Imagine an Earth-massed moon orbiting around a Saturn-massed planet in a period of 28 days. This calculation tells me that is possible if the moon is 1.8 million km away. If so, the moon must be moving extremely quickly, since it is 4.7 times farther than Luna from Earth and yet takes slightly less time to complete an orbit.

First question is: Would inhabitants of this moon experience any consequences of that rapid speed?

Second question: Is the moon far enough away to reasonably avoid becoming tidally locked to its planet within the time span necessary to evolve a complex ecosystem of flora and fauna?

  • 3
    $\begingroup$ The thing with speeds is that they are alway relative to something. For example, right now both of us are moving at a speed of about 140 km/s (500,000 km/h, 300,000 mph) with respect to Barnard's Star. Does this rapid speed affect us in any way? $\endgroup$
    – AlexP
    Mar 20, 2021 at 4:23
  • $\begingroup$ Your satellite would have linear speed of 4.6 km/s, which is negligible on cosmic scale. $\endgroup$
    – Alexander
    Mar 20, 2021 at 4:35

3 Answers 3


Short Answer:

It might be possible that such a moon would be habitable. But it might not be possible for any moon in such a situation to be habitable.

Long Answer:

I don't think that you want a habitable moon of a gas giant planet to avoid being tidally locked to the planet. After all, if the giant planet and the habitable moon are in the circumstellar habitable zone of a star, and if that star is dim enough, the moon would be tidally locked to the star unless it was tidally locked to its planet.

If a habitable moon becomes tidally locked to the planet, it will have a day-night cycle equal to one of it's "months" or orbital periods around the planet, and so it will have alternating days and nights over every part of its surface instead of having one side always face the star and one side always facing away from the star.

So being tidally locked to the planet is much better for habitability than being tidally locked to the star.

If the planet has a day 28 Earth days long, will it get unbearably hot during the 14 day day and unbearably cold during the 14 day night? Maybe.

In Habitable Planets for Man, 1964, Stephen H. Dole believed that a planet habitable for human beings would need to have a day shorter than 96 hours or 4 Earth days.


But maybe the atomosphere and hydrosphere of the moon will distribute heat enough to keep the temperature changes reasonable.

And possibly tidal interactins with other large moons of the planet might give the moon a shorter day than its orbit around the planet.

And you should look up scientific articles about the possibilities of habitable moons of giant planets. Articles by Rene Heller,for example.

Here - https://www.researchgate.net/profile/Rene-Heller-2[2] - is a link to many papers that Heller is the author or co author of. Some of them discuss the possibilities of habitable exomoons, and I list some but not all of those papers below:

"Exomoon habitability Constrained by Illumination and Tiadal Heating" Rene Heller and Roy barnes, Astrobiology, volume 13, number 1, 2013:


"Exomoon habitability Constrained by Energy Flux and Orbital Stability" Rene Heller, 2012.


"The Effect of Multiple Heat Soures on Exomoon Habitable Zones", Vera Dobbs, Rene Heller, & Edwin L. Turner, 2015.


"Magnetic Shielding of Exomoons Beyond the Circumplanetary Habitable Edge". Rene Heller & Joge Zuluaga.


Rene Heller and co authors created the concept of a "habitable edge", a limit to how close a large moon could be to its planet without suffering a runaway greenhouse effect due to tidal heating.

In "Magnetic Shielding of Exomoons Beyond the Circumplanetary Habitable Edge", Heller & Zuluaga calculate that an exomoon around a Neptune sized planet would either be too close to it and suffer a runaway greenhouse effect or be outside the protection of the planetary magnetic field. They calculate that, other things being favorable, a large exomoon orbiting a jupiter sized planet at a distance of 5 to 20 times the radius of the planet could avoid a runaway greenhouse effect while also being shielded by the planet's magnetic field.

I am not certain whether a Saturn sized planet could protect any large moons with its magnetic field.

Even if Saturn would be large enough to form a strong magnetic field covering it's moons, since Saturn has an equatorial radius of 60,268 kilometers or 37,449 miles, any habitable moons of it would have to orbit between 301,40 kilometers or 187,495 miles and 1,205,360 kilometers or 748,980 miles to avoid a runaway greenhouse and to beshield by Saturn's magnetic field.

The question is about a habitable moon orbiting a Saturn sized planet at a distance of about 1,800,000 kilometers in order to have a month about 28 Earth days long. So even if Saturn sized planets are as good as Jupiter sized planets at protecting their moons with magnetic fields, the moon in the question would be outside the magnetic shield of Saturn and would need to be large enough and rotate fast enough to have its own strong magnetic field.

So you need to find out whether it is possible for an exomoon of a Saturn sized planet to have a strong enough magnetic field.

