Yet another question that follows from “why do planets have moons?” is “why don moons have moons?
It all gets back to the fact that when gas clouds collapse in space they form central concentrations, which become stars, these stars are spinning and are surrounded by spinning gas disks; amazingly flat disks. This is a configuration of stability of all self-gravitating systems. Gas and particle lumps of higher density within disks can become, through accretion, solid bodies (planetesimal) which further accrete via gravity into planets with moons. So the Solar System formed in a two-step dance where first the Sun and surrounding disk formed and then the planets formed with surrounding disks and then these disks became the moons. In general it was a big house cleaning project with the dust bunnies clumping up to be planets and moons and the debris that was left over became the comets and asteroids. All this has to do with how gravity makes the shapes it does in the Universe. This pattern in the structure of the Universe of central objects orbited by other smaller particles in flat (dis
