Moonmoon:
Moons can come in all manner of configurations, too. Two of Saturn’s – Janus and Epimetheus – almost share an orbit. But it could get weirder than that. “In principle, you could have crazy things like rings of moons around planets, like Saturn’s rings but moons instead of tiny little particles,” says Sean Raymond at the University of Bordeaux in France. Along with Juna Kollmeier at Carnegie Observatories in California, Raymond has even postulated that, under the right conditions, moons could have their own moons. These are called moonmoons. [“Why we’re finally on the cusp of finding exomoons around other planets,” Jonathan O’Callaghan, NewScientist (2 Mar 2024, paywall)]
