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Is it plausible for a mini-Neptune to form around another gas giant? How big would the main gas giant be to accomodate such a moon?

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    $\begingroup$ Remember to show your prior research when posting. Related blog: How Big is the Biggest Possible Planet?. $\endgroup$ Commented Jan 24 at 1:39
  • $\begingroup$ if a star is orbiting another star... do you call it a planet or a binary sys? $\endgroup$
    – user6760
    Commented Jan 24 at 2:09
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    $\begingroup$ @user6760 Ganymede is the size of a planet, we don't call Jupiter a Binary do we? $\endgroup$ Commented Jan 24 at 2:19
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    $\begingroup$ @asteroidbelt: based on the definition of a planet then Ganymede is not a planet also see this ext link: Could a gas moon exist? ;D $\endgroup$
    – user6760
    Commented Jan 24 at 2:32
  • $\begingroup$ The Roche limit is an important consideration to these planets, particularly if these two bodies formed nearby at the same time. Also, two bodies sharing an atmosphere both tend to slow and get dragged inward. $\endgroup$ Commented Jan 24 at 3:58

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This is an interesting question. I'd argue that the answer is only a maybe through standard moon formation channels, but a clear yes if you accept more exotic pathways.

The giant planets in the Solar System have moons which, relative to their parent bodies, are much lower mass relative to the moons of terrestrial planets and bodies like Pluto. For instance, the ratio between the mass of the Moon and the mass of Earth is $f\sim0.012$, whereas the ratio between the mass of Jupiter's largest moon, Ganymede, and Jupiter is $f\sim7.8\times10^{-5}$. It turns out that the sum of the masses of the satellites of each giant planet divided by that planet's mass end up all being pretty close to $f\sim10^{-4}$.

What's up with that? Well, it's been suggested that this isn't a coincidence; rather, this could be a natural consequence of the accretion processes that form gas giant moons (Canup & Ward 2006). If this is true, then for an exoplanet to have a moon with a typical mini-Neptune mass -- say, about 10 Earth masses -- the exoplanet would need to have a mass of $10\times10^4=10^5$ Earth masses. This is roughly $0.3M_{\odot}$, the mass of a small star! The most massive exoplanets are around 10 times the mass of Jupiter, and so we'd expect the most massive moons to be roughly the mass of Earth.

That said, we have evidence to suggest that this may not be the case. Five years ago, there was a claimed detection of a possible exomoon, Kepler-1625b I. The moon was thought to be approximately the mass and size of Neptune, while its host exoplanet was likely several times the mass of Jupiter. Reexaminations of the data indicate the candidate should be treated with caution. However, other exomoon candidates of similar size have surfaced, including Kepler-1708b I, which would be quite close to the mini-Neptune range you're looking for. None of these candidates have been substantiated, and I know a lot of people who'd give a lot of money to confirm them one way or the other. But they're pretty tantalizing.

This raises questions. Maybe the accretion models that appear to yield a maximum limit on the masses of a giant planet's moons are invalid. More intriguingly, maybe high-mass moons can be acquired through other channels, accretion physics aside. For example, the moons of Mars, Deimos and Phobos, are thought to be asteroids captured by Mars long ago. It's quite conceivable that a very massive exoplanet could capture a mini-Neptune-sized neighbor in the turmoil that young planetary systems experience. Our own Solar System went through a period of chaos that rearranged the orbits of the outer giant planets.

I think the latter scenario is the most feasible one for you to invoke. Gravitational interactions are tricky and pretty perilous, but in the universe the size of ours, I'd be shocked if this didn't happen somewhere. Is it a likely outcome in any given planetary system? No. But is it possible? Definitely.

As for the mass of an exoplanet needed to harbor such a large moon -- I can't help you. I'd suspect that the more massive the better, since that would make gravitational capture more likely (and should make any moon systems it forms naturally more massive!). Opting for something like 10 Jupiter masses, near the upper end of the exoplanet mass spectrum, would be a smart idea. But I don't know enough to say anything definite.

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    $\begingroup$ Perhaps capture of a mini-Neptune rather than forming in situ is more reasonable? $\endgroup$
    – Jon Custer
    Commented Jan 24 at 2:44
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    $\begingroup$ @JonCuster Yeah, that's basically what my proposed second scenario is. $\endgroup$
    – HDE 226868
    Commented Jan 24 at 3:06
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    $\begingroup$ Formation process similar to Earth's moon? An oblique collision between two gas giants in similar orbits, followed by something Neptune-sized condensing around a super-Jupiter? $\endgroup$
    – nigel222
    Commented Jan 24 at 10:17
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    $\begingroup$ @nigel222 My concern with that scenario is that gas drag from their atmospheres might prevent the remnants from forming a proper moon-planet system, and we'd instead wind up with a single object. But I don't really know enough to say that with certainty. $\endgroup$
    – HDE 226868
    Commented Jan 24 at 15:51
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    $\begingroup$ Regardless of how it happens, I suspect that rather than it being a moon, you'd end up with a double planet. $\endgroup$ Commented Jan 24 at 20:10

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