My question is in the context of a fictional super-Neptune that lies in the habitable zone of its host star system.

Giant planets only form beyond the Frost Line of their protoplanetary disk, and can then migrate inward to establish habitable orbits. Research that I have done on the topic suggests that in the case of Jupiter, the planet took its already formed icy moons inward with it when it migrated into the inner solar system.

Does this mean that migrated giant planets will only be able to harbour watery moons? Or, is there a mechanism by which a giant planet can form or acquire a rocky moon post migration?

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    $\begingroup$ Possible duplicate of Habitable moon of a gas giant: working out the sizes and distances $\endgroup$
    – Mołot
    Dec 3 '18 at 22:23
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    $\begingroup$ It's important to realize that what we know about space might fill books, but what we don't know about space would fill libraries - and space is currently having a lot of fun proving that we don't know much. We're regularly surprised by what we find, having previously believed it to be impossible. Granted, I'm not a specialist in orbital mechanics - but my gut tells me you're describing something that would be perfectly believable in a story. I'd vote that you make it so and roll with it. $\endgroup$ Dec 3 '18 at 23:15
  • $\begingroup$ AFAIK Io and Titan are rocky. If Jupiter or Saturn moved inwards, there you'd have it. $\endgroup$ Dec 4 '18 at 1:29
  • $\begingroup$ Could you link to something that describes why giant planets should NOT have rocky moons? $\endgroup$
    – bukwyrm
    Dec 11 '18 at 7:47
  • $\begingroup$ @Mołot OP is just after rocky moons, your linked Q/A deals with big, habitable moons with tectonic movement and a magnetosphere. $\endgroup$
    – bukwyrm
    Dec 11 '18 at 7:49
  1. After a water moon dries off (and it will, because of higher temperatures and star radiation), a rock moon+oxygen are left. If the moon is small, then oxygen flees, too.
  2. The giant planet, moved to the star, can easily obtain some ready Earth type planets as satellites.
  3. If the system has more than one star, or really a large giant planet, something as 10-50 Jupiters, planets and satellites can be shuffled in very peculiar ways.
  • 1
    $\begingroup$ Not too peculiar. Simple orbits are the most likely to produce stable systems. $\endgroup$
    – StephenG
    Dec 4 '18 at 0:09

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