Determining the orbital distance of planets from their star depends on a fairly intricate mechanism known as Bode’s Law. According to Bode’s Law, planetary orbits follow a recognizable mathematical pattern of development; this system replicates it.
Roll 1d6 to create a “seed” number. (The Sol system’s seed number
is 3.) Beginning with 0 and then the seed, run a series of doublings out for as many planets as your system has. (For the Sol system, that series is 0, 3, 6, 12, 24, 48, and so on.) Now roll the die again, and add that constant to the seed series. (The Sol Bode’s constant is 4, which gives 4, 7, 10, 16, 28, 52, and so on.) Now divide the new series by 10, and that’s your planetary
orbit pattern in AU. (Again for the Sol system, we get 0.4, Mercury; 0.7, Venus; 1, Earth; 1.6, Mars; 2.8, the
asteroid belt; 5.2, Jupiter, and so on.) Even the Sol system pattern breaks down with Neptune, so you can
vary the Bode’s result if you like.