Expert: Philip Stahl - 3/3/2004

Question
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Followup To
Question -
Orbital speed decreases as the distance from the orbital focus increases.  As rocky planets like the earth rotate, different levels from the surface to the center, must be trying to rotate at different speeds relative to their distance from the center (as in orbits) rather than rotate as a whole as they do.  There must be an upper limit to this ability to rotate as a whole, before the orbital forces involved tear the planet apart.  Could this be part of the reason that the large planets are gaseous rather than solid?
Answer -
So far as we believe, and I underscore this is still tentative *theory*, the outer planets formed from the earliest residues of the collapsing solar nebula. So they would naturally have had more methane, ammonia, etc. in their atmospheres than the inner planets (which accreted last)

Thus, their gaseous nature has more to do with the epoch in which they formed, retaining most of the existing light gases in their atmospheres.

Additionally, their relatively greater gravities and enormous distances from the Sun, inhibited out-gassing.

Could you please comment on my "reasoning" about orbital stress on rotating planet-size solid objects?  Thank you.

Answer
Hello.

You're reasoning is actually faulty because it's based on a false analogy. You are likening the "rotations" of layers about a planet's core to the orbits of planets in the Solar System. Say along the lines of the period and semi-major axis relationship expressed by Kepler's 3rd law. In fact, no such similar relationship exists for planetary layers or 'shells'.  Or at least, has ever been reliably demonstrated.

In the case of an active magnetic core planet like Earth, there is actually NO evidence that any particular sediment level, or layer - rotates like any planet in the Solar System. Yes, the core itself rotates, but that's because the Earth rotates - and the two share the same sort of dynamical relationship.

There is no evidence that any intervening layers "rotate" independently at all. In order for your hypothesis to have any basis, various layers would have to be able to rotate *independently* of each other.

Thus, there is no real worry about any alleged internal, "rotational" forces doing anything to tear a planet (like the Earth, or Mars) apart.