it became clear that they really weren't like planets. It had nothing to do with their being numerous or in a belt together, which had been known for a century. It was that they were small and lumpy and didn't have anything going on internally. And when this became clear, astronomers stopped including asteroids as planets pretty darn fast. Now these problems don't apply to Ceres, but I guess Ceres just got swept along (although some people continued to argue that Ceres ought to be considered a planet).
OK, but what about moons? Why did astronomers stop considering those to be planets back in the 1920s? Well, this wasn't a sudden change like with asteroids, but rather seems to have happened more slowly via a process of diffusion. OK, but where did it start? Well, there's something important I've left out of the above -- above I talked about what scientists considered to be planets. But there's a parallel story of what the public considered to be planets, and that goes a little differently.
Whenever an existing word becomes a technical term, there's likely to be some disagreement between the existing usage and the technical usage. Sometimes, those using the term technically are able to convince the public to go along -- e.g., just about everyone these days appreciates that whales are not fish; the technical usage has overtaken the original common usage. And sometimes they coexist harmlessly. But sometimes it's a bit more troublesome than that.
See, knowledge of heliocentric astronomy didn't filter down to the public so quickly. It became generally accepted among astronomers in the early 1600s, but it didn't filter down to educational materials until the early 1700s, and it wasn't until the second half of the century that the general public started to really accept it. And when they did -- well, the general public may finally have been ready for heliocentrism, but they weren't really ready for the full implications of the Copernican Revolution. While the scientific notion of "planet" included moons, and later asteroids, the popular notion would exclude both of these.
Why? Well, there were two main reasons. The first was astrology. Having too many planets isn't a problem for astronomy... but it is for astrology! Astrologers went along with Uranus, and some of them went along with the initial 4 asteroids for a while, but a whole asteroid belt? Ridiculous. These had to be excluded from the planets; astrologers couldn't possibly come up with meanings for so many bodies.
Similarly, moons, in addition to being potentially numerous, had the problem that they were astrologically pointless -- they wouldn't get far in the sky from their primary, so there was no point in considering them separately. Earth's Moon is the exception there, of course, but that had to be excluded for consistency. It was still plenty significant astrologically, of course, but it wouldn't get counted under the planets anymore, just as the Sun no longer was.
(Actually, the authors show there was a transitional period; astrological almanacs went from including the sun and the moon as planets, to including the sun and the moon and the Earth as planets; to including the Earth as a planet and classifying the Sun and the Moon as non-planetary bodies.)
The second was theology -- the cosmos had to be orderly! People just weren't ready for a messy solar system where planets could be kicked out or captured; there had to be a few main planets to maintain the same orderliness that the geocentric cosmos had had. Thus, planets were the big things that orbited the sun; moons were not planets, that was too messy.
So what we seem to have here is a case where, instead of the scientific notion pushing out the folk notion as with "fish", instead the folk notion pushed out the scientific one. Starting in the 1860s, educational materials seem to start using the folk notion of planets over the scientific one. Then from about 1900-1950, astronomy (and planetary science in particular) seems to go into a bit of a lull, and in the 1920s, the folk notion of "planet" starts to take over there (with the exception that only moons are getting excluded, not asteroids). And then in the 1950s asteroids get excluded when it becomes clear they don't belong.
(...and then in the 1960s planetary scientists apparently go "hey, why aren't we considering moons to be planets? Let's include them", but nobody else listens.)
So this brings us to 2006, and the IAU vote. What happened here? And by that I mean two things -- why did the vote go the way it did, but also, why did the vote happen at all?
As I said above, it was clear that a 9-planet list was not tenable. The question, then, was the appropriate solution -- redefine "planet" with an orbit-clearing criterion, to exclude Pluto? Or -- alternatively -- expand the notion of planet to include Ceres, moons, etc, much like it had used to; and instead come up with a new term -- say, "major planet" -- to denote the 8 big boys? (Apparently Alan Stern suggested "uberplanet", which, uh, I dunno, I think "major planet" is better.)
The former was chosen, and the authors contend that reasons for this include the whole thing being rushed leading to things getting heated with people not having proper time to consider things, and also the fact that planetary scientists mostly aren't members of the IAU! But wait -- why settle this via vote in the first place? That's not how science works, right?
The authors spend a fair bit of time on this point, pointing out that taxonomical disputes should be settled via the normal process of argument and slow consensus over time, i.e. the normal progress of science, just as it happens in biology with its many taxonomical disputes; there isn't any need to get some sort of formal resolution via a vote, and to do so is contrary to the process of discerning good categories via study and argument. When the authors say that the vote was a mistake, they don't just mean that it went the wrong way, but that it shouldn't have happened at all. Moreover they say that the IAU should rescind its definition, and not put a new one in its place; how to form taxa of astronomical bodies is a question for the normal process of science, not for the IAU.
So why did the vote happen, then? There isn't any other similar case of a vote happening, and it doesn't seem like there was some urgency forcing a decision. Well, according to the authors of this paper, the real reason for the vote was that there was urgency of sorts... to get a name put on the body now known as Eris! Why does naming a Kuiper Belt object require deciding what's a planet and what isn't? Because planets and minor bodies have different naming conventions.
If the authors are correct and that really is the real reason for the vote, that's, uh, obviously a pretty silly reason. The authors then discuss how this illustrates that having naming conventions tied to taxa like this is quite a bad idea, because taxa often change quite a bit as we learn more, and doing this sort of thing requires making decisions prematurely, and also helps fix them in place. So... huh. That isn't somewhere I expected this to go.
Anyway, yeah -- up till now I'd always taken the point of view that the 2006 IAU vote was pretty sensible, because, well, I hadn't really ever seen the good arguments for the other side. Now that I have... I'm pretty convinced. The notion of "planet" should be broadened; instead of saying there are only 8 planets, we should say there are only 8 major planets.
...I mean, unless there turn out to be more, which if you're not aware is fairly likely! But 8 that are known at present...
-Harry
