19: Moons, Exomoons & Moonmoons

The Kepler mission found loads of exoplanets, and now astronomers are digging deeper into the data in the search for exo*moons* around those planets. All of which sends Chris and Emily on a deep dive into the detail of definitions: what's a moon, anyway? Do we know what a planet is? Or a star? Do astronomers actually understand anything at all?!