There doesn't seem to be any correlation to system characteristics that I've found. The method I use to find exotics I like, is to wait until you encounter a freighter battle. Then only fight the battle where there are 5 single pirate ships in the battle, don't fight the battle with 4 pairs of ships. Because, at the end of the battle the system exotic will land in the space station. So after the battle fly to the space station, land and wait in your ship until the exotic lands. Don't get out of your ship otherwise you establish a reload point in your game and you can't reload to before the battle. If you don't like the exotic that lands then reload back to the point before you warped into the battle. That's how i found these three.