Prior to stardate5730, this world was established as a vital transfer point for the regular space commercial lanes. The population was counted to be one billion. 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")