How likely would a habitable world be that orbits a gas giant that itself is orbiting a larger gas giant? Assuming one exists, what are the likely conditions?
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$\begingroup$ Welcome to the site Rolf De Dog, please take the tour and read up in our help centre about how we work: How to Ask Possible duplicate: worldbuilding.stackexchange.com/questions/26634/… $\endgroup$– Escaped dental patient.Mar 16, 2019 at 20:43
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$\begingroup$ Don't take this as an answer, but I think for one gas giant to orbit a gas giant, the larger of the two would need to be a star ;-) (Another problem would seem to be that the planet's orbit would be in an area that contained massive radiation 'exchanges' between the planets themselves) $\endgroup$– Giu PieteMar 16, 2019 at 21:13
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$\begingroup$ @GiuPiete The masses are not that much of a problem: True, the larger gas giant could be classified a very small brown dwarf. But that would be pretty much indistinguishable from Jupiter, except for its weight. It won't even be much larger than Jupiter. $\endgroup$– cmaster - reinstate monicaMar 16, 2019 at 22:32
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$\begingroup$ @cmaster I'm no astrophysicist, it seems to me that masses have to be a problem, if they're in the habitable zone they're closer to the sun than jupiter is currently, if they're closer to the sun then for 'Jupiter' to gain and retain control of another gas giant the sun has to be smaller, if the sun has to be smaller then jupiter has to be yet closer to the sun, etc etc ad nauseum,. $\endgroup$– Giu PieteMar 16, 2019 at 22:37
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1$\begingroup$ @GiuPiete Well, if those habitable moons happen to be in orbit around that 15x Jupiter, I don't see a problem from an orbital mechanics standpoint. It's the factor of the sizes of the required orbits that give me headaches: The sub-Saturn would need to be in low orbit around the 15x Jupiter, and the habitable moon in even closer low orbit around the sub-Saturn. Either you get brutal tidal locking, or you don't get stable orbits. Not so much because of the weight of the super Jupiter, but because the sub-Saturn must orbit in too close proximity for there to be enough room for a double moon. $\endgroup$– cmaster - reinstate monicaMar 16, 2019 at 22:55
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It's hard to say without any actual examples of moon of moons in our own solar system to use as models, but I'd expect it to be pretty rare. The first factor to consider is that your habitable world would be far more inclined to orbit the bigger gas giant meaning that smaller one would likely need to maintain a wider orbit than any moon in our solar system to keep its big brother from stealing its moon. The farthest moon from Jupiter has about a 14,600,000 miles orbital radius and even it does not have a moon of its own. If you need a 15x super jupiter as is implied in the comments, then the minimum orbit could be significantly greater than this. Small fast orbits won't help you as much as some have theorized, because the stronger pull of a larger planet will still rip the small moon away regardless of speeds if the big gas giant is too close to the smaller one.
So, the biggest problem here is that you want a habitable world. Even with an orbit as small as Callirrhoe you'd be looking at massive seasonal changes as the world gets closer to and farther from the star by a factor of ~0.3au per year. If you tried putting that gas giant system into the goldilocks zone, winter and summer would be like going from Venus's orbit to Earth's Orbit every time the small gas giant orbits the bigger one. So, even if life could evolve here, human life probably could not survive.
for the best chance for this to be possible, main sequence stars are pretty much out. Yellow suns are too small for that kind of variance to not be devastating and blue suns tend to go boom way to fast for life to have much time to evolve. A red giant might work, but it is my understanding that red giants have periodic novas that would likely kill off the planet every so often.
answered Mar 18, 2019 at 20:03