And since the potentially habitable exomoon of a saturn sized exoplanet would proably have to rotate a lot faster than once every 28 days to generate a strong enough magnetic field to protect it from the solar wind, you need to find out whether it is possible for a moon 1,800,000 kilometers from Saturn to not be tidally locked.

I think that all the moons of Saturn out to Titan at 1,221,870 kilometers are tidally locked, having a rotation period the same as their orbital period around Saturn - 15 days and 22 hours in the case of Titan.

The moon Iapetus, orbiting Saturn at a distance of 3,560,820 kIlometers, is also tidally locked, having a rotation period equal to its orbital period of 79.3215 earth days.

But Hyperion, which orbits Saturn at a distance of 1,481,009 kilometers, even closer than the desired 1,800,000 kilometers, is not tidally locked. Its rotation period of about 13 Earth days is shorter than its orbital period of 21.276 Earth days.

The Voyager 2 images and subsequent ground-based photometry indicated that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable. Its Lyapunov time is around 30 days.[21][22][23] Hyperion, together with Pluto's moons Nix and Hydra,[24][25] is among only a few moons in the Solar System known to rotate chaotically, although it is expected to be common in binary asteroids.[26] It is also the only regular planetary natural satellite in the Solar System known not to be tidally locked.

Hyperion is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near a much larger moon, Titan. These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely. The fact that its rotation is not locked probably accounts for the relative uniformity of Hyperion's surface, in contrast to many of Saturn's other moons, which have contrasting trailing and leading hemispheres.[27]


One problem with using Hyperion as a justification for making a habitable moon of a Saturn sized planet rotate much faster than its obital period is that a habitable moon, even it is only habitable for microbes, would be very large for a moon, probably much larger than any moons in our Solar System. And a moon which interfered with it's rotation enough to keep it from becoming tidally locked would probably be many times the mass of the habitable moon, and thus might be much too massive to exist in orbit around a Saturn sized planet.

So if you care enough about scientific plausiblity in your story you might want to have an astrophysicist - like Sean Raymond at Planet Planet https://planetplanet.net/about/[8] - calculate whether your moon is possible.

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    $\begingroup$ Awesome in-depth reply, thanks! Would it change the magnetosphere part of your answer if our Saturn-sized planet had no rings and fewer moons to absorb energetic particles? I'm no astrophysicist, but I read that's a reason why Saturn has a relatively weak magnetosphere. Saturn's extends 1.1 million km. If we can extend our Saturn-size planet's magnetosphere to just 1.8 million km or more by removing its rings and such, that would protect our moon. Of course, not sure what the effects of being right at the very edge of the magnetosphere would be. Mutated magneto-saurs, I assume. Ha ha. $\endgroup$ Mar 20, 2021 at 22:13

Would inhabitants of this moon experience any consequences of that rapid speed?

On an individual level, no. Their scientists might figure relativity in less time than it took us because they will have astronomical bodies moving very fast, much closer to them, but that's it.

Is the moon far enough away to reasonably avoid becoming tidally locked to its planet within the time span necessary to evolve a complex ecosystem of flora and fauna?

I don't think so. Titan is at ⅔ the altitude of your moon and is very massive (about 1.8 lunar masses), yet it is also tidally locked to Saturn. At the very least your moon would have a very slow rotation.

  • $\begingroup$ Thanks for the helpful reply! $\endgroup$ Mar 20, 2021 at 22:15

First question is: Would inhabitants of this moon experience any consequences of that rapid speed?

Nope. For one thing. everyone is moving pretty fast... for example, Earth's orbital speed is ~30km/s around the Sun.

For another thing, speed is relative. I'm not gonna try and get into the details of relativity here (because I'll probably get it wrong anyway), but it isn't unreasonable to say that one speed is pretty much like another, all else being equal.

If the two moons are pretty much the same mass and size and material and rotate about their axes at the same rate, you'll have a tough job telling them apart other than by the view and weather.

Second question: Is the moon far enough away to reasonably avoid becoming tidally locked to its planet within the time span necessary to evolve a complex ecosystem of flora and fauna?

That's two different questions, on top of the question you've already asked!

Tidal locking is a complex subject, and has been the subject of a lot of questions on this site https://worldbuilding.stackexchange.com/search?q=tidal+lock (and relatedly exomoons, but especially habitable moons). Suffice to say that it is very difficult to arrange a moon that does not get tidally locked to its planet, and yours most certainly will be.

It is worth remembering that tidal locking doesn't necessarily mean that the world will be hostile to life... thick, super-rotating atmospheres will distribute heat well, and the moon could well be within its parent's magnetosphere, and have a short enough orbital period that it doesn't have super long nights, and so on. Have a read of the various other colours and flavours of question and answer on those subject before asking a new question on the subject, though... there's a wealth of useful information there.

  • $\begingroup$ Great reply. Very helpful. Thanks! $\endgroup$ Mar 20, 2021 at 22:14

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