I am currently creating a world for a series of fantasy short stories (and hopefully movies one day), and I need some help with the science. Ideally, I would like the story to take place on a moon that is slightly smaller than the earth, with a similar atmosphere and temperature, that orbits a gas giant, which in turn orbits a star similar to the sun. The moon must be able to support human life. I have several questions that I have been unable to answer on my own:

  1. What sizes and masses of the star, planet, and moon would make this possible?
  2. What would the orbits of the planet and moon be (shape, distance, and speed), and how would I calculate the length of a day, month, year, and the seasons on the moon using this information?
  3. How would the gas giant affect the tides of oceans on the moon, as well as the seasons and how much would it light up the moon at night?

Since this is for a fantasy world, it doesn't need to be 100% realistic, but I would like to be as scientifically accurate as possible.


  • $\begingroup$ I'd start by looking at Europa as an analogue: en.wikipedia.org/wiki/Europa_(moon) While it doesn't exactly have the characteristics you're after, it's a good starting point. $\endgroup$ Jun 15, 2016 at 8:32
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    $\begingroup$ This question is similar; see also links therein. $\endgroup$
    – HDE 226868
    Jun 15, 2016 at 20:42

4 Answers 4


It is quite possible.

For a model, I'm using Callisto. It is the second largest of Jupiter's moons, and the furthest large moon from the planet. It takes 16 days to orbit Jupiter and so suffers from less tidal heating than the other moons. It is also further away from Jupiter's radiation bands. (This last is an important point. Radiation around Jupiter is severe) We don't want tidal heating of the planet, as we have the sun to warm our atmosphere. Tidal heating will just make more volcanos.

You can be quite flexible in your choice of star. Something sun-like is safe. Dimmer stars are also possible, (though the planet must be closer to the star, so flares are more of a problem). Very bright stars probably don't last long enough. The length of the year would be shorter if the sun were dimmer longer if the sun is brighter, from a few Earth months to a few Earth Years.

The planet can be anything from Saturn sized to bigger than Jupiter. The mass will affect the moon's orbital period (for the same sized orbit, more mass=quicker orbit) But the radius of the planet will be about the same.

The moon will be tidally locked to the planet, so the planet will only be visible from one side of the moon. This may be a big deal for the inhabitants (do those born on the "far" side even know the planet exists?) Tidal locking also means that the day length = the orbit length. About 16 Earth days for Callisto. The nights will be long and cold. If that is too long you can increase the mass of the planet to shorten the day a bit.

Since the moon is tidally locked, there won't be much tide. The seas won't change in height a little due to solar tides, and perhaps small effects from other moons, but nothing like Earth tides. Also there wouldn't be any months.

The moon will orbit in the equatorial plane of the the planet (unlike our moon which orbits in the plane of the ecliptic) periodically the moon will enter the shadow of the planet causing a eclipse. Those on the far side probably won't even notice, but it will be a major event for those on the the planetside. The planet will be bright, perhaps too bright to allow stars to be seen by those on planetside except during an eclipse. Eclipses will occur during the planets equinox, twice a year,

Other moons are likely to exist, and may be quite significant sky features, clearly showing a disk to observers.

So to summarise

  1. You can have a sun-like sun and a Jupiter like planet, and an Earth-like moon.
  2. The planet's year could be about 360 days, the moon's day and month are equal at about 16 days.
  3. The planet would be a major feature in the sky of half the moon.
  • $\begingroup$ James - only problem with this answer is your 3rd paragraph. For orbits where the primary is much bigger (eg the Sun) the orbital speed is not related to the smaller objects mass. Vo = sqrt (G.Mass of sun/radius) $\endgroup$
    – Rory Alsop
    Jun 15, 2016 at 16:27
  • $\begingroup$ I mean orbit of the moon around the mass of the central gas giant planet. The moon will orbit faster if the mass of the planet is faster $\endgroup$
    – James K
    Jun 15, 2016 at 17:20
  • $\begingroup$ ah - okay. That makes sense - in your answer it's unclear. $\endgroup$
    – Rory Alsop
    Jun 15, 2016 at 17:23
  • $\begingroup$ The Jovian radiation belt is created by Io's interaction with the Jovian magnetosphere, thus not having an Io equivalent will reduce the radiation hazard a lot. $\endgroup$ May 7, 2019 at 20:17

