Horizon was the Federation code-name for the planet Silmareno, "on the edge of the spiral rim" and the only habitable planet in Zone Nine. It was visited annually by a Federation freighter to take back monopasium-239 mined by the natives. Nominally ruled by Ro, a native trained at the Central Educational Complex, the Federation's Colonial Service kept its own representatives and a small garrison present at all times. The planet was protected by a magnetic barrier, and only Liberator's force wall prevented the ship from being atomised.
Once the Federation personnel had been eliminated, Ro installed himself as the true ruler. The name Horizon was, however, retained. (TV: Horizon)