Some Moons Have Moons

Moons orbit planets, but could a moon have its own moon? Surprisingly, yes—at least for a while. Astronomers call these “submoons” or “moonmoons,” and while none have been confirmed in our solar system yet, simulations suggest they should be possible. A 2019 study found only four moons here that could hold on to one: Titan, Callisto, Iapetus and our own Moon. The problem? Tidal forces from the planet usually rip them away. But for a brief period, a moon could have a moon of its own—just don’t expect it to last forever.