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Originally Posted by Ken G
Actually, that is a fairly good definition of isotropic pressure. After all, once you overcome rigid forces, what do you think is left?
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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. 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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But if you are saying that you feel this should be the sole definition, then I tend to agree.
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I'm not. 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 wouldn't split hairs about any definition that looks only at the internal physics and not the motion of the object.
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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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That's why I absolutely think that large moons should be considered planets, and I think that will be even more patently obvious when we start landing on them.
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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. Personally, I would restrict the word Moon to objects that meet a self-gravitationally-round criteria, using another term for smaller natural satellites. 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.