Ceres by itself doesn't really challenge Pluto's old classification. Pluto has 14 times the mass of Ceres, and five moons. Ceres is the only "dwarf planet" classified so far that has zero moons. There are still good arguments for a definition that doesn't consider Ceres a dwarf planet.

This helps illustrate how arbitrary these classifications are. They depend entirely on what properties are included in the definition. The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.

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kergonath a day ago | parent | next [-]

So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?

Yes, those classifications are arbitrary, but then any classification is. If you want to consider Pluto as a planet, you have to include quite a lot of other bodies, which are all qualitatively and quantitatively different from the 8 (which themselves are in 2 qualitatively and quantitatively different groups. Or 3, depending on who you ask).

In the grand scheme of things, it does not matter and astronomers won’t care about the general public’s feelings. It is utterly pointless to get worked up about these things.

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antonvs a day ago | parent | next [-]

> So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?

Why not? It's an arbitrary line, and there has to be a lower limit somewhere.

In the IAU definition, hydrostatic equilibrium loosely defines a lower limit for all planets including dwarf planets, but at the limits that's a fuzzy property that gets debated for small bodies. In fact Ceres is less massive than what some studies have found to be required for hydrostatic equilibrium, e.g.: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3 . The abstract of that paper starts out with, "The current IAU definition of 'planet' is problematic because it is vague and excludes exoplanets."

If we had more small planetary candidates in the solar system, they would have had to come up with some additional constraint to separate planets from non-planets.

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cwillu a day ago | parent | prev [-]

The problem is not experts having precise terminology in technical settings; the problem is the army of “experts” who will descend upon you if you use the incorrect arbitrary term in colloquial settings. The same sort that will descend on you if you dare refer to a planet orbiting a different star as a planet, not an exo-planet.

	▲	Timon3 a day ago \| parent [-]
		A lot of comments here explicitly complain about the precise terminology of experts in technical settings.

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fc417fc802 a day ago | parent | prev [-]

> The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.

I wholeheartedly agree with this take. I have something of a personal hatred of poorly executed retconning thus I present the following proposal.

For the sake of simplicity we should use even powers of 10 with earth as the primary reference point. Note that the earth is 10^24, mars is 10^23, and the earth's moon is 10^22 (pluto is also 10^22 and less massive than the earth's moon). It seems "obvious" then that 23-25 are planets, 22 or less are relegated to dwarfs, and 26 and over are giants (jupiter is a 27 and the sun a 30 for reference).

By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.

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Timon3 a day ago | parent | next [-]

Following this logic, a black hole with earth's mass would be a planet.

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antonvs a day ago | parent [-]

You could add a hydrostatic equilibrium requirement, which is what the IAU definition does. It doesn't really affect anything else the GP was saying.

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Timon3 a day ago | parent [-]

Why did we add this requirement? To fit how you want to classify planets? That retcon was not executed well.

It shows that the line of reasoning makes no sense, because any classification is constructed to fit some of our preconceived notions. Choosing a classification that builds off our social history is perfectly fine, as long as the rules for classification are applied consistently.

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antonvs 14 hours ago | parent | next [-]

Hydrostatic equilibrium is the most defensible of the requirements to be a planet.

The IAU wording is, "...has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."

In this case it just means that the planet's mass and therefore gravity is sufficient to give it a spheroidal shape. (Technically the equilibrium is with the opposing push from the planet's material, which resists gravity's attempts to compress it. The result is a sphere, made spheroidal in real planets by rotation.)

Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons.

This isn't really social history but simply classifying the objects according to intrinsic properties, which seems fine to me. This classification could be applied to objects anywhere in the universe without any problem.

The other two requirements are more problematic and arbitrary. One is that it has "cleared its orbit". Earth, let alone Mercury, would be unlikely to qualify for this if it were at the same distance as Pluto, for example. Rogue planets also can't satisfy this one, and we've observed those.

The other is that it has to be in direct orbit around the Sun, not a satellite of some other object. This excludes exoplanets as well as rogue planets. The rationale for this is that they didn't have enough observational info to say much about exoplanets, but that seems like a flimsy excuse. A good definition would apply to exoplanets by virtue of being theoretically sound, and not having been tuned to fit the way they were trying to classify solar system planets.

	▲	Timon3 13 hours ago \| parent [-]
		> Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons. Okay, and how did you arrive at the hydrostatic equilibrium requirement? What law of nature told you that planets must be spheroidal? You're only adding this requirement so the outcome fits your pre-existing desired results, since you're literally arguing for the requirement based on what objects it excludes. And what are the consequences of such arbitrary decisions, according to you? "You can't get away with that in, say, physics and still end up with useful theories[...]" Do you get the issue with this line of critique? There is literally no useful definition of "planet" that is non-arbitrary.

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fc417fc802 a day ago | parent | prev [-]

> any classification is constructed to fit some of our preconceived notions

That is not the concern here. It seems to me that you're failing to account for intent. Classifications exist to serve a useful purpose. What I object to is mental gymnastics used to gerrymander a particular outcome. These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.

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Timon3 19 hours ago | parent [-]

> These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.

That is literally what happened - a definition for "planet" that is as minimal as possible, while staying as close to our historical categorization as possible.

Now, I know you'll object to the "while staying as close to our historical categorization as possible" part, but that is literally how scientific fields always approach arbitrary classifications. Without taking our historical use into account, there is no "objective" definition you can derive.

If I'm wrong, please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.

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fc417fc802 10 hours ago | parent | next [-]

> but that is literally how scientific fields always approach arbitrary classifications

No, not always. What you are describing there are shoddy classifications. I already addressed this exact point in my previous reply when I stated that classifications are crafted to serve a useful purpose. It is sometimes useful to hew close to the historical norm; other times it serves no purpose.

