How to Form a Moon Using Gravity

If a clump of dirt and rocks, when the planet is forming, it will clump together before it clumps with the Earth. And that's how you get like a protoplanetary disk. The planet forms and you have some stuff out there that hasn't fallen into the planet. So initially, it's like a disk and that can pull itself together using gravity into a ring. Sometimes it stays a ring and sometimes it forms a moon. That depends on how close you are to that planet. If you're really, really close to that planet, close to what we call the Rosh limit, then tidal forces from the planet are too strong. But if you're out past