| |
| ▲ | 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. | | |
| ▲ | 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. |
| |
| ▲ | 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. | | |
| ▲ | Timon3 20 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. | | |
| ▲ | 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. |
| |
| ▲ | antonvs 19 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... | | |
| ▲ | Timon3 19 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. | | |
| ▲ | 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 15 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. |
|
|
|
|
|
|
|
