Let's say that in my planetary system there is a gas giant around the size and distance from the Sun as Jupiter. I need the planet to be similar enough to earth to be able to be inhabited natural by humanoids. There are a few problems that I see with this below are the questions.

  • How do I keep the moon from becoming ice locked like Europa or Triton? I need it to be catch and keep enough heat to be habitable.
  • How do I protect the moon from both stellar radiation and the radiation of the gas giant?
  • How do I keep the orbit stable enough to avoid catastrophic failure?
  • How would the tides be affected by the gas giant (Assuming that they would be in the first place)?
  • How do I keep the temperature semi-regular to allow life to evolve?

The closer to Earth you can make the moon the better.

This question differs from: Habitable moon of a gas giant: working out the sizes and distances in that I am asking about atmosphere, radiation, stability, tides and tempurature not distance or size

  • $\begingroup$ Why not just have the moon as a captured planet? $\endgroup$ Commented Oct 15, 2015 at 20:37
  • $\begingroup$ Is that possible? $\endgroup$
    – TrEs-2b
    Commented Oct 15, 2015 at 20:47
  • $\begingroup$ Yeah, another solar system passed by too close, the resulting astrodynamics left a small planet in a weird position. $\endgroup$ Commented Oct 15, 2015 at 20:48
  • $\begingroup$ Thats a good theory how about turning it into an answer $\endgroup$
    – TrEs-2b
    Commented Oct 15, 2015 at 20:50

3 Answers 3


Since you want this planet to be more or less Earth in orbit around a gas giant, let's do that, as a gedankenexperiment, and see what shakes out.

  • Currently, Earth is in the "Goldilocks Zone" of radiant energy from its star. too close and it would be too hot; too far, too cold ("too hot" and "too cold" generally having the bounds of the temperatures of liquid water). Mars is nominally inside the butter zone too, but too small to keep the necessary insulating atmosphere (that requires a planet between about 0.75 and 1.8 Earth masses); Venus is both a bit close and too insulating with the methane-heavy atmosphere.

    Jupiter is too far out, on the opposite side of the asteroid belt, but if our star were a bit larger and hotter, that would compensate. Additionally, Jupiter re-radiates quite a bit of the energy it absorbs from the Sun, which helps to warm its moons (not nearly enough, but it could help in a differently-arranged system elsewhere in the galaxy).

  • Earth is protected from most of the cosmic radiation our star emits in two ways. First, its magnetic field directs charged particles in the outer atmosphere toward the poles. The aurora borealis is the result of the Sun's emitted radiation collecting at the poles and its energy levels being moderated by the atmosphere to non-ionizing levels, producing visible light photons in the process.

    Second, the atmosphere itself and the ozone layer in the upper stratosphere moderate EMR in the lower ionizing levels of the near-ultraviolet spectrum, in a process that also makes the sky blue (known as Rayleigh scattering).

  • Every orbiting body of the Solar system by this point is pretty stable, including all of Jupiter's moons. There are some notable exceptions, a few comets and asteroids on highly elliptical orbits that haven't - yet - crossed paths with a planet, but the majority of the cataclysmic instabilities worked themselves out in the first few hundred million years of the Solar System's existence, and what's left is pure survivor bias. Any orbiting mass long-lived enough to produce a sentient race (or to be around long enough for humans to turn up) should be pretty safe.

  • The tides on our Earth-like moon of a gas giant would be significant but not destructive. The Earth-Moon mass ratio is about 6:1, and the orbital distance is around 450,000 km. The Jupiter-Earth mass difference is much larger, something like 315:1. However, Callisto, one of Jupiter's moons, orbits 1.8 million kilometers from Jupiter. Gravity, like most forces, follows the inverse-square law; all other things being equal, the magnitude of the force decreases on the square of the distance between particles on which the force is acting. The distance term, then, is critical to getting an Earth-like tidal balance using Jupiter instead of our Moon. An Earth-like planet orbiting at Callisto's distance might have even gentler tidal forces than Earth's.

  • Maintaining Earthlike ambient temperatures is a simple matter of making sure the orbiting body is close enough to the star to be warmed just the right amount to allow liquid surface water, then having an atmosphere with enough greenhouse gases to manage the temperature swing once you lose direct solar illumination of the surface. The big trick is going to be moving behind the gas giant. The most likely scenario is a highly off-eclipic orbit of our Earth-like moon, reducing the duration of time that the moon spends behind the giant relative to the sun. It would still get pretty chilly; for a few Earth weeks or months every few Earth months or years, depending on the exact orbital period and plane of the moon, you'd lose the sun's lumination completely and the world would experience a severe winter-like season. Native plant and animal life would have to evolve to survive these deep freeze cycles even if the moon were slightly warmer than Earth the rest of the time encouraging more tropic or desert-like life. Mechanisms like the weta's ability to be frozen solid and then revive when thawed might be widespread among native life forms. A natural antifreeze component in biological fluids of life forms on this planet, such as a naturally occurring alcohol, might be another coping strategy.

  • $\begingroup$ It would be interesting seeing when animals develop intelligence, how there approach to winter changes (even though you can keep your house warm, do you still like a winter's nap?) $\endgroup$ Commented Oct 16, 2015 at 0:08
  • $\begingroup$ It would. It's thought that true sentience as we know it developed in the homo genus prior to or coinciding with the last ice ages around 10,000BCE, so our race had time to develop to that tipping point during a period of relative stability of climate. These winter freezes in my answer, however, would be happening on a fairly regular cycle of maybe a few decades (depending on how off-plane the moon's orbit around the gas giant is), so whatever creature rose to sentience would require eons of evolution to get that far, and may not bother with any new strategy. $\endgroup$
    – KeithS
    Commented Oct 16, 2015 at 0:18
  • $\begingroup$ Even though you can keep your house lit, do you still like a darkness-period nap? @PyRulez $\endgroup$
    – Joe
    Commented Oct 16, 2015 at 3:15

The easiest way I can think to make a moon Earth like is to just drop a planet into the orbit of the gas giant.

Drop a planet? I hear you scoff; I know we're not gods, but we can be mathematicians and astrophysicists.

There's really two ways we can do this; we can move a planet from the local system into the orbit of the gas giant, or we can just allow a rogue planet to happen upon the solar system and get captured in the gas giants gravity well. Moving a planet takes a little more effort, the 'best' way I can see to do this is our system passes close to another star with enough gravitational force to change the normal orbit of the earth like planet. We can say for arguments sake that the gas giant was on the other side of the solar system for this and hence experienced a reduced effect.


How do I keep the moon from becoming ice locked like Europa or Triton? I need it to be catch and keep enough heat to be habitable.

Jupiter's (or another gas giant like it) is sufficiently large enough, for it's gravitational forces to cause pull and friction on the planet, thus heating it up. This is true for one of Jupiter's moons already, allowing for liquid water to flow.

How do I protect the moon from both Solar radiation and the Radiation of the gas giant?

You will need to terraform a new magnetosphere, which is what planet earth has to protect against radiation.

How do I keep the orbit stable enough to avoid catastrophic failure?

You're planet should live either in the "Goldie Locks" zone of it's parent star, or sufficiently close enough to the gas giant, that it stay in it's orbit permanently (which gives it the gravity pull it needs anyway)

How would the tides be affected by the gas giant(Assuming that they would be in the first place)?

The tides would behave as they do here on Earth, although, the tide might be much higher or lower.

How do I keep the temperature semi-regular to allow life to evolve? By maintaining it's orbit to either the parent star or the gas giant. That is where the warmth comes from.


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