In a recent newsletter, the team elaborates on the Stellar Forge system that it uses for most of the game's celestial bodies: "In Elite: Dangerous, when we are generating a system procedurally, each planetary system is formed from first principles. Bodies are gradually aggregated over a very long simulated time from available matter, taking into account its chemical composition. Depending on the angular momentum, this might begin to form into a single central body, or into multiple co-orbiting bodies."
This explanation goes on in detail -- great detail -- and if you haven't appreciated how planets are made before now, you certainly will by the time you finish reading it.
[Thanks to Cotic for the tip!]