Quote:
Originally Posted by Ken G
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.
|
Degeneracy pressure is a pressure whose form is isotropic. All isotropic pressure means is a pressure that is the same in all directions. This is the case for the internal pressure of a body for which gravity dominates, but is not true for all smaller objects. In such smaller solid bodies, rigid-body forces dominate. These forces are electromagnetic, and once you get these, then together with gravity there is nothing else in play until the onset of electron degeneracy.
Quote:
|
Then we are in agreement that differentiation is a better criteria than force balance.
|
No we are not. How you get from my pointing out that the criteria in the IAU definition is not 'nearly round' to my agreeing that differentiation is a better criteria than force balance is beyond me. Don't put words into my mouth.
Quote:
|
I agree, 'nearly round' sounds like an aesthetic consideration, rather than a physical one.
|
Whether it is or is not, the point is that it is not the IAU criteria.
Quote:
|
....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.
|
Which is an arguable point of view. But not one a lot of astronomers would agree with (certainly not of those at the last IAU Congress). I think people's stance on this may have something to do with what they are interested in, or study about, planets. If it's the objects themselves, you prefer to highlight their internal physics. If it's their relationship to other objects (like for orbital dynamicists), it's where it is, etc.
Quote:
|
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?
|
I don't think it's a scenario we need worry about.
Quote:
|
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.
|
That's the main difference if you're thinking about the objects themselves. If you're thinking about their relationship to others, the fact that one orbits the Sun and the other does not would be the main difference. Other people have different points of view.
Quote:
|
The orbit around Jupiter likely affects [Titan's] 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.
|
You mean Saturn. Jupiter and Saturn are themselves satellite planets (of the Sun), but, yes, you could sub-divide the 'planets' like that. I'm not saying you cannot come up with a classification scheme on those lines. But I'm not the one you have to convince.
Quote:
|
On the contrary, pre-existing usage in scientific circles is to call all these things planets.
|
Which scientific circles? Given the strength of feeling displayed at the IAU, I doubt very much if this was anywhere near universal.
Quote:
|
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.
|
Yes, there's quite a lot of tolerance in seminars for obvious errors! Possibly a slip of the tongue. Or perhaps the speaker is of the same mind as you as to definition of a planet. But evidence as to a specific instance of a particular usage is not evidence that that usage is a general one.
Quote:
|
... and don't be surprised to find reference in the scientific literature to Titan's "planetary atmosphere" or "planetwide cratering".
|
I'll be surprised to find it from now on, as editors will stick to the IAU definition (though the odd example might be missed). You could say that before because there was no scientific definition of planet. There is now.
Quote:
|
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.
|
Er no, according to the IAU there are only eight planets. The IAU adopted a definition. As it's the IAU one, it's the scientific definition, and certainly those astronomers who voted for it thought so, presumably on partly utilitarian grounds. Elegance, like beauty, is in the eye of the beholder, but I haven't seen any alternative definition that is more elegant (though there are simpler ones).