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Originally Posted by mosaix  Chris, is it plausible that a Jupiter-like planet could gain the extra mass by colliding with another planetary sized body, triggering the fusion process? I know that such a collision would probably involve some changes in orbit but could a star system end up with two suns in this fashion? |
Or by obelisques from outer space compressing its matter into a smaller space? Mathematically, yes; the compression wave and the infall energy could trigger off a chain reaction. But why (unless you're intending to revamp our solar system) look for complicated methods of doing it, when it seems extremely likely (from the number of multiple stars we can observe) that the cosmos, given the right starting conditions, can do the job on its own. A gas-giant planet is not all that different from a small star, after all.
And, personally, I don't like the idea of gas-giant sized bodies, just failed stars, supermaxicomets or whatever you want to call them, floating around loose in the Galaxy. I would hope they'd stay in the correct star lanes, like well behaved cosmic junk.
Besides "some change in orbit"? You'd probably get something as eccentric as pluto. And the original moon system's orbits would be all smashed to b… all messed up, too.