J.@sri-unix (06/26/82)
This was prompted by the business about the brown dwarf; it's probably completely harebrained, but here goes: as I understand it, the current theory among geo-whatevers (I may be out of date here, corrections/updates requested) is that the reason the inner planets aren't gas giants has to do with the fact that a) they are smaller, and b) they are at a higher temperature; the combination (varying frlmo planet to planet) of the smaller potential energy well and the higher particle speed allows all the light gasses to escape. Now, I might believe this for Mercury, but it seems a bit dim for Mars, which is pretty cold; it requires some mechanism to explain why all the inner planets are much smaller than the outer ones. What crossed my mind is the possibility that at some point in the past the sun was somewhat larger and cooler than it is now for a brief period. The inner planets might have actually been inside the tenous outer layers of the sun briefly, and would have had most of their light gasses boiled off, which would explain why the switch to gas giants is so abrupt. Now, according to classical stellar evolution theory, the sun's headed there anyway as a red giant, but couldn't have been there yet and will take a while to get there. I can, however, conceive of several possible ways for this to happen very briefly in ways that would not show up on the long term scale. One would be a massive flare or instability of some sort, of short duration (of the sort that some people used to think killed the dinosaurs, but on a larger scale), but this seems far-fetched. That sort of energy output should have affected the outer planets as well, although I suppose if it was brief enough it might be possible. The other one (that I like better) is that the proto-sun stage would also have been about red giant size and somewhat lower temperature, but very short-lived as the material coalesced inward heating up until the sun entered the main sequence with gravitational and radiation pressures equalized. Anyone know if this is in fact part of the current explanation, or if it's reasonable? -------