This is a progression from my first question, here (if this is too similar to my original question, please let me know of a better way to address this, as I am not getting the answers I need on that first question to be able to answer this one, and they seem separate enough to me.)

As with my first question, I found many questions and answers on this site that have related information, but don't directly answer my specific question. And I seem to be finding incomplete, incorrect, or conflicting information. Most specifically, this was prompted by a comment about Io being a major factor in Jupiter's radiation belt, and I attempted to research along those lines, but failed to find the information I needed, hence this new question. See these:

Conditions for human life in a Jupiter-like system

Captured Earth-Like Moons around Gas Giants

Naturally making a gas giant moon habitable

Calculating Radiation Zones of Gas Giants and the Effects on Its Moons?

Some of my research seems indicate that gas giant radiation comes almost exclusively from the planet itself. Others indicate that it's mainly a function of material contributed by orbiting bodies like Io. And nowhere did I find any attempt to balance it like I want to. So here's the actual question:

Is there a distance between the planet and the moon where the moon is far enough from the planet that the radiation from the Planet will not be able to get to the moon's surface in any quantity that is detrimental to human life, while at the same time the moon is close enough to the Planet so that the planet's magnetosphere is still providing the same level of radiation protection from the star's radiation as Earth's magnetosphere provides to Earth?

In other words, is there a gap between the lethal levels of radiation from the star and the lethal levels of radiation from the planet, and could a moon orbit the planet in that gap?

If such a gap is possible, what 'type' of Gas Giant is needed to create it?


  1. Assume that the distance from the star is appropriate for life in this system, (in the Goldilocks zone, adjusted appropriately for a moon instead of a planet, etc.)
  2. Assume the life on the moon is Earth life, disregard how it got there

  3. Assume the moon has all necessary life supporting functions, including the same level of radiation defense as Earth's atmosphere, with the sole exception of it not having any magnetosphere of its own.

  4. Assume no other moons or rings orbiting the Gas Giant are contributing anything that could significantly affect radiation levels in the system, the only radiation considerations are the star and the gas giant itself.

  5. Disregard the reason for the missing magnetosphere on what is essentially Earth orbiting a gas giant in all other life support related aspects. though the moon does not need to be the same size, mass, density, etc., as Earth, if such adjustments are needed to allow a change in the orbital distance from the gas giant to put it in the gap in the radiation, and such changes can be 'hand-waved' to have no impact on the other life support functions of the planet.

  6. The Gas Giant can be 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.

Desired information:

  1. Is there any Gas Giant that could have such a small/weak radiation belt system that an Earth-like atmosphere could protect life from it, and a large/strong enough magnetosphere at the same time that the star's radiation would not strip away that atmosphere within the time frame of earth life evolution from single cell to human, to produce such a gap?

  2. Is the gap appropriately shaped (distortion due to stellar wind) that a moon could make a complete orbit of the planet without leaving the gap, or without leaving it for a long enough time to be detrimental to earth life on an evolutionary time scale?

  3. If the answer to the above is affirmative, what is the Gas Giant like (mass, volume, density, composition)? or is this something that most gas giant could have, and the ones without such a gap are the exceptions, and not the rule?

  • 5
    $\begingroup$ Earth's magnetosphere does not provide that much protection against "radiation"; moreover, there are places near the magnetic poles, for example the Labrador peninsula in Canada, where the magnetosphere protection against "radiation" is very very small, if any. ("Radiation" in scare quotes because the magnetosphere cannot protect against X-rays and gamma rays at all.) The overwhelming majority of radiation protection is due to the atmosphere. The importance of the magnetosphere consists in protecting the atmosphere against the solar wind. $\endgroup$
    – AlexP
    Sep 21, 2018 at 19:40
  • $\begingroup$ See, I've seen comments and answers explaining exactly the opposite, indicating that the planet's mass is what retains the atmosphere, and that the magnetosphere provides negligible protection from atmospheric stripping, but that the main function of the magnetosphere is radiation protection. And this type of conflicting information is exactly why I asked this question. So, in my scenario, the atmosphere on the moon will be comparable to Earth, the magnetosphere will not. Can earth life survive on this moon? or will radiation from the planet or star kill it? $\endgroup$
    – Harthag
    Sep 21, 2018 at 20:08
  • 3
    $\begingroup$ The planet's mass is what retains the atmosphere, true. But the solar wind will strip is away over millions of years, little by little; the magnetosphere works by redirecting the plasma of the solar wind, so that it doesn't hit the atmosphere head on. And there is zero doubt that the magnetosphere cannot protect at all against electromagnetic radiation; it's basic physics. It protects only against charged particles, such as the plasma which constitutes the solar wind. Where the magnetosphere does not protect against them you get polar auroras, but life on the surface is still safe. $\endgroup$
    – AlexP
    Sep 21, 2018 at 20:22
  • $\begingroup$ So the specific effect is different, but the end result remains, and so the question remains valid: Can earth life survive on this moon? or will radiation from the planet kill it outright, or will radiation from the star kill it "by stripping away the atmosphere"? question details (specifically "desired information" section) edited to reflect this $\endgroup$
    – Harthag
    Sep 21, 2018 at 20:35

2 Answers 2


First, it's important to discuss what radiation belts are and how they form. Radiation belts are formed by charged particles that are trapped by a planet's magnetic field and, due to the shape of that field and their own initial velocity, tend to collect in certain regions. The main source of charged particles in e.g. Earth's Van Allen belts is the solar wind: particles emitted from the Sun.

