I am aware of being a newbie in the field I am asking my question... please be kind and explain what I did wrong if I make a mistake.

The idea from where the problem bloom

I love worldbuilding. Whenever I have a spare moment, I try to invent unique features for my world and how I can make them believable. This time around, the idea came from an article that talked about moons of gas giants habitable by humans. Immediately, I told myself 'wow, that's awesome! I need my world to be one of those!', and got into the process of collecting information about how to make it believable.

Quickly, I found a lot of useful information (in particular questions on Stack Exchange, and some blogs found thanks to Google). Then I saw an article that made me think of another crazy idea :

  • What if they were not one, but two habitable moons orbiting the same gas giant, like Janus & Epimetheus do -- swapping orbit regularly ?

For a crazy worldbuilder always in search of new unique features, this would be the straw on the cake. I would have two worlds for the price of one, and inhabitants filled with dreams at the sight of their twin world crossing their path and showing the lights of a more advanced civilization.


I am not a clever man. I did a lot of research, but could not even find a start to begin my calculations to see if my idea was realistically feasible. I knew I probably had all the necessary information to make it, but I can't seem to approach the start of an answer. So in the last approach, I am asking for help to fool-check my concept.

What I am seeking

In short: Is my concept realistically possible? And what are the differences between Earth and one of the two habitable moons (related to the impact of the inhabitants everyday life -> aka. the difference in the weather, the look of the sky at night and daylight, can people see the giant storms of the gas giant, does the tides change as the twin cross the path of the other, are meteorites more likely...) ?

Longer version: I know there is a lot of parameters to take into account when creating a whole system like this one. The idea is only about finding one setup where it can work. Here is what I can comment on, and what I know is needed :

  • Mass of the objects, distance to the sun/giants: This is a significant factor, but for this situation, it doesn't matter what values are taken as long as it sticks together.

  • Atmosphere: I'm unsure about this one. While I do know that moons can have their own atmosphere, I've also heard that the atmosphere of the giant can encompass the satellites.

  • How the moons formed: I've found somewhere that habitable moons were more likely when they formed in the habitable zone of the system, and were captured afterward by the gas giant when she would 'migrate' toward the habitable zone. For this question, I will simplify by saying that it doesn't matter. Let's just imagine that everything magically appeared in the best setup possible.

  • Tidal locking: This is a regular, and inevitable occurrence of moons. If I am correct, the question would be whether the planet can be habitable when it is already tidally locked, and the consequences of this. Like: where are the north and south poles equivalent, do inhabitants of one side never see the gas giant whereas inhabitants of the other side see it every day?

It's everything that came to my mind as of now. If I remember anything else, I'll edit the question to add it. Feel free to guide me to resources I can use if you know some, an incomplete answer is better than no answer at all.

Thank you in advance !


  • 2
    $\begingroup$ I'd start with reality. What we find is strange enough as it is. I would recommend that you pick either HD 34445b or HD 34445f as the host planet for you twin moons. Both gas giants are in the habitable zone. openexoplanetcatalogue.com/planet/HD%2034445%20f $\endgroup$ May 29, 2018 at 21:43

1 Answer 1


Wow... good question with a lot to answer... i'll try my best, but i admit, i'm not an Astrophysicist.

Short Answer

We don't know. But the more systems we look at and manage to find exoplanets, the more weird and wonderful things become, so there's no reason why not...

Another option could be 2 equally sized Planets orbiting each other around a star, that way there would be closer to each other but never really far apart so that interaction between them would be possible once they've reached the space age.

Long Answer

Let's name the planets to make life a little easier.

  • Host Planet, Gas giant the moons are orbiting.
  • Moon A, closer Moon to the Gas Giant.
  • Moon B, moon further from the Gas Giant.


Host Planet, yes Gas Giants can exist in the habital Zone of a Star, so that's a good start.

Moon A, will most likely (if you're wanting to make it look as cool as possible) have an orbit that cuts a swath through the rings of the Host Planet, (because rings are always cooler than no rings). This is seen around Saturn, now the star would most likely need to be an F type star, the sun is a G type star, F types have higher mass therefore greater gravity, and are hotter, therefore the distance from the star would be increased, this is important because the Host Planet would need to be larger than Jupiter, in order to support an Earth Sized Moon. And it would need to be further away so that the moons themselves are not quite as effected by the star's gravity as to throw the moons orbits out.

Moon A would orbit faster than Moon B due to its lower orbit, Moon B would need to be at least twice the distance from the host planet as Moon A, this would make 1 orbit take about 4 times as long as Moon A, but they would not get as close to each other as Earth is to the Moon, the gravitational effects would change their orbits too much over time. So unfortunately they would probably not be able to look up at the advanced planets night lights with the naked eye, maybe with decent telescopes though.


Moon A would most likely be slightly smaller than Moon B, so lower gravity and slightly thinner atmosphere, similar to Mars, but still with an active core to provide the protective Magnetic field, this would protect the planet from the strength of the solar wind and therefore allow it to keeps its atmosphere, it would also have some effect from the gravity of Moon B, the tidal forces allowing it to keep its molten core. Both would be enough to sustain liquid water, important for life as we know it.


You've actually got that pretty right, although this is just best guesses, as we haven't Exactly seen it happen. As when the host planet formed, most of the material would have been pulled into the planet instead of the moon.

Tidal Locking

Moon A is more likely to be tidally locked then Moon B due to Moon B having that effect, but not definitely, it depends on a variety of factors, for the sake of the story I would suggest that moon A is not tidally locked but close to it, so it does rotate, that way you still get the planet rise.

The Issue

For potentially large parts of the orbit the planet would be blocking the sunlight from the planet so day night patterns would be completely out of wack compared to earth,

It would also lead to more dramatic cooling when in the dark which means as it enter sthe light and starts to heat up then big big weather. but the closer to the planet they are the more dramatic the changes, so again Moon A would have the more dramatic weather then Moon B

If I think of anything else I'll let you know, but this is at least a basis some ohters can build on.

  • $\begingroup$ The moons don't necessarily need their own magnetic fields, since some gas giants have their own very powerful ones. (This would create powerful van Allen belts, which would make travel between the moons difficult.) As for the moons regularly experiencing solar eclipses, that depends on how inclined the orbit is compared to the ecliptic and how far away they are. $\endgroup$
    – ltmauve
    Jul 27, 2018 at 20:46

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