I ask this as a basis for a World (in this case Moons) Building I'm doing. I came up with a couple of different problems and some possible solutions to those problems:

  • All of these questions presume the moons are artificially created or modified by a far superior race and placed into orbit artificially. That being said, they can bend physics, not break it. So I'm asking these questions based on that idea.

  • If the moons orbited on the ecliptic, there would be protracted periods where the moons would be eclipsed from the primary. Unless... they orbited in a circumpolar fashion. Would there be any problems with this? If circumpolar was too far fetched or had hidden physics problems, could the axis of rotation of the jupiter style planet itself be close to that of the ecliptic so its moons would orbit in a manner similar to circumpolar?

  • Could multiple Earthish-sized moons with an Earth-like atmosphere exist in orbit around a Jupiter-sized Gas Giant? Or would the tidal pull strip the atmospheres?

  • Assuming the moons had active tectonics like Earth, would the magnetosphere of these moons be strong enough to protect those living upon it?

  • Can you identify any other problems livable moons circling around a gas giant would have?


  • $\begingroup$ Active tectonics isn't a requirement for a magnetosphere...rotating iron core is the key there. I really can't find much for interactions between a moons and the planets magnetic fields and how they interact though. $\endgroup$
    – Twelfth
    Commented May 22, 2015 at 21:43
  • $\begingroup$ The eclipses wouldn't be a major problem, as they'd be ony a few hours duration during a 'month' - the orbital period of the particular moon. Think about how short even a lunar eclipse is on Earth. The other questions I think all depend on how long ago the moons were placed in orbit. Loss of atmosphere, tidal locking, and so on ought to take at least hundreds of millions of years. A sensible advanced race would also clean out orbital debris, so it would take considerable time to accumulate more. $\endgroup$
    – jamesqf
    Commented May 22, 2015 at 22:24
  • $\begingroup$ All of those are excellent points. I looked up the amount of time it takes a moon to Tidally lock and this limit goes up rapidly the higher the mass of the moon. So, I think it's safe to say, that recently placed moons would remain orbiting for quite some time only slowing down a few minutes a century (if my math is correct). As far as atmosphere... it's anyones guess - I have not yet found the equations for this. $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 22:30
  • $\begingroup$ Another question is could a Jupiter sized Gas Giant exist that close to the Sun? The increased temperature is going to help the escape of the H and light gases even from a large planet, and reduce the size. $\endgroup$
    – Oldcat
    Commented May 22, 2015 at 22:47
  • $\begingroup$ I've been reading through the blogs based around the Kepler satellite. I was sure I'd read there had been a larger-than-Jupiter gas giant found orbiting around a nearby star... but my google skills are lacking, apparently because I can't seem to find the article again. $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 23:22

2 Answers 2


If the moon was large enough to hold an atmosphere, then in principle there is no reason that it could not be capable of being terraformed. There would be some interesting complications, however.

First off, a moon orbiting a Gas Giant would likely be tidally locked with one face towards the primary. Days would be very short, but the hemisphere facing the primary would be illuminated by the primary itself, as well as the primary and the sun during part of the orbit, and in darkness for a very short period of time. The side facing the primary would have the "hot pole" where the primary is at the zenith, while the opposite side would have the "cold pole", so atmospheric and hydrospheric circulation and heat flows would be dominated by this.

The leading hemisphere of the moon would be bathed by the energetic radiation trapped in the primary's magnetosphere, while the trailing hemisphere would be relatively shielded. The interaction between the energy deposited from the hot pole and the "leading pole" could be defined as a series of concentric bands at 90 degrees from each other, leaving the moon covered in a sort of checkerboard of ecosystems based on energy inputs.

Depending on the numbers of other moons, the core of this moon might be "kneaded" by multiple and overlapping gravitational pulls during its orbit, making the moon quite active tectonically. Lots of volcanoes and active plates would make the surface quite active, as well as subducting lots of water and carbonate rocks. The hydrothermal and carbon dioxide cycles on this moon would be much faster than on Earth.

Since we are in the middle of a very deep gravity well, you should expect the moon is also subject to lots of collisions with asteroids and comets. This would add lots of water to the moon, but also reset ecological "squares" that were hit, meaning evolution would be going in fits and starts if the moon already had or was seeded with some sort of "native" life.

