What is the difference between a moon and a planet? If you look at a picture of Venus and a picture of Jupiter’s moon Io next to each other, they look pretty much indistinguishable. Rocky composition, general size, orbital behaviors—a lot of those qualities can be the same on both planets and moons. For instance, our little rocky Earth is a lot more like the Moon than it is, say, Saturn or Uranus. Watch Pop Mech’s Jackie Appel discuss these definitions in more detail, nail down what a planet really is, and decide if there’s a fundamental difference between planets and moons at all.