Looking at current Jovian satellites, Europa and Io give you some idea of what you'd need to consider. Starting with the Solar system model:

  • Europa is (as Aaron commented) the most likely candidate, but is probably too small (little under the size of our moon) and too cold - but it probably has a water ice crust, which means it is not far off the necessary temperature.

  • Io maintains a slightly higher temperature, but is the most volcanic object in the solar system (these 2 facts are linked with the proximity to Jupiter - Io is the closest of the Galilean satellites)

  • So in an exact analogue of our solar system you probably would need this moon to be similarly close to Jupiter to allow for tidal heating, volcanism would be likely (and possibly help with warming if sufficient greenhouse gases were a part of this) but your real challenge in an exact Jovian copy is that you are so far from the sun, so solar heating will be tiny, photosynthesis will not provide plants as we know them, and hence oxygen will need to be sourced from somewhere.

The solution to the temperature/energy problem is to have the gas giant much closer to the star, or have a larger star. You do run up against a minimum orbital distance limit for gas giants, however if the gas giant was the only planetary body in the system, it could certainly be much closer than Jupiter is in our solar system.

So, a Jupiter-sized planet orbiting a little outside Mars' orbital distance, with a moon a fair bit larger than Europa could suit your needs. So you could use a year around 3 Earth years long for the gas giant, but need to look at how orbiting that gas giant affects things (an orbital period of 3 days or so is well within bounds for this model) in addition to the rotational period. For interest you could look at having the moon's orbit at a high inclination to the gas giant - this could simplify some of the concepts of days and years.

Rotational period may be very long - tidal locking is likely. If the moon isn't locked then the tidal effect on not just any liquid water, but the moon itself will be extreme.

Reflected light will make for impressively bright night skies on the side facing the gas giant (see how bright Jupiter is from Earth for an indication) - by default, gas giants have a large albedo so if you have tidal lock here the side facing the gas giant will only see darkness when eclipsed by the gas giant.

  • $\begingroup$ If "your real challenge in an exact Jovian" means 'the real challenge with a planet identical to jupiter' I have nothing to say. but the wording is prone to confusion. As you yourself say, Jovian planets are not restricted to cold parts of a system. They can surely exist within habitable zones. Even closer. So you can have a jovian moon with enough irradiation to sustain plant life. $\endgroup$
    – Oxy
    Jun 15, 2016 at 10:56

Either you wait for these guys who know the math, or you take the easy-going approach I can offer.


So, the inhabitable world should be at least as heavy as Mars (in terms of mass), but better be in the same weight class as earth - less mass would allow fancy jumping combat, but you may lose your atmosphere, more mass would... do many things. To break things down: aim for a surface acceleration close to ours to get the most earth-like appearance. Something between Earth and Mars in terms of mass and size may work. When in doubt, add superheavy elements to the core ;)

About the other protagonists... here you should aim for something similar to our system. Bigger suns may offer brighter days but also more radiation... which may be nullified if your Earth-Moon is inside the gas giants magnetic field. At the other hand that would mean that it will so close the rotation went to a bound one (one side is facing the gas giant all the time). Just the same our very own moon (Luna) is doing after all. When you want this moon to have a more earth-like rotation, it have to reside in a pretty big orbit, but... I thing there might be a limit of some kind. Here the guys with the correct formulas on hand can offer better explanations: how far it can go of without saying farewell to the gas giant completely?

So, have a modest sun, place that gas giant inside its habitable zone, add a moon that is a bit smaller than Earth but a bit bigger than Mars and here we go. Should work and can offer pretty impressive polar lights under the right circumstances.

