I'm thinking of an idea for a Red Dwarf star to have one (perhaps even two, but that would obviously be very unstable) Jovian Gas Giants with Mars- to Earth-sized moons in the habitable zone. The idea is that the Gas Giant's strong magnetic field protects the atmospheres of those moons from solar flares and thus keeps them habitable. Those moons primarily orbiting the planet instead of the star itself would also help give them somewhat of a day-night-cycle.

TL;DR: Can a tidally locked Gas Giant planet (I'm assuming the Gas Giant would be tidally locked to the star) still have many moons like Jupiter or Saturn, or would the tidal locking of the planet somehow prevent that?

  • $\begingroup$ Please read tag description and follow what it says $\endgroup$
    – L.Dutch
    Oct 2, 2023 at 18:48
  • $\begingroup$ Not sure if that is worth an answer, but your setup may be doomed even before you ponder the placement of the moons; a red dwarfs habitable zone is quite small, and putting two gas giants inside that would most likely mean, that their gravitational influence on EACH OTHER is considerable enough to disrupt their orbits each time they have a conjunction at any place. Even more so, if you stick to the first answer here; brown dwarfs and M class stars are in the same mass ballpark, so calling this system "unstable" would be... unfitting. $\endgroup$ Nov 28, 2023 at 6:13

2 Answers 2


Use a brown dwarf, not a gas giant

The magnetic moments of dipoles (reasonable approximations of planetary magnetic fields) fall off with the cube of distance. The strongest known planetary magnetic field, as far as I'm aware, is that of Jupiter: a magnetic moment roughly 20,000 times more powerful than that of the Earth's (control-F "Jupiter’s dynamo").

At the radius of Jupiter, that falls off to roughly 4.71 gauss (bottom of page 5), which is Jupiter's magnetic field strength at its equator. The weakest the Earth's magnetic field gets at its surface is approximately 0.25 gauss. ((4.71-0.3)^(1/3)) ≈ 1.646, meaning that the distance at which Jupiter's magnetic field = 0.3 gauss is 1.646 Jupiter radii away from its equatorial surface. 1.646 Jupiter radii away from Jupiter's equatorial surface is 2.646 Jupiter radii away from its core, or about 190,000 kilometers away from Jupiter's core.

As 190,000 kilometers is well inside the orbital radius of Io, which is slightly larger than the Earth's moon and yet still rendered incredibly volcanically unstable by tidal forces, there's no way an Earth-sized planet is going to survive here — 190,000 km is likely inside the Roche limit for a body that size. And even if it somehow did, the radiation belts would likely render it uninhabitable.

Thing is, though, Jupiter is the planet with the strongest magnetic field discovered so far. Certain brown dwarfs — weird sort of hybrids between ultra-large gas giants and ultra-tiny stars — likely have enormous magnetic fields, on the order of 1,000 times more than that of Jupiter. Given that magnetic moments decrease with the cube of distance, that represents a magnetic field which is ~0.25 gauss at approximately 10x the distance at which Jupiter's is ~0.25 gauss — or, in other words, ~1.9 million kilometers, greater than even Callisto's orbital distance.

Back-of-the-napkin math indicates that:

  • the ratio of mass between Jupiter and Io is comparable to the ratio of mass between a particularly large 70-Jupiter mass brown dwarf and an Earth-sized planet
  • the ratio of density between Io and Jupiter is greater than the ratio of density between a brown dwarf and an planet with Earth's density

...both of which put together mean the Roche limit for an Earth orbiting a brown dwarf is proportionately less than the Roche limit for an Io orbiting a Jupiter, which means that this planet is less likely to be subject to extreme tidal forces.

In other words, you don't want a gas giant. You want a brown dwarf with an ultra-strong magnetic field and Earth-like planets orbiting it at a radius of over 1% of an AU, where that magnetic field is relatively weak.

Note that the largest brown dwarfs likely glow to a small extent in visible wavelengths (after all, there is fusion going on, just deuterium and lithium fusion rather than hydrogen fusion) and therefore that anyone on these planets will see that, as well as the fact that, within the skies of these planets, it will likely appear comparable to or slightly larger than our Moon.

  1. The gas giant may not be tidally locked.

This is difficult to model, because tidal effects on big blobs of gas and liquid and other exotic things are hard to compute at the best of times. Using a fairly simply approximation for tidal locking timescales from wikipedia using Jupiter as a model gas giant, some estimations for the magic $Q$ and $k_2$ parameters from some random papers that I found on the internet (and eventually decided on values of and 5x105 and .54 respectively) a sensible parent star mass (say, .6 solar masses, like Lacaille 8760) and not-totally-implausible semimajor axis (.268 AU, a sensible distance for Lacaille 8760) delivers timescales on the order of trillions of years. Even if the error bars are very large indeed (and they're likely to be; this is a difficult problem with many hard-to-estimate parameters) you can see that there's a good chance that a Jupiter-sized planet might remain tidally-unlocked for enough billions of years for life to have a chance of evolving on its moons whilst it spins.

(also, if the moon were sufficiently massive relative to the gas giant, it can help the primary avoid tidal locking too, by handing over some of its own angular momentum, though that's tricky for a Jupiter-scale planet with Earth-scale moons).

  1. A tidally locked gas giant might not have any moons anyway.

Tidal deceleration occurs when something orbits faster than the thing it is orbiting rotates. This is the effect that will eventually cause Phobos to convert itself into a ring system and ultimately crash into Mars. A big enough moon to have a habitable atmosphere is definitely at risk of this effect if its parent planet were tidally locked, and whilst I'm not going to try and calculate how quickly it would meet its demise, I suspect it may be too quick for complex life to evolve in an Earthlike way.

  1. The gas giant's magnetosphere may be a hazard in itself.

Jupiter has the biggest magnetosphere in the solar system after the Sun itself. It also has the biggest and most dangerous radiation belts.

A gas-giant moon, snuggled in close to its parent, will be tidally locked and therefore won't have a significant magnetosphere of its own and as such will be continually subjected to those radiation belts. This isn't good for the longevity of your atmosphere, and depending on the thickness of your atmosphere and the composition of the radiation belts, it might be unhealthy for things living on the planet, too (and your Jovian's local space is likely to be hostile to living and electronic spacefarers, if you have any of those in your setting). Things living in the sea are probably fine, and you'd probably get some cracking aurorae by way of compensation.


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