Arietus-1 lies between the twin stars of Arietus Beta and Arietus Charlie, which causes an erratic and unpredictable orbit, as it is juggled between the two stars. Its orbital position also results in a varying day-night cycle, ranging from constant daylight to near-total night - a fact that adds to the punishment of those imprisoned on its surface.

For Arietus-1's radiation blasted by intense solar radiation is home to 'The Hole', a large prison complex built into a crevasse that holds many maximum security prisoners, and those deemed to have a high escape risk. The desolate landscape and erratic solar cycle serve to discourage escape attempts.

A major shipping and trade hub for the nearby star systems, Arietus-2 is positioned far enough outside of the orbit of the triple suns that it has a stable environment and day/night cycle, with only minor changes in day length throughout the year.

The largest and oldest settlement on the planet, appropriately named First Landing, acts as the de facto capital and main trading post. The need to receive massive cargo landers, troop carriers, and merchant vessels resulted in a ring-shaped city surrounding massive landing fields. With such a high volume of traffic, First Landing always has a large population of visiting Astra Militarum and Navis Imperialis personnel, who are sometimes permitted a brief period of R&R before continuing to their destination.

This fragmented husk of a world is the remains of the original capital of the Arietus System. Approximately 3,000 years ago in a series of events that remain unclear to this day, the citizens of the planet rose up against the Lord Governor, executing him and his council. The rebellion quickly seized total control of the planet from the PDF, and commandeered its significant manufacturing capacity. In light of the speed of the takeover, and the risk of further rebellions and heresy spreading throughout the system, an unknown Inquisitor ordered that the planet be subjected to Exterminatus; the assembled warships in orbit bombarded the planet for several solar days, finally smashing the crust with a series of coordinated lance strikes.

Now all that remains is the shattered and burnt-out husk and a large debris field, the latter of which is an important source of materials for the rest of the system. There are rumours that some still call the airless world home, though these rumours so far remain unconfirmed...

A unique arrangement of planets, Arietus-4 is actually 2 planets orbiting a common barycentre, along with several moons (some say that the smaller planet is in fact a mere moon, a fact disputed by its inhabitants). Arietus-4A, the largest of the two bodies, is a small gas giant, whilst Arietus-4B is an important industrial world.

The major strategic value and production of the two planets is the drydock sitting in the L1 Lagrange Point between them. Fed by the resources mined in the remnants of Arietus-3 and brought in by interstellar shipping, these dockyards produce ships for both the Imperial Navy and Merchant Fleets. Arietus-4B houses most of the population of the twin planets, as well as vast manufacturing facilities to refine materials and produce components for the ships assembled in orbit.