The periphery would would have to have something very important, Harjel, Germanium, A WOB or 1st SL Weapons Supply Cache/Factory/Depot, A Castle Brian, or Castle Port. The IS has people, the IS has common resources in vast numbers, without the cherry on top there would be no reason for a house or even a clan to annex a periphery world. The MOC, Taurians, OA, they my bring more planets into the fold, but it would either need to be a formerly abandoned world or have a cherry on top.