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Originally Posted by CuddlySkyGazer
I'm sure isotropic pressure occurs in other scenarios. The next thing after rigid body forces would be electron degeneracy pressure, but not really relevant when considering a lower limit for planets.
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No, the next thing is normal nondegenerate forms of pressure. Degeneracy pressure would come much later, and it is also a form of isotropic pressure.
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The point is that the mass must be sufficient to overcome rigid-body forces if it has to - it wouldn't if the body solidified from liquid for instance, as that would already be round, much less mass being needed for self-gravity to make a liquid object round.
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Then we are in agreement that differentiation is a better criteria than force balance.
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I was just commenting on the fact that a lot of people seem to get hung up on 'nearly round', when it's not the criteria.
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I agree, 'nearly round' sounds like an aesthetic consideration, rather than a physical one.
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I would want a classification scheme that distinguishes objects both on relevant internal characteristics but also on where they are and how they formed. Which is the upper-level classification, and which the lower, I'm not fussed about.
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Well, that's what has to be decided-- it's a part of the classification scheme to decide which is the upper level. On the surface, there's no need to get "fussed" about any classification, we'd eventually figure out what the other person is talking about even if we had no classification schemes at all. But having schemes changes how we think about the universe, and so does the choice of the upper level scheme, and I argue the upper level scheme should always be based on the physics that is internal to the object, not the environment it happens to find itself in. For example, if the Earth is ever ejected from its orbit into interstellar space, and humanity continues to live on underground using fusion energy or some such thing, do you really want their astronomers to have to say "oops, we're not living on a planet any more" just because there's no more day and night?
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I would still want a classification scheme that distinguishes between objects orbiting stars and objects orbiting planets (or other planets, as the case may be), because I think that's an important difference.
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I agree, but that is what I am arguing should be a lower level aspect of the classification, just as is whether a planet is a terrestrial planet or a gas giant planet. Making that part of the upper level scheme falls victim to all the pitfalls I've mentioned above, the biggest being, it artificially changes the way we think about the Earth and, say, Titan, when it shouldn't. The main difference between Earth and Titan has to do with distance to the Sun and the smaller size of Titan, which may alter the degree of complete differentiation. The orbit around Jupiter likely affects its icy outer layers, but the crust of any planet will be affected by its surroundings. If we call it a planet, then we can further classify it as a satellite planet in honor of its relation to Jupiter.
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You would call them planets, but, although there are many logical classification schemes, I think it best not to conflict with pre-existing usage where possible.
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On the contrary, pre-existing usage in scientific circles is to call
all these things planets. I was at a seminar the other day where an approach to Titan was described as "approaching the planet", and not an eye was batted. That's as it should be, and don't be surprised to find reference in the scientific literature to Titan's "planetary atmosphere" or "planetwide cratering". It is only in the popular literature that "planet" is reserved for the big 9, but now apparently the IAU wants to adopt the popular usage in the place of an elegant and useful scientific definition.