When planets have orbiting bodies, we call them moons. But what do we call things that orbit those moons?
Astronomers Sean Raymond and Juna Kollmeier thinks that they should be called "moonmoons." And hey, theoretically, that can happen:
Their analysis suggests that moonmoons are possible, under the right circumstances—if, for instance, the large moon is quite large, the small moon is quite small, and both are sufficiently far away from the host planet. Moons that are too close to their planet risk losing their submoons to tidal forces from that planet, resulting in the submoon being shredded up, shot out into space, or sent careening off course and potentially crashing into their moon and its planet.
Image: Moon! by Nathan Davis