Post #4by Spaceman Spiff » 12.02.2005, 14:17
Hmm, yes, there might be no reason that two forming gas giants collide and form a gigantic Earth-Moon style 'double planet', if the gas giants were mostly formed, and they are not too close to the parent star.
I say "not too close" because going back to the original post, mentioning 51 Peg, I would expect two gas giants being, say, 300,000km apart would be prone to such a great difference in gravitational pull from 51 Peg just a few million km away - that is, 'tidal disruption' across the pair of gas giants as they orbit their common barycentre. It's probably unstable compared to the lifetime of the star.
Double gas giants at the distance of Jupiter would be very stable though. If Jupiter could hold into Io through to Callisto OK, then it can hold on to a Gas Giant too...
Finding the limit of stability between 5 A.U., and 0.05 A.U. is something many detailed and complicated calculations would be needed for.
By the way, there's one of those 'did you know...?' facts about the Earth and Moon. The gravitational pull of the Sun on the Moon is twice as strong as the gravitational pull of the Earth on the Moon. In that sense, the Moon really orbits the Sun, and the Moon and Earth travel together in a common orbit. I think chris made a post about replotting the Moon's orbit about the Sun instead of the Earth, and was surprised to see the Moon never moves backwards in it's true orbit around the Sun. This is indeed the case.
I think Solar tidal disruption from the Sun was once an explanation for why Venus and Mercury don't have moons. However, I think the Moon could orbit Venus quite stably just like the Earth. The Solar tide increases by only about 2.7 and Venus has 89% the mass of Earth, making the Solar force 6 times greater than Venus. For Mercury, the tide would increase much more. With perihelion at 0.29 A.U., and Mercury weighing 5.5% of Earth, the competing force grows to 750 times...
So, when you feed that back into the moonless Mercury and Venus puzzle, it's not so clear cut. Mars' moons are probably captured asteriods. More likely terrestrial planets simply don't normally form moons.
Maybe double gas giants near to their parent star could do orbits like an Earth-Moon with each other. I wouldn't expect much room for stable moons though...
Well, that's my opinion.
Spiff.