A.K.A. - PRings, Prings or PRs
These are the backbone of a higher level player's base defenses, these are things that usually solely damage attacking fleet, where other ships and base defenses are there to distract attacking fleets. If a player has 3 or more levels of Planetary Rings, they are usually not attacked as most players know this will not be profitable. Never go past level 30 without these or you will regret that decision... You will be farmed easily.
SImilarly to Planetary Shields, Planetary Rings have a reduction to their cost when built on moons and asteroids. Some players like to use asteroids with mulitple levels of Planetary Rings, Planetary Shields, and Command Centers as 'fortresses' for the production of their fleets, since asteroids have high levels of metal for use in production.
Planetary Rings also take up no space on your base, since they are classed as an orbital facility.