I've been trying to figure out a good modern model for procedural planet generation that takes into account the rare hot Jupiters and Neptunes as well as super Earths. I want to model the grand tack thing scientists think happened with Jupiter and Saturn, with the rest of the system literally being leftovers.

So, we seed the star system with an initial clump of accretion planetary disk material in two basic flavors; the rocky material, dust, and the hydrogen and helium gases which make up most of the mass of the universe but which is blasted out of the inner part of the planetary disk, within the so-called frost-line, somewhere roughly inside of the orbit of Jupiter in the solar system. It's important to decide if we have a uniform disk density by linear distance and orbit or by the surface area of the disk. A line model of a uniform mass per Astronomic Unit is what I'm going with, form an inner limit where the star's mass tears the clumping material apart and an outer limit. A clump gets most of the dust near its orbit, with the rest getting kicked out of the system or into the star itself, and the first clump outside of the frost line gets most of the gas.

We will set up these clumps based on the initial clump's orbit, moving in and out one step at a time, based on the multiplicity or number of planets. Initial orbit is some random position unless we already know there is a planet closer than Mercury, as we have previously established. Is there one or more planets? If so, is there a planet inside the orbit of Mercury at roughly 0.4 AU? Otherwise the initial clump, or the one outside of the orbit of Mercury, is at some random distance between the inner and outer limit. Then we randomly look for another orbit inside or outside of that, splicing them into a list until we're out of planets to place, so the second or third orbit will more likely be outside if there's a more room outside, and inside if there's more room inside. Once we have orbits. We work up mass by orbital widths and randomly tossing dust into the star or out of the system, followed by gas.

The lazy GM/World Builder in me just wants to splat something out, so N planets randomly dividing the system into N orbital bands with random capture of dust and gas within each orbital band and actual orbits halfway between the inner and outer edge of each band!