Ariannus was established as a vital transfer point for the regular space commercial lanes prior to stardate 5730. At that time, there were a billion people living on the planet. The nearest starbase was Starbase 4.
In 2268, Ariannus was infected by a bacterial invasion that threatened to destroy all life on the planet. The crew of the USS Enterprise, working in tandem with the planet's ministry of health, conducted an orbital decontamination mission to save the planet's inhabitants. (TOS: "Let That Be Your Last Battlefield")
In 2293, the location of Ariannus in the Milky Way Galaxy was labeled in the star chart "The Explored Galaxy", which was on display in James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)