Earlier this week, Nivarias revealed some new content that is bound for Stellaris soon. This week's dev diary explains it in detail, with Orbital Rings being one of the new updates.
Orbital Rings are a Tier 3 Voidcraft Engineering technology requiring Starholds, Galactic Administration, and Ceramo-Metal Infrastructure. Like Habitats, they do not require Mega-Engineering.
Orbital Rings are treated as a variant of Starbases, and while system control is still primarily determined by the actual Starbase of the system, the planets they surround cannot be invaded until the Orbital Ring has been disabled.
Initially, your Orbital Ring will have two module slots and no building slots. As you gain additional Starbase technologies, the Star Fortress and the Citadel, and improve the planet’s capital building you can upgrade the Orbital Ring through two additional tiers, adding one module and building slot at each tier.
Most of the Orbital Ring modules are similar to Starbase modules. Defensive modules trade piracy protection for extra hull and armor, and the Habitation Module is a Ring specific module that adds a district slot to the planet below.
Systems with multiple habitable planets can become an exceptionally thorny obstacle if you build multiple defensive orbital rings supporting a bastion starbase at the center. Having a large conveniently placed ring around your planet provides an opportunity to enhance the planet with some interesting buildings. These stack with similar planetside buildings.
Many standard starbase buildings can also be placed on an Orbital Ring - though some are now limited to one per system. Orbital Rings fill the same orbital slot as habitats, so you’ll have to decide which of the two you want over your worlds, and they can only be built around colonized habitable planets.