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Let’s say we have a Star system with a gas giant about 30 AU from the local sun. This is way too far for this particular sun to warm any moons to habitability (unless life inhabits subsurface oceans) but what if the planet could act as a sun?

We have a moon called Pontus III, about twice the mass of the Earth’s moon and rocky. It orbits even closer to the gas giant Pontus than Io does to Jupiter. Because it’s orbit is ever so eccentric, this places it under huge tidal stresses, which cause increased vulcanism and warms the planet sufficiently for liquid water to exist on the surface.

Could this arrangement work to make Pontus III habitable?

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  • $\begingroup$ When you say habitable... It's never going to hold onto an Earth-like atmosphere an human-comfortable temperatures, there's simply not enough gravity to keep an atmosphere that's breathable - could you clarify what you mean by habitable. $\endgroup$ Dec 28, 2022 at 10:13
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    $\begingroup$ ..... Unless it's artificial and made of Osmium or something sufficiently massive. $\endgroup$ Dec 28, 2022 at 15:33
  • $\begingroup$ I recall answering a very similar question on here previously. I think about a moon heating mars. Tidal heating isn't particularly useful because it is a low amount of energy per time, and because it heats the interior where the heat takes a very long time to get to the surface. $\endgroup$
    – BillOnne
    Dec 28, 2022 at 20:24
  • $\begingroup$ @BillOnne Do you mean this question? worldbuilding.stackexchange.com/questions/237167/… $\endgroup$
    – Alastor
    Dec 29, 2022 at 9:25
  • $\begingroup$ @ArktourosUltorMaximus7600 That's the one. $\endgroup$
    – BillOnne
    Dec 29, 2022 at 23:33

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Tidal heating can be large enough to make a moon of a gas giant habitable without the influence of the sun. The paper The effect of multiple heat sources on exomoon habitable zones by Vera Dobos, René Heller and Edwin L. Turner describes the regions where habitable moons can exist around gas giant planets. The image below shows the distance a moon can have from its gas giant and its sun. Both the green and orange striped regions illustrate areas where a planet has enough heat for liquid water to exist. The graphs only go out to 8 AU, however, tidal heating exists irrespective of the host star. Therefore, a moon huddled close to a gas giant can have water on its surface even with the host star being 30 AU away.

Moon Habitability around a Jupiter mass exoplanet orbiting a sunlike star

You are going to have more issues with the mass of your moon. A 2 lunar mass moon is not large enough to hold an atmosphere over geologic timescales. Generally speaking, planets/moons need to have masses of at least 0.1 earth masses in order to hold an atmosphere for millions to billions of years. Any object smaller will loose its atmosphere rapidly to Jean's escape.

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