I've already asked a similar question but i'd like more details.

This moon would be orbiting around a saturn like planet without rings. The host planet would be located in the habitable zone, allowing few of its moon to sustain life.

The moon would be 12000 kilometers wide, so it is big enough to have its own small magnetosphere. Thanks to that, it can orbit far away from its host planet.

I now want this moon to have a tilted orbit in such a way that when it orbits around the host planet, it will always receive light from the star because the light won't be blocked by the host planet when the moon is behind it.

Now I would like the moon to be non tidally locked so that it takes a few month to orbit around the host planet, but it would rotate on itself like earth would do in 24 hours, to have a day and night system.

how can this moon be non tidally locked? how can it be stable? I don't want the planet to be at the beginning non tidally locked and then become tidally locked after a certain amount of time, like normal moons would do. Phoebes or hyperion also are non tidally locked moons, but they are located far enough from saturn and are very small, therefore, my moon is 12000km wide!

How could it be a stable non tidally locked moon, that would rotate on itself in 24 hours? can it be caused by an asteroid that is orbiting around the moon and that's dragging it, causing it's rotation? Or a nearby other moon that is pulling the moon with it and causing also it's rotation?

I don't know how a moon like this could be stable and complete all these criterias so I would like a solution and details. Thank you!

  • at 12,000 km diameter, it's not so much a moon as a dual planetary system. – Serban Tanasa Jul 21 '17 at 14:02
  • Do you just want any life or do you want something more specific? Life relatively similar to what we know could exist on a couple of moons in our solar system outside the "habitable" zone. – Raditz_35 Jul 21 '17 at 14:27
up vote 4 down vote accepted

Let's break this down into the individual requirements.


The moon would be 12000 kilometers wide, so it is big enough to have its own small magnetosphere.

That's nearly twice the diameter of the largest known planetary moon, but it's not out of the realm of plausibility, so sure. You could say it's a captured planet or something.

Thanks to that, it can orbit far away from its host planet.

This is where you might run into problems. Going by the answer to your previous question, your moon would be orbiting at a distance of around 2 million km, so its distance from the Sun is going to vary quite a lot over the course of its orbit. And that's not taking orbital eccentricities into account. Depending on how quickly it orbits the planet, you might get some rapid fluctuations in temperature.

Fortunately, your next requirement makes this slightly less of a problem:

I now want this moon to have a tilted orbit in such a way that when it orbits around the host planet, it will always receive light from the star because the light won't be blocked by the host planet when the moon is behind it.

I can't think off the top of my head how much you'd have to tilt the orbit in order to avoid this, but a safe bet would be to give it the same tilt as the Earth's axis: about 23.4° As noted in the comments (thanks @pablodf76 and @bendl), you can achieve this with a tilt as small as 1.7° (even smaller if the orbit is further out). The larger the orbital inclination, the less distance there'll be between your minimum and maximum distance from the sun, but you'll start running into problems with parts of the planet being permanently light/dark.

Now I would like the moon to be non tidally locked so that it takes a few month to orbit around the host planet, but it would rotate on itself like earth would do in 24 hours, to have a day and night system.

You've answered your own question here. As you note, Phoebe and Hyperion are non-tidally locked moons, so this is entirely possible. Indeed, the answer to your previous question notes that larger moons will resist becoming tidally-locked for much longer, so your supermoon can very easily sustain its own axial rotation.

How could it be a stable non tidally locked moon, that would rotate on itself in 24 hours? can it be caused by an asteroid that is orbiting around the moon and that's dragging it, causing it's rotation?

Again, refer to your previous question. It must have already been spinning that fast (or faster) when it was captured by your gas giant. An asteroid isn't going to have enough mass to pull your planet round in circles like that.

Or a nearby other moon that is pulling the moon with it and causing also it's rotation?

Another nearby moon might affect its rotation around the planet (this is called an orbital resonance, and can be seen in the Galilean moons of Jupiter), and would also cause tidal flexing that would increase the moon's geological activity (again, see Io), but it wouldn't affect the planet's rotation around its own axis.


TL;DR: Can you have a habitable moon orbiting a gas giant, with an inclined orbit and its own day/night cycle? Yes, yes you can.

  • I'm not sure why you bring up axis tilt there. The Moon's orbit is inclined by only 5 degrees with respect to the ecliptic and yet lunar eclipses are relatively rare, and solar ones even more so. As long as the planet is far away there won't be many eclipses (and these will be short). – pablodf76 Jul 21 '17 at 11:44
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    At this distance from a saturn sized planet, the planet's angular diameter would be 3.4367 degrees. About 13.2x bigger than the moon. A really rough calculation in which i probably made a mistake makes the minimum orbital tilt about 1.7 degrees for this moon always to have full view of the sun – bendl Jul 21 '17 at 12:47

A resonant lock, like Mercury's, can stabilize the moon's rotation. With an eccentric orbit the same hemisphere (or its opposite, as with Mercury) can end up facing the primary at each periapsis. This will tend to lock the rotation in place and keep it from progressing to a 1:1 lock.

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    Actually, this answer seems a solution to my question but I don't completely understand (because I'm not such a scientist) so may you please simplify the answer, because it looks very interesting and i would like to understand it completely. There's some vocabulary I unfortunately don't know ( such as "resonant lock", "eccentric orbit" or "periapsis" ). Thank you for your help! – Mathis Jul 22 '17 at 12:13
  • An eccentric orbit is one that is not circular, but is an ellipse. For instance Mercury's orbit is 29M miles at it's closest point (periapsis, or in the case of the Sun, perihelion - periapsis is the general term and perihelion refers specifically to our Sun; for orbits around the Earth the term is perigee), and 43M miles at its farthest point - apoapsis or aphelion. This results in tidal forces acting strongly on denser portions of Mercury which tends to keep it locked at its 3:2 rotation rate. – Brook West Jul 23 '17 at 7:06
  • If the rotation rate were to change for some reason, tidal forces would act upon the planet or moon to keep it at the same rotation rate it had before - thus locking it into that rotation rate. This is called a resonant lock. – Brook West Jul 23 '17 at 7:09
  • Thus your moon could be locked into a rotation rate such that the same face is presented to its primary each time it reaches the closest point in its orbit. It could rotate multiple times during each orbit and the resonant lock would keep it at that rotation rate. This does require an eccentric (ellipsoid or non-circular) orbit. – Brook West Jul 23 '17 at 7:15
  • The primary would be at one of the two foci of the elliptical orbit. – Brook West Jul 23 '17 at 7:16

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