> But remember, it must not take into account what we today consider "planets" to be.

An arbitrary restriction that I do not accept.

> please give me an example of what kind of definition for "planet" you'd be happy with

I already did this in my original comment. I proposed an earth centric system that bins mass by powers of ten. My proposed range for planets was 10^23 <= x < 10^26. Hydrostatic equilibrium works well as a constraint to eliminate things that are clearly not earth like (such as black holes) but alternative formulations such as stipulating an acceptable density range are possible (such cross cutting subcategories might even be preferable).

Note that this would result in ganymede being classified as both a planet and a moon. I think that makes perfect sense given how similar in size it is to mars. In fact ganymede being excluded as a planet is precisely the sort of gerrymandering I object to.

As it turns out my proposed criteria also relegates pluto to the status of dwarf planet. I think that's sensible given that it's both smaller and lighter than the moon. However I'd be willing to entertain a set of criteria that acknowledged both pluto and the moon as proper planets - the important thing to me is consistency.

	▲	Timon3 3 hours ago \| parent [-]
		I don't see how your proposed criteria are any less of a shoddy retcon.

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antonvs 18 hours ago | parent | prev [-]

> that is literally how scientific fields always approach arbitrary classifications.

That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories, because your theories quickly break down due to inconsistencies. You can't design working scientific theories by committee, because votes don't affect fundamental physical reality.

Much of astronomy is a hard science, which is why this definition came in for pushback even from astronomers themselves. Here's astronomer Phil Plait's take:

> "For some time now I, and many others, have stated that the IAU definition is silly for many reasons, including a line that says that planets can only orbit the Sun. If that’s true, then exoplanets — worlds orbiting other stars — aren’t really planets!" -- https://www.syfy.com/syfy-wire/mea-culpa-are-exoplanets-plan...

The article goes on to clarify that the IAU explicitly limited its definition to solar system planets - in fact, the resolution is titled ""Definition of a Planet in the Solar System."

As such, it's not even a universal scientific definition for "planet".

Another quote:

> "At [the IAU] 2006 international meeting in Prague, they drafted a series of defining characteristics for planets and put it to a vote. The definition passed, and the outcome was entirely predictable: They made things a lot worse."

> "...the more proximate problem is that these definitions are, um, silly."

-- https://www.syfy.com/syfy-wire/redefining-planets-answer-sea...

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Timon3 18 hours ago | parent [-]

> That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories,

Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith. No theory in physics will break if you change the criteria of an arbitrary classification.

I'll pose the same challenge to you: please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.

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antonvs 15 hours ago | parent [-]

> Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith.

Nice false dichotomy and binary thinking.

You're right that arbitrary classifications are arbitrary, obviously. My point is that you can't get away with arbitrary classifications in the context of hard science theories.

There's an example of a definition of "planet" in one of those Plait articles I linked:

> "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

That comes from the following paper: https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf

The Plait articles are worth reading, because they might help disabuse you of the notion that the IAU definition is a good one, and explain why it would make sense to have a better one.

	▲	Timon3 14 hours ago \| parent [-]
		> My point is that you can't get away with arbitrary classifications in the context of hard science theories. Arbitrary classifications don't factor into hard scientific theories, so I have no idea how this is relevant. Or what "hard science theory" are you thinking of that relies on whether we classify an object as a planet? We can modify arbitrary classifications without any theories being affected, because the theories don't rely on arbitrary classifications. We arbitrarily decided what "one meter" is, yet we use "meters" all over physics. We could change the definition of "one meter" tomorrow, and all our theories would stay valid, because they don't rely on arbitrary classifications. > There's an example of a definition of "planet" in one of those Plait articles I linked: >> "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters." How does one arrive at this definition, without taking into account what we today consider "planets" to be? What principle of physics tells you that a planet must be sub-stellar mass, or that it has never undergone nuclear fusion? You'll have to actually explain how this definition is any less arbitrary, but I don't think you'll be able to, because every useful definition falls under your objection.

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antonvs a day ago | parent | prev [-]

It's easy to exclude the moon, like the IAU does, by requiring that planets orbit the Sun. (Although you might want to avoid the IAU's parochialism, which excludes exoplanets, and say "a star" instead.)

> By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.

Ganymede is larger than Mercury, so a definition that doesn't specify orbiting a star would have to include some moons, or exclude Mercury.

This exposes another problem with this kind of definition - it seems to make more sense to define a body in terms of its intrinsic properties rather than whatever location it happens to be in.

For example, a rogue planet is not a planet by the IAU definition. You'd have to call it something like a free-floating planetary-mass object. ("Free-floating" is not the right term either, but I don't have the research funds to come up with a better one.)

> I have something of a personal hatred of poorly executed retconning

Totally agree. Wanna join me outside George Lucas' house to protest the Kessel Run?

	▲	fc417fc802 a day ago \| parent [-]
		> orbits and rogue planets I think that's an example of coming to a useful realization while working through the process. It should serve to illustrate the importance of not attempting to engineer a particular outcome. If the earth were wandering the galaxy clearly it should still be considered a planet, no? And were we living on a body the size of mars that orbited a gas giant we would still be said to occupy a planet. At least assuming the goal is to classify things of a vaguely earth-like nature. I think the more difficult question is what exactly qualifies as a moon. Anything orbiting a planet, including a smaller one? That seems perfectly reasonable to me. (Yes yes I know the barycenter matters.) And yes, I think ganemede should be considered both a planet and a moon. That thing is on the same order of magnitude as mars. What useful purpose is served by excluding it as a planet?