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Science-based answers, please.

Key points:

  1. the main concern is radiation protection for sustainability of human life (and accompanying plant and animal life ecosystems, whether earth-like or not).

  2. atmospheric retention is a minor concern (assume other factors have (mostly) taken care of the atmosphere, so it would only be a bonus if the parent planet's magnetosphere helps with this retention).

  3. assume that similar atmospheric protections as are present on Earth (ozone, etc.) are also present on the moon, but that nothing extra exists on the moon itself to provide any additional radiation protection, and the moon has no magnetosphere of its own.

  4. answers can assume a moon with any combination of factors within these ranges: mass ranging from .5 earth mass to 3 earth mass; diameter ranging from 3500 km to 14000 km; 75% earth gravity to 125% earth gravity

  5. answers can assume a parent planet of any size/composition/configuration that current scientific understanding deems could plausibly exist, and that any layman could reasonably accurately label as a 'gas giant', regardless of accepted scientific definitions and terminology (including brown dwarf, ice giant, etc.) but that a layman would not even accidentally believe is a star or rocky planet.

  6. answers can assume that radiation and stellar wind and any similar phenomena that are reaching the magnetosphere of the planet (and would otherwise reach the moon) from the star in this system is comparable to what Earths magnetosphere participates in defending humanity from, regardless of the actual size or type of star or the planet's or moon's distance from the star.

The research: The most relevant information I was able to find here are these questions which cover related and similar topics, but do not answer this specific question

Making a planet habitable for humanoids: The planet

Can a gas giant have its own habitable zone?

Habitable moon of a gas giant: working out the sizes and distances

The desired information:

  1. Could a moon within the diameter/mass/gravity/etc. ranges mentioned above orbit any plausible 'gas giant' planet close enough to the planet to be protected by the planet's magnetosphere from the radiation and similar hazards created by the star without other adverse affects (roche limit, etc.)?

  2. Is there any plausible composition of a celestial body that could be called a 'gas giant' and that could produce a large enough and strong enough magnetosphere to protect this orbiting moon, while not emitting or re-emitting so much radiation itself that it defeats this purpose?

  3. bonus points for details on distances between moon and planet, orbital times, and similar considerations

This is my first question on stackexchange, so please be gentle, but don't hesitate to provide constructive criticism if I'm doing anything wrong.

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    $\begingroup$ +1 for all the research you've clearly done! $\endgroup$ – Qami Sep 13 '18 at 21:06
  • $\begingroup$ The importance of the magnetosphere for radiation protection is vastly overstates. The principal defense against solar and cosmic radiation is the atmosphere; the magnetosphere is secondary, and, moreover, it does not help at all against UV light, X-rays, and gamma radiation. $\endgroup$ – AlexP Sep 13 '18 at 21:56
  • $\begingroup$ Thanks Alex. I know it is often overstated, and I had hoped that the details I included in my question, about the atmospheric defensive properties also being present would be enough to show that I was aware of it. Apparently I was mistaken in that hope. Having said that, the question still remains, can that protection, however significant or insignificant, be provided by the parent planet's magnetosphere in the absence of a magnetosphere generated by the moon itself, without other overly severe adverse effects? $\endgroup$ – Dalila Sep 13 '18 at 22:03
  • $\begingroup$ I don't have any orbital calculators on hand to check if this is feasible, but if the orbital period of the moon around the planet is the same as the orbital period of the planet around the sun, then the moon could live in the shadow of the planet's magnetosphere, thereby being protected by it all the same. $\endgroup$ – ColonelPanic Sep 14 '18 at 10:53
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Jupiter's magnetosphere encompasses all of its Galilean satellites

Jupiter's magnetosphere has a dipole moment 18,000 times greater than Earth's and encloses all four of its major moons. Callisto's orbital semi-major axis is 1.8 million km; so this gives a wide range of potential orbits for an Earth-sized planet. A planet larger than Jupiter, perhaps one with an even greater magnetic field, could have an even wider range of potential orbits for an Earth-sized planet.

Jupiter has a radiation problem

One of the downsides to Jupiter and its large magnetic field, though, is that it produces a lot of radiation. Io, the closest in moon, recieves a quite deadly 3600 rem per day; probably deadly within the hour. Callisto, though, farther away from the planet receives only 0.01 rem per day. This is still at least 10 times what you would expect to see on a sunny day on Earth, but certainly feasible for life adapted to it.

Jupiter's intense radiation has a lot of do with Io. Escaped voclanic particles, blasted from that moon's surface, form a plasma torus around Jupiter, accreting in belts inside the orbit of the major moons. Interaction of that plasma with the magnetic field is why Jupiter produces such intense radiation. An alternative way to remove the radiation on your planet from a large magnetic field is to not have any potential plasma sources like Io orbiting nearby.

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  • $\begingroup$ I knew Jupiter had a radiation issue, hence the second point in my 'desired information' section. But I didn't know about Io participating in the radiation issue. Thank you. I assumed, based on Jupiter and brown dwarfs, that radiation was a relatively common issue with the larger 'gas giant' type celestial bodies, which became increasingly more common with increasing size. I'll rethink that assumption and investigate that angle more. $\endgroup$ – Dalila Sep 14 '18 at 20:33

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