First of all you should look at this: https://forum.playboundless.com/t/wiki-worlds/6488
The planets in the outer ring are the “moons” how people call them, which are medium hostile. The ones in the middle are the starting, earth-like planets which are quite peacefull (except from some annoying spitters) and the planets in the first ring are the so called ring planets which are the most hostile and connect all the other planets together in form of a ring, hence the name.
The Moons you pointed out currently doesn’t seem to have any purpose but to make the sky look more interesting but who knows what the devs have in plan.