A heavily populated planet and vital transfer point on Federation shipping lanes, which the Enterprise is sent to decontaminate when a bacterial invasion threatens its people in Let That Be Your Last Battlefield.
Ariannus is an inhabited world that serves as an important transfer point on the commercial space lanes. In the Original Series episode Let That Be Your Last Battlefield, a bacterial invasion threatens to wipe out its population, and the Enterprise is ordered there on an urgent decontamination mission.
The errand is interrupted twice. First the ship intercepts a shuttlecraft stolen from Starbase 4 and carrying the fugitive Lokai, and then his pursuer Bele seizes control of the helm to force a course for Cheron. Kirk wins the ship back only by starting the self-destruct sequence.
Bele allows the mission to go ahead, and the Enterprise sprays the planet from orbit, ending the danger before the quarrel between the two men of Cheron resumes.