For a big narrative project of mine I and my team developed an habitable moon that revolves around a gas giant in a star system with an F-star. I wanted to know what you think about it and if you think it would make sense to have the moon habitable.

I'll start with the star. It would be an F-star 1.3 times the mass of the sun and of an age of 2.5 billion years.

Next there would be the gas giant. The gas giant would be 3 times the mass of jupiter and orbit at a distance of 2 AUs from the star being it's 5th overall planet. It also has an axial tilt of 20 degrees.

Finally the moon. The moon is about 0.7 times the mass of the earth and orbits at a distance that allows for a 36 hours long orbital period and thus day since it's tidally locked, it has an orbital inclination to the giant's orbital plane of 10 degrees, which combined with the gas giant's axial tilt makes for a 30 degrees inclination to the star's plane making seasons possible and more 'extreme' than earth's. It would be one of the many moons the gas giant would have thanks to it's mass, which I figured would be about 10 or so plus a hundred of minor ones. The moon itself would have a thick breathable atmosphere rich in CO2 and screening gases such as ozone and would feature a magnetic field capable of shielding the moon from the gas giant's and the star's radiation, thanks to the higher percentage of iron in it's core and mantle and thanks to the graviational interference of the other moons and the gas giant.

Will my moon be habitable?

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    $\begingroup$ Welcome to WB.SE. Just a warning, the community here tends to dislike 'What do you think?' type of questions. Could you reword it to reflect your specific problem? For example, 'Will my moon be habitable?' $\endgroup$
    – Otkin
    Aug 13, 2021 at 17:05
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    $\begingroup$ Do you want microbial life, macroscopic life, or just a moon that can be colonized? Macroscopic life on Earth did not evolve until it was already 3 billion years old; so, evolving anything interesting by 2 billion years might be unrealistic (though it is hard to say with only 1 data point). $\endgroup$
    – Nosajimiki
    Aug 13, 2021 at 17:26
  • $\begingroup$ Nosajimiki it depends on the generation time required for intelligent life. The assumption is about 4 billion years based on anthropic principles but that is as you say the only data we have. Perhaps evolutionary pressures result in the evolution of intelligence more quickly than that. I think the greater leap is probably multicellularity. $\endgroup$
    – KodiakMFL
    Aug 13, 2021 at 19:42
  • $\begingroup$ Nosajimiki, I'd want to go for humanoid intelligent life and a biosphere akin to earth's. Full disclosure, the planet's abiogenesis and evolution was 'aided' making the multicellular leap that as I understand is the most difficult one easier and I did figure that the evolutionary pressures the planet would be subjected to would favor adaptability and intelligence. Finally I guess that I could increase the star's age to 3 billion years, but wouldn't that make the star a bit too old? $\endgroup$ Aug 14, 2021 at 10:55
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    $\begingroup$ Keep in mind that your question will get more traffic and helpful input if you don't accept an answer so soon. P.S. Also I hope you consider fun implications like eclipse season! $\endgroup$
    – BoomChuck
    Aug 16, 2021 at 2:18

1 Answer 1


Hi there and welcome to Worldbuilding! I hope this leads you in a good direction and doesn't sound too critical - your moon seems like quite a fun place to go. And these are all napkin calculations, so take them with a 1 AU-wide grain of salt!

According to this Wikipedia article, the star's luminosity 2.86 times our sun's (=1.3^4), and according to this one, the habitable zone would be centered at 2.26 AU (=1.34 x square root of 2.86). So sitting at 2 AU is pretty similar to the earth, which is 0.34 AU inside our goldilocks center. But the huge gas giant would probably cause quite a bit of tidal heating on the moon, so you might want to move it somewhat farther from the star, perhaps closer to 2.26 AU or a little farther. That would probably mean dimmer sunlight than we're used to on earth.

As some have commented, the age is rather young for life to process that comfy atmosphere, but maybe factor it into your story - e.g. life was seeded or cultivated there, high adaptation pressure, etc.

The planet's large magnetic field is a double-edged sword. It will protect the moon from a ton of cosmic and solar radiation, but it will accelerate any sources of free particulate to create its own dangerous radiation. Jupiter accelerates Io's sulphur volcanism output into a belt of very destructive ionizing radiation. Basically the neighborhood must be very tidy. 10 major moon neighbors are each sources of radiation ammo, so maybe fewer or very far orbits. An easily pickpocketed thick atmosphere on a moon smaller than earth is more of a threat than a blessing. Instead, maybe protect it from the F-star's heavy UV radiation by moving farther out in the goldilocks zone and lean into tidal heating for warmth.

At 2 AU and 3 Jupiter masses, the planet is quite close to squeeze 4 planets next to a star even bigger than the sun. Its gravity would probably have an enormous affect on such close planets; note that Jupiter is at 5.2 AU and the closest thing to it is a crumbly asteroid belt centered at 2.7 AU. This is just guesswork, but I'd say move the planets beyond the gas giant. Maybe 1 interior planet and 1 belt? But who knows? Put a lampshade on it and it could work.

That orbital inclination is nuts. Most solar system planets are within about 2 degrees of the invariable plane, which is in fact set by the mass of our gas giants. See this table. Basically, where your gas giant goes, so do the other planets. So you're gonna need something quite significantly bigger than your gas giant elsewhere in the system to set the baseline differently, and some crazy impact or stellar flyby story to explain how a 3-Jupiter gas giant in the inner planets is more out of whack than Pluto.

The 36-hour orbital period might be cutting it very close to the gas giant. Just comparing Jupiter's major moons puts it relatively closer than Io, the closest, which is ravaged and torn by its proximity to Jupiter. But your moon is bigger, and it would take more math than I have time for to be sure.


  • $\begingroup$ Thank you a lot for this very well thought out response, though I also realized I made a mistake. The Gas giant's orbit isn't inclined...I was talking about the planet's axial tilt, I'll change it now and also mention that the moon has an orbital inclination of 10 degrees, and 30 degrees to the orbital plane of the star making seasons possible. How does this correction affect your assessment? $\endgroup$ Aug 14, 2021 at 10:01
  • $\begingroup$ And on a side note, one of the reasons why I am a bit adamant about the moon not having a magentic field is that I am concerned about how that might affect the development of technology on the world. I envisioned it to spawn a pretty advanced civilization all the way to space faring but if the gas giant's magentic field disrupts electronics that might be hard, what sorts of protections would you suggest if not a local magnetic field? $\endgroup$ Aug 14, 2021 at 10:13
  • $\begingroup$ You're very welcome! Unless the moon was captured like Triton, probably wrecking your other original moons, its axial and orbital tilts would likely closely match the planet axis as most solar system moons do. Google them and research for more info and ideas. So I'd just put all 30° into the planet tilt. I just googled more and learned tidal locking doesn't always kill magnetic fields so that might help some with the radiation problem. Magnetism will be nuts either way, but their technology certainly doesn't have to be electronic like ours. $\endgroup$
    – BoomChuck
    Aug 16, 2021 at 2:12
  • $\begingroup$ BoomChuck I'll make the necessary arrangements, however I should have mentioned that the moon was indeed captured but at a stage in which the other moons were still forming, while there was still a large debries field orbiting the planet. The idea was that a considerable metallic mass interfered absorbing a lot of material to form a considerably massive moon, without necessarily destroying the others. About the technology...I don't know, what other mediums are there that can be as good as electronics? Photonics pheraps? $\endgroup$ Aug 16, 2021 at 10:08
  • $\begingroup$ That sounds like a different question! Search through the worldbuilding stack exchange; I'm sure someone has asked something like that before. $\endgroup$
    – BoomChuck
    Aug 16, 2021 at 17:13

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