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https://academic.oup.com/mnrasl/article/483/1/L80/5195537
> For more massive stars, the habitable zone is more distant and habitable zone planets have wider Hill spheres. Large moons can therefore survive on more distant orbits and have larger Hill spheres such that submoons can exist on wider orbits where submoon–moon–planet tidal evolution is far slower, as compared with lower mass stars.
(Yes, the Moon could have had a submoon.)
> For the case of the exomoon candidate Kepler-1625b-I (Teachey & Kipping 2018) we showed in Fig. 2 that the largest stable submoon is Vesta- to Ceres-sized. The largest possible submoon would thus be roughly 5–10 km in radius. For much less massive, Solar system-like moons the largest stable submoon was ∼10 km such that the largest possible subsubmoon would be sub-km-sized.
Amazing! Add some trojan neighbors here and there and it would be a fun place to navigate around. Something like Interplanetary Transport Network, but crunched into a cisplanetary space.