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I'm working out a concept for a story where there's a gas giant with at least 6 habitable moons (willing to use teraforming tech to get over any big hurdles) and the planet itself has a ring of rocks and minerals around it, like Saturn but closer to a sun. Are there other concerns to have (gravity, for example, if the moons are too small)?

Thanks!

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  • $\begingroup$ Welcome to the site, prodigeek. Questions like, "Is there something I didn't think of?" tend to be too broad for the site and attract close votes from the community until an edit is made to limit the scope of the question. We strongly prefer specific questions that produce detailed answers; you can always ask multiple questions and link them together if they add detail the world under construction. Feel free to take the tour to get a better understanding of the site. $\endgroup$ – Frostfyre Nov 23 '17 at 5:01
  • $\begingroup$ There are already a lot of questions about habitable moons of gas giants on this site. I would recommend that you search "gas giant habitable moon" and read through them. There's a 99% chance this question is fully answered among them (habitable moons are a popular topic). $\endgroup$ – JBH Nov 23 '17 at 5:11
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Probably.

One of the most habitable moons of Jupiter is Europa:

A base there would also offer up a spectacular view: Jupiter would look almost 24 times larger in the sky than Earth's moon appears from the planet's surface. "It wouldn't be a boring place to drink your afternoon tea," Vance said.

However, residents of Europa would have to deal with the blistering cold. The temperature averages about minus 260 degrees Fahrenheit (minus 160 degrees Celsius) near the equator and minus 370 F (minus 220 C) near the poles, Pappalardo said.

Aside from the radiation and cold, a few other dangers exist on Europa. Modest ice quakes — which might feel somewhat like an earthquake on Earth — could occur every now and then, and you'd definitely want to watch out for giant plumes of water that could violently shoot out of the ice. There would also be a small chance you'd be hit by a small meteor or particles from space.

Europa's gravity is only about 13 percent of Earth's, resulting in a virtually nonexistent atmosphere. Because of this, Europa, like Earth's moon, has no weather, wind or sky colors. And walking on Europa would feel similar to walking on Earth's moon.

If you went for a stroll, you'd probably want to visit Europa's "chaos terrain," where the normally flat ice has been fragmented into jumbled blocks. "It might look like you're in Monument Valley," Pappalardo said. You'd also want to see the moon's depressions and ice domes, which are up to a little more than a half-mile (1 kilometer) high.

But, neither Europa nor Jupiter are as big as a moon around a gas giant or a gas giant can get. The maximum possible mass of a gas giant before it ignites into a brown dwarf star is 11-16 times the mass of Jupiter and the diameter if Jupiter is as big as a gas giant can get (adding mass would only increase it density).

But, a moon around a large gas giant can get as large as the planet Neptune ("surface" gravity 110% of Earth's) or Uranus ("surface" gravity of 88% of Earth's). And, while Jupiter has 60 moons, nothing in the laws of physics (ideally helped along with a little orbital engineering) prevents it from having fewer, larger moons than it does. It would be tricky to figure out how to keep any of the planets from colliding into each other for a very long period of time, but not impossible by any means.

It would be more realistic, however, to have six more or less habitable moons and at least a dozen or so more small moons that were not habitable.

With larger moons, the gravity and atmosphere problems can be solved. The right rotational speed on its axis and right tilt of its axis and atmosphere combined would moderate the temperatures greatly. You would need the gas giant to be much closer to the Sun, or it would have to actually be a brown dwarf rather than a gas giant, however, to make the moons sufficiently warm.

Their speed of rotation around Jupiter respectively would have to be fast enough to prevent the moons from getting too cold when they were in Jupiter's shadow relative to the sun (although terraforming with a particularly thick atmosphere could also help with this issue), if you want to avoid the very strange and possibly unstable dynamics of having the entire system on a huge axial tilt relative to the star around which it orbits.

The ring of rocks and minerals wouldn't be very habitable, but there would be no problem with having one, and indeed, you would expect one.

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  • $\begingroup$ There's no issue with the planet having an axial tilt. Saturn's is 26.7, about 3 degrees more than Earth's, and Uranus's is famously 98 degrees. $\endgroup$ – Keith Morrison Nov 23 '17 at 23:05
  • $\begingroup$ Planets can have an axial tilt, but the axial tilt impacts its weather. You have to have the "planets aligned" properly to get decent weather without too much of a variation between hot and cold. $\endgroup$ – ohwilleke Nov 27 '17 at 2:51
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Titan is roughly the same size as our moon, but it's atmosphere is actually thicker than earth's, so holding an atmosphere is feasible. The moons would probably be tide locked to the planet. There wouldn't be much in the way of seasons. Nights might get quite cold, but there would be a near constant wind as the cool dawn air warms and the warm evening air cools.

Some of Saturn's rings are now known to be debris from specific moons (Iapetus for example). The rings themselves are about 10 feet thick (if memory serves) and composed of gravel and smaller particles (dust). They pose no hazard to a moon worth an atmosphere.

Radiation would be an issue. Gas giants are large enough to sustain limited nuclear reactions - they put out more energy than they receive from the sun, but nowhere near enough to make the transition to being a star. In their own right. Atmosphere and magnetosphere protect against charged particles and neutron emissions (the auroras would be spectacular), but are less effective against xrays.

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  • $\begingroup$ Perhaps he could terraform some genetically engineered photosynthetic air born plant matter optimized for x-rays instead of visible light? $\endgroup$ – ohwilleke Nov 23 '17 at 5:54

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