Jupiter is the largest planet in the Solar System and the location of an ongoing battle among four factions of Mages. It is also linked to the Shade Realm of Matter.
While Jupiter itself is a ball of gas with no surface, its many moons are more amenable to supporting visitors. Here, beyond the Horizon, mages can work extraordinary feats of magic without fear of Paradox. Each of the so-called Gallilean moons (those large and bright enough to be visible from Earth with a telescope) is, at present, controlled by a different faction of mages, though footing shifts quickly and frequently. Portals between any two moons of Jupiter or Saturn are easy to open, so heavy fortification is necessary to keep invaders out.
The Traditions, mostly Sons of Ether, maintain a foothold on Ganymede, while the Technocracy controls Europa. A group of Marauders controls Callisto, while Io (a world of sulfur volcanoes and constant earthquakes) became a base for the Nephandi after they were exiled across the Horizon. The many smaller moons change hands frequently as the four factions harass one another.