A bit of background:

I'm in the beginning stages of creating a habitable moon orbiting a gas giant. The conditions I have so far for this are:

  • The moon itself is more or less Earth-size, and the gas giant orbits the sun-like star at a distance of 1 AU or the equivalent (translation: I don't really want to fiddle around too much with things like gravity or different star types)
  • The moon is not tidally locked to the gas giant planet, with a day length of 24-36 hours. From what I've read on other posts here, the most realistic condition for this would be that the moon was a planet that was captured by the gas giant at some point in the past.
  • Ideally the moon would have an axial tilt similar to Earth's, such that seasons would be a thing
  • The moon orbits sufficiently close to the gas giant that it's quite large in the sky, and eclipses are a daily or at least very common occurrence, at least around the equator.

So the questions I have are:

  • Assume that the moon was already habitable and tectonically active back when it was a planet. What would happen to the tectonics and climate of a planet as it's in the process of being "captured" by a gas giant moving inward towards the star? Would it continue being habitable, or would there be huge planet-wide shocks that would cause mass extinction? Would plate tectonics go haywire or otherwise be disrupted during this process?
  • Would the axial tilt of the moon be plausible? Or would this have other implications for its orbit/have different climactic consequences?
  • Assuming axial tilt is plausible, would the position of the gas giant in the sky change depending on the season? Would the zone of daily eclipses change based on the season?
  • What would a reasonable orbital period and distance from the planet be? Assuming a more-or-less Earth-sized moon around a more-or-less Jupiter-sized gas giant, would an orbit ("month"?) of about 8 days with the planet occupying about 12 degrees in the sky be plausible?
  • Would other large-ish moons like the Galilean moons be possible around the gas giant, or would the process of capturing the habitable planet disrupt the orbit of all other moons and make their formation unlikely?

Apology for the many questions! Note that this is a fantasy world, so I'm willing to tolerate some hand-waving, although I'd like the planetary setting to be more or less plausible aside from magical elements.

  • $\begingroup$ Welcome to worldbuilding. Our model is "one worldbuilding question per post", so that it's easier to answer and to pick a single "best" answer. Please narrow down your post to a single question. You can find more info in the help center $\endgroup$
    – L.Dutch
    Commented Mar 6, 2021 at 17:28
  • $\begingroup$ You've got a whole raft of probably irreconcileable requirements there. But to start with: if your planet had an evolved biosphere before being captured, it must have been in a stable situation for billions of years. What was so cataclysmic that it upset its orbit, but so gentle that it wasn't resurfaced? Have you thought about the habitability of the gas giant's orbit? If the gas giant moved inwards, what stopped it moving further in? etc etc. $\endgroup$ Commented Mar 6, 2021 at 17:33
  • $\begingroup$ Have a read of this question (and especially the links in M A Golding's answer) if you haven't already done so... there's some consideration of non-tidally locked moons. TL;DR: you probably need a really massive planet (brown dwarf, not gas giant) and your habitable moon probably needs a moon of its own. At that point you can consider the possibility of life having evolved on it in situ, somewhat simplifying your creation story. $\endgroup$ Commented Mar 6, 2021 at 17:39
  • $\begingroup$ VTO, despite many questions it basically just one question what for a system it will be like, and most of the questions easy to answer as they are yes answers. To OP most stuf looks okay on a first glande, especially with handwavium. Tectonics will be more active, weather handwave, tilt plausible, position will change, orbit needs just calculation, other moons are plausible. There can be some nuances but nothing drastic with handwavium. $\endgroup$
    – MolbOrg
    Commented Mar 6, 2021 at 23:45
  • $\begingroup$ @MolbOrg Failing the book test is precisely one of the reasons for keeping close as too broad. Each of those question would require writing a book to be answered. $\endgroup$
    – L.Dutch
    Commented Mar 7, 2021 at 6:44


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