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My story takes place on the moon of a large gas giant planet. I have been trying to decide on a way for the moon to be as Earth-like as possible, but I have now decided to ask the community for help, because I can't seem to figure it out.

It should be a very stable system that has been in place for billions of years, and should be comparable in age to the real-life Earth system.

The Earth-like moon should:

  1. Be able to support life
  2. Have Earth-like climates
  3. Have yearly change in seasons
  4. Have differences in temperature at different latitudes (ie. hot at equator, cold at the poles)

The list shown above is ordered from most important to least important. So being able to support life is the #1 most important detail. The first two are non-negotiable and should be included in any solution provided.

These are the two scenarios I have come up with, neither of which seem to satisfy my conditions:

Scenario 1

The Earth-like moon is in orbit around gas giant Planet A. The Sun is much hotter than our Sun, which allows Planet A to be further from the Sun and yet still inside the habitable zone. The change in seasons occurs due to the distance variation that results from not being directly orbiting the star. However, as a result the temperature changes at a global scale every year. I'm not sure how this would affect the climate and weather. The poles are roughly the same temperature as the equator.

Scenario 2

The Earth-like moon lies in the L1 Lagrangian point of gas giant Planet A. Change of seasons, temperature differences between latitudes, and climates are all Earth-like. As viewed from the Earth-like moon, Planet A has no phases and is always full, setting as the sun rises and rising as the sun sets. However, this formation is unfortunately not scientifically plausible, as the L1 Lagrangian point is very unstable.

How can I have an Earth-like moon orbiting a gas giant?

My goal is to make the moon as similar to Earth as possible within the realm scientific plausibility. I am open to any ideas the community has.

Answers should not say things like "that's not possible" without offering an alternative scenario. My goal is for the moon to be as Earth-like as possible; it doesn't have to be 100% identical to Earth in every possible way.

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  • $\begingroup$ Is there a reason you can't simply have a gas giant orbiting inside the star's habitable zone with an Earth-sized moon? $\endgroup$
    – Arkenstein XII
    Commented Oct 29, 2019 at 21:56
  • $\begingroup$ @ArkensteinXII Wouldn't the varying distances of the moon to the Sun cause problems? $\endgroup$
    – overlord
    Commented Oct 29, 2019 at 22:13
  • $\begingroup$ If the difference of distance to the Sun between the equator and the poles is enough for a large difference in temperature, wouldn't much larger distances cause problems due to huge differences in temperature? Or is the Earth hotter at the equator mostly due to angle of incidence of sunlight? $\endgroup$
    – overlord
    Commented Oct 29, 2019 at 22:16
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    $\begingroup$ While the difference in distance from the sun between the equator and the poles on Earth is no greater than about 6,370km, the radius of the orbit of your moon is probably going to be about 550,000km for a Jovian-sized host planet. I cannot discount the possibility that this would have a temperature effect, as that is about 0.35% of the distance between Earth an the Sun. I don't know the math required to determine it though. Perhaps someone who does will oblige. $\endgroup$
    – Arkenstein XII
    Commented Oct 29, 2019 at 22:56
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    $\begingroup$ For the gas giant, I am using Jupiter's mass exactly. The orbital radius of the moon should be a distance appropriate for an orbital period of almost exactly 360 Earth days. (I have done these calculations but I don't have them on hand) $\endgroup$
    – overlord
    Commented Oct 29, 2019 at 23:00

2 Answers 2

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If the radius of orbit around the gas giant is 550,000km there is no problem: the difference between Earth's aphelion and perihelion distances is 5 million km, and we do not experience problems - the seasonal effect on temperature owes more to orbital inclination than actual Earth-Sun distance. Also, the orbital period of an earthlike moon around a Jupiterlike gas giant with an orbital radius of 550,000km is 2.6 days, so much shorter than any relevant thermal time constants.

One of your comments stated you want the moon to orbit the gas giant with a period of about 360 Earth days. That requires a semi-major axis of 14.6 million km, which is 0.1 AU, or a third of the way between Earth and Venus. Unless you're planning a true 3-body system with no other significant bodies other than the star, the planet and the moon (which wouldn't really correspond with current planetary formation models), a moon at such a large distance from its parent planet would rapidly be perturbed out of orbit by interactions with other bodies and just become a separate planet.

For comparison, the orbital radius of Ganymede (the largest of Jupiter's moons) around Jupiter is 1.07 million km, which if it had 1 Earth mass would give an orbital period of roughly a week.

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I don't see a problem.

  1. The planet being habitable is a function of its distance from the sun, if it's in the Goldilocks zone and has liquid water, than seems likely that life will develop sooner or later.

  2. Earth like climate - if you have liquid water that's essentially Earth like climate for all intents and purposes

  3. Seasons - that's function of the planet's axis tilt.

  4. Same as #3

Distance from the sun and the type of star do not really matter that much as long as you are in the Goldilocks zone.

The gas giant can be anywhere in the solar system. We've detected hot Jupiters, gas giants very close to their star, as well as more "typical" ones similar to our own solar system. As a matter of fact, it's theorized that our own Jupiter might have been much closer to the Sun and migrated out over it's life.

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