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What is the phenomena behind gas giants like Jupiter and Saturn having a large number of moons as compared to other planets. Were the present state moons a part of their parent planet long time ago ?

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While this is a good question, I also think it is currently largely open. Someone who knows more about the current theories of planet formation might give a more thorough explanation but I think the short answer is we don't really know. – Warrick Jun 7 '12 at 7:16

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You could look at the question from the other perspective i.e. why don't the inner planets have moons? The Earth only has a moon because it (probably) got hit by another planet while it was forming, and Mars' moons are probably captured asteroids.

Having said this, we don't know the answer to this question either. It's been suggested that the inner planets formed a lot closer to the Sun than they are now, and migrated outwards due to interactions with dust surrounding the Sun (the solar wind has since blown away this dust). It's possible that all the small bodies that hadn't aggregated into planets got ejected from the inner Solar System by interactions with the inner planets.

If you accept this argument then I guess the outer planets got moons because they were more widely spaced so the system was less chaotic. There's also a lot more junk floating about because the solar wind was too weak to blow away all the small particles.

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I just came from a talk by Dave Stevenson where he put forth the idea that a planet like Venus could have had a moon that was disrupted by close encounters with large planetesimals (after all, for every collision there will be many near misses). Earth may have been lucky in that our moon formed late (for which there is evidence), so there were fewer things around to perturb the system. – Chris White Jan 29 at 18:46

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