IF the Primary was tipped over like Uranus and the moons were in orbiting facing the sun at all times, then the positions of the "Hot" and "Leading" poles would be different. There would actually be a third pole, where the primary is overhead at all times, with the sun coming down at a high angle but also being permanently illuminated by the primary as well. There would be no diurnal cycle as we understand it, but the solar and hot poles would always be illuminated, while the dark pole (opposite the solar pole)would be in darkness and the cold pole would only have solar illumination, but more constant than the cold pole of the first example. Trying to trace the energy flows in these moons would be very interesting indeed.

  • $\begingroup$ I was thinking that only the moons closest to the Roche limit would be tidally locked. The other, outer, moons should be able to rotate freely. But, to be honest, I'm not sure of this as I haven't worked out the math. $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:19
  • $\begingroup$ And yes, I was aware all of these moons would have very active tectonics. Not sure HOW active, but pretty darned active, to be sure. $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:20
  • $\begingroup$ Lastly, I was hoping a strong moon magnetosphere would alleviate most, if not all, of the Gas Giant's magnetosphere; thus keeping the energy levels down to somewhat reasonable levels. Is this too naive a conclusion? $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:22
  • $\begingroup$ The numerous amounts of collisions was a problem I hadn't thought of, to be honest. What about a trailing Gas giant to help strip asteroids out of the playing field like Jupiter does for us? $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:24
  • $\begingroup$ All of Jupiter's major moons are tidally locked, so even if they start with rotation, over the eons they will become locked. A strong magnetosphere will alleviate the primary's field to some extent, but not 100%. More gas giants may set up a chaotic situation in the outer system, and lots of comets and asteroids in erratic orbits would be more of a problem. Even so, the moons would be habitable, just dangerous. $\endgroup$
    – Thucydides
    Commented May 22, 2015 at 21:28

We have ice giants with the planet's plane at near right angles to the sun's, so no problem.

I don't think tides would destroy the atmosphere. It's the small size that makes it prone to losing the air to space.

One of Saturn's moons has a thick atmosphere. One of Jupiter's moons has a magnetic field, but it's not enough to shield it from Jupiter's radiation. For planet-scale engineering, look for Robert L. Forward (et al.)'s plan on how to drain Earth's Van Allen belts. An upsized version could remove the radiation around a giant, and supply power.

Earth is about as small as you can get and have plate techtonics. However, that is not the same as having a magnetic field.

Maybe (with the Forward Shunt) the giant's magnetic field could protect all the moons, instead of creating a hazard as with Jupiter.

  • $\begingroup$ Cool! I've read Forwards work - Dragon's Egg. I thought it was quite good. Unfortunately, I'm not familiar with the Forward Shunt. Do you have a reference? $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:09
  • $\begingroup$ Also - the little and very sketchy understanding I have of Van Allen belts, the Jupter-like planet would not be very conducive to happy living on the moons. Unless the moons were within Roche's limit (and thus, not a moon anymore) they are gonna get zapped by all of those energized particles. I think they'd need their own, very powerful, Van Allen belt of their own. I was referencing tectonics because active tectonics means a molten core. Which means you get the more powerful magnetic field. Planets like Mars have a field, but it's much weaker, IIRC. $\endgroup$
    – Hirahito
    Commented May 22, 2015 at 21:12
  • $\begingroup$ @Hirahito , I admit I don't understand this that well, however, Saturn's magnetic field and radiation belts are not nearly as intense as Jupiter's. It is possible that other gas giants resemble Saturn and it's environs rather than Jupiter, in which case this becomes a much more inhabitable region. $\endgroup$
    – Jim2B
    Commented May 23, 2015 at 4:01
  • $\begingroup$ s/it's/its/ its not "it is" :) $\endgroup$
    – JDługosz
    Commented May 23, 2015 at 4:24
  • $\begingroup$ WRT radiation belts &c, the premise is that the moons were placed in orbit (& terraformed?) by an advanced civilization. Said civilization certainly would have tuned the gas giant's magnetic field and so on to whatever was needed to make the planets comfortable - for them, if not necessarily for humans :-) $\endgroup$
    – jamesqf
    Commented May 23, 2015 at 5:03

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .