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I was wondering about a setting where a moon orbiting a gas giant can sustain human life on its surface, i.e. with nearly identical conditions compared to Earth (e.g. size, gravity, mean temperature, oceans covering at least 70% of the planetary surface, same atmosphere composition, etc.).

The gas giant orbits a star in the habitable zone, so the moon is in the habitable zone too. Also, the moon would be most likely tidally locked to the gas giant, with an orbiting period of nearly 24h in order to have an Earth-like day/night cycle.

Due to this small orbiting period, I guess the moon would be very close to the gas giant, most likely the innermost natural satellite in this system. Additionally, I'm assuming a (nearly) 0° orbital inclination. Because of this, some conditions would be like those of Jupiter's moon Io:

  • Stronger tidal forces resulting in intense volcanic activity;
  • Stronger magnetic effects, maybe resulting in intense auroras around all the globe;

Additionally, the moon's side facing the gas giant would receive far less light from the sun because of a daily eclipse that would last at least several hours each time. How can this affect life forms development? Could the reflected light from the gas giant replace to some extent the direct sunlight, in order for photosynthesis to work?

Are there any other effects I should account for? If these effects can prevent an Earth-like environment capable of sustaining (human) life, what additional conditions must be met in order to counter them?

Should the gas giant itself have some particular features? I guess that it must be at least comparable to Jupiter in terms of both size and mass, in order to guarantee a stable orbit to its moons.

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  • $\begingroup$ I don't think a Jovian can exist within the habitable zone. The star would strip it apart into something far smaller. $\endgroup$ – Mormacil Apr 7 '17 at 16:24
  • $\begingroup$ Jovian worlds have massive magnetic fields, which protect them from the solar wind. $\endgroup$ – Joe Kissling Apr 7 '17 at 16:26
  • $\begingroup$ Once formed, but they can't form that close to the star. $\endgroup$ – Mormacil Apr 7 '17 at 16:54
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    $\begingroup$ @Mormacil Solar system formation is very dynamic, and we're fairly certain that planets don't always stay where they're formed. So it's entirely possible that the gas giant forms further out and migrates inwards. In fact, we've found many "hot Jupiters" - gas giants that are orbiting extremely close to their star. $\endgroup$ – Phiteros Apr 7 '17 at 19:49
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    $\begingroup$ @Mormacil Hot Jupiters can range up to 11 $M_{Jup}$, and can orbit as close as 0.05 AU. Most of them have strangely low densities, and there's a lot we don't understand about them, but they definitely exist. $\endgroup$ – Phiteros Apr 7 '17 at 20:36
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Orbit around a gas giant is dangerous due to the presence of radiation in belts around the planet. In one sense the plane's atmosphere is protected from the sun by the Magnetosphere of the gas giant, but it may not be protected from the intense radiation produced by it. The thick atmosphere of Earth will protect life from this radiation, but it may have an eroding effect on the atmosphere. The bigger issue is the interactions with the atmosphere that this radiation has such as destroying the ozone layer or the production of toxic compounds. These charged particles may be able to get through earth's magnetic field, some of the moons of Jupiter are bombarded with radiation even through their magnetic field.

It's even possible that the planet may lose its geodynamo which is an added layer of protection. Io doesn't have one for example.

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  • $\begingroup$ Tidally locked does not mean that the planet is no longer rotating. In fact, if the planet was not rotating, it would mean that it would not be tidally locked. Tidal locking means that the rotational period is equal to the orbital period. $\endgroup$ – Phiteros Apr 7 '17 at 19:46
  • $\begingroup$ Good point I'll readjust my answer shortly $\endgroup$ – Joe Kissling Apr 7 '17 at 19:51
  • $\begingroup$ «dangerous due to the presence of radiation in belts around the planet» that's how we see it. They might say that life on a primary planet is impossible because it relies on life-giving radiation belts. Remember, Oxygen is a toxic compound, and higly corrosive at that. $\endgroup$ – JDługosz Apr 8 '17 at 8:55
  • $\begingroup$ Life as we understand it doesn't tolerate radiation very well, that is to say carbon-based and DNA containing. That doesn't mean that is the be all end all for the form life may take, but chemistry favors carbon for biological process and there are only so may ways to arrange molecules. So it's likely (especially since in this case it's an earth like planet) that chemically speaking the life is going to be identical to what we have here on earth. Which means it's not going to like the radioactivity. Of course life will find a way, radiation just makes it much harder. $\endgroup$ – Joe Kissling Apr 8 '17 at 13:46

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