The N body problem has no closed solution, and in many cases closely spaced bodies around a major body kick members out. In our solar system we have Titus Bode relationships that space orbiting bodies out in a geometric series. Various resonances are common. In Trapp1st 1, the planets are .011 .015 .021 .028 .037 .045 .06 AU, but most of them have earth like masses. With the difference between a and b being .004 AU, compared to the .3 AU difference for Earth/Venus, the interplanetary forces will be (.3/.004)^2 = 5700 times as great. I wasted an afternoon one day playing with an orbital simulator trying to find out if all those sci-fi novels with multiple moons showing disks were possible. I was unable to find any. Anyone on this list have access to the equivalent of the Digital Orrery who can plug in the numbers for Trappist -1 and see if it's stable? Secondly: Any generalization about under what conditions large moons of a planet are stable in numbers larger than 1?