In the case of Jupiter, though, that's only part of the equation. Most of the material in Jupiter's radiation belts, especially the strong, close-in plasma torus, comes not from the Sun or Jupiter itself but from its moon Io - as much as a metric ton per second, mostly in the form of ionized gas ablated from its surface. Saturn has smaller, less charged plasma tori generated from some of its own moons. On the other hand, Neptune and Uranus have no analogous moons, and so their magnetospheres lack powerful internal radiation belts.

It seems reasonable to expect, therefore, that an otherwise Jupiter-like gas giant (or any other type of gas giant) that lacks an Io analogue will also lack the plasma torus that makes Jupiter so inhospitable. It may even be that the presence of an Earth-sized moon in the gas giant's system would make an Io-like orbit unstable, giving it added protection against the possibility of radiation belts.

Finally, don't discount the moon's own magnetosphere. Ganymede, another Jovian moon, has - uniquely of moons in the solar system - its own permanent magnetosphere that shelters it from the radiation environment it's in, in much the same way Earth's shelters it from the solar wind. A large body of Earthlike composition (differentiated, with a molten iron core) would absolutely have its own protection from stray solar particles or those ablated from other moons.

To sum it all up:

  1. Yes, it's possible to have a gas giant without a strong internal radiation belt that would pose a hazard to its moons, while still having a magnetosphere.

  2. Yes, it should be well within the gas giant's magnetosphere at all times, and it would also have its own magnetosphere to consider, which could offer additional protection.

  3. Potentially any. The key factor appears to be the presence of close-in moon(s), rather than anything intrinsic to the gas giant. Some types of gas giants might be more or less likely to have such moons, and the formation or capture of an Earth-sized moon will probably have some effect, but it may simply be down to luck.

  • $\begingroup$ An edit and an upvote! ;-) $\endgroup$
    – Fabby
    Sep 22, 2018 at 2:06
  • $\begingroup$ Note that the absence of an Io-like moon just means Earth-like radiation belts, not no radiation belts. Still not a healthy place to hang out, but you won't get fried the way you would at Jupiter. $\endgroup$
    – Mark
    Jul 26, 2019 at 2:35

So a potentially habitable exomoon of a giant exoplanet might be protected from the radiation zone by not orbiting with the radiation zone, which the giant exoplanet might not have, depending on various factors, and if the giant exoplanet does have a dangerous radiation zone that zone might not cover all the region where a habitable exommoon should orbit its planet, depending on various factors.

A habitable exomoon might also be protected from the dangerous radiation it is planet's radiation belt by the exomoon's own magnetosphere. So the exomoon might possibly orbit in the dangerous radation zone without any ill effects.

An exomoon with the right mass and composition that rotated at the right rate could have a very strong and powerful magnetosphere. That magnetosphere would be generated by the rotation of the exomoon's core, which would also be necessary to drive plate tectonics, which might also be necessary for the exomoon to be habitable.

There is an article:

"Exomoon Habitability Constrained by Illumination and Tidal heating" by Rene Heller and Roy Barnes, Astrobiology, January 2013.


In section 2, Habitability of Exomoons, they discuss the mass range necessary for hypothetical exomoons to be habitable in the sixth paragraph:

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms≳0.1M⊕; Tachinami et al., 2011); to sustain a substantial, long-lived atmosphere (Ms≳0.12M⊕; Williams et al., 1997; Kaltenegger, 2000); and to drive tectonic activity (Ms≳0.23M⊕; Williams et al., 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Gurnett et al., 1996; Kivelson et al., 1996), suggesting that satellite masses>0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy—such as radiogenic and tidal heating, and the effect of a moon's composition and structure—can alter the limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the planet's interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M⊕ (Gaidos et al., 2010; Noack and Breuer, 2011; Stamenković et al., 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al., 2012).

The upper limit of about 2 times the mass of Earth should hold for exoplanets as well as exomoons.

Heller and Barnes give the source for the importance of plate tectonics for habitability as:

Williams D.M. Kasting J.F. Wade R.A. Habitable moons around extrasolar giant planets. Nature. 1997;385:234–236. [PubMed] [Google Scholar]

Heller and Barnes give the sources for an upper mass limit at about 2 Earth masses as:

Gaidos E. Conrad C.P. Manga M. Hernlund J. Thermodynamics limits on magnetodynamos in rocky exoplanets. Astrophys J. 2010;718:596–609. [Google Scholar]

Noack L. Breuer D. Plate tectonics on Earth-like planets [EPSC-DPS2011-890]. EPSC-DPS Joint Meeting 2011, European Planetary Science Congress and Division for Planetary Sciences of the American Astronomical Society; 2011. [Google Scholar]

Stamenković V. Breuer D. Spohn T. Thermal and transport properties of mantle rock at high pressure: applications to super-Earths. Icarus. 2011;216:572–596. [Google Scholar]

It is possible that the importance of plate tectonics for habitability, and the upper mass limit of about two times the mass of Earth for plate tectonics, are not accepted by all scientists interested in astrobiology, but I have not researched that.

Anyway, it seems possible for exomoons to have the right mass, composition, rotation, and orbit to be safe from the effects of their planet's radiation zones, and also to have other necessary qualities for habitability.


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