Something more: big gas giants will heat up your moon by their own. Not like a true sun, but sufficient to shift the orbit a bit more away from the sun. If you install a brown dwarf instead, chances are high that life would flourish even outside the habitable zone up to some million kilometers. But it would be pretty dim all the time.


Oh... math. As Aaron Lavers in his comment said, you could look up whats happening with the big moons in our system. The gas giant might need more or less a year (less, if the sun is smaller than ours, way more if its bigger, when its orbit is adjusted to stick inside the habitable zone). The planet of yours (the moon)... I guess he will take more than a month for a full round, but way less than a year. Three to four months (real earth months)?

Seasons would be pretty interesting, by the way, because you may have some days your moon would stick in core-shadow of the gas giant. Reminds me about the movie Pitch Black with Vin Diesel.

Well... to stick with the four month orbit (its good for explanation), you would have one month for northern hemisphere for Winter, one for Spring, one for Summer and one for Fall... and imagine the gas giant need twelve to orbit the central star and have some orbital excentricity of its own, you could have three "winter" which range from "cold as fu**" to "pretty hot", while three times a year you suffer from total darkness for a couple of days... If that happens in the "cold-winter" your people would be... very poor off, while in the "hot-winter" it could be more decent. At the other side during a "hot-summer" they will enjoy the off time from scorching temperatures.

To conclude this: your moon will have summer, fall, winter and spring three times a year, one in a hot variant, one in a cold variant, one in a medium variant... and I thing these will shift over the years. Correct calculation will be a nightmare if you decide to go for super realistic orbital times... Maybe its a good idea to stick with the 4-months orbit for the moon and the 12 (or maybe 16) month orbit for the gas giant, so you can determine more easily how things are looking at that moon.

EDiT: Turns out I forgot something: to have long orbital times for the moon, that gas giant can't be pretty big (the bigger, the faster you have to run on your orbit to avoid crashing down). So either move away from the giant (which will make the ecpises less intense), or shrink the giant... hm. Titania from Uranus does have a similiar orbit to the one our moon has but still, it just takes about a week to do a full orbit. That will be tricky. Maybe a 4-month orbit is way outside any posibillity after all... or the gas giant would be as small as our moon at the night sky and lose its appeal ;)


Expect giant tides. And bright nights. The latter will only occur when your moon is between the gas giant and the sun. When its a bright ball of Helium and Hydrogen like Saturn, Reflection will cause nights that are as bright as days (with a few lumen off), while something more... Neptunish will cause more dark nights.

As soon as your moon is behind his gas planet (lineup: sun-gas-moon), nights will suddenly get pretty dark and even days can get very dark, when the moon goes for a full sun eclipse.

Well... enough for now. Coworkers already starting to wonder why I'm this busy typing all of a sudden. My answer might not be the most accurate, but I hope most effects had been covered so you can start expecting how thing would look like. Now wait for these guys with the formulas and small pictures of orbits. Or drop all this, because if its a fantasy-world without modern technology stuff like this wont matter enough to waste lines describing accurately how the solar system does look like.


As a practical matter it is possible to write science fiction and fantasy novels set on habitable, earth like moons of gas giant planets and get them published.

For example, Lin Carter's Callisto series of eight novels is set on Callisto, a real moon of the real gas giant planet Jupiter. They are described as science fiction novels of the sword and planet subgenre. The sword and planet subgenre is definitely close to the rather vague line between science fiction and fantasy.

And what many people might think pushes the Callisto series over the line into fantasy is that Callisto is described as a habitable world, which violates everything which science has discovered about Callisto by 1972 to 1978, when the novels were published, and since then.

Setting a fantasy novel on a fictional habitable moon of a fictional gas giant planet in another star system seems with as reasonable to me as setting a fantasy novel on Earth in the past or future, or on a known or unknown alien planet, or on a flat disc world, or in an alternate dimension, or in an unspecified setting, or using any of the usual fantasy settings.

And it has the bonus of having a large object, the gas giant planet, visible in the sky of half of the moon and making a very impressive sight even if you don't make it's angular diameter larger than is scientifically possible.


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