The Arhennius system was a solar system in or near Federation space, centered around the Arhennius star. The Arhennius system had no habitable planets.
The Arhennius system had been scanned by Federation sensors nearly a parsec away, and by passing ships, looking for lifesigns and habitable worlds, but there were neither; the system contained two gas giant planets roughly the size of Saturn and two small rocky planets slightly smaller than Venus without atmosphere. There was no record of any Federation, Romulan, or Klingon vessel ever entering the system. No large explosions had been recorded, but there was evidence of a half-dozen low-yield photon torpedoes detonating near the end of the 22nd century.
The innermost planet contained amounts of dilithium ore in quantities too small or deep to be detected at long range.
In an alternate timeline where Montgomery Scott rescued James T. Kirk before he could be absorbed into the Nexus, the USS Enterprise-D found evidence of a Klingon mining operation that had been abandoned at roughly the same time as the recorded torpedo detonation. The Klingons also mined the system with interphase cloaked torpedoes, one of which caught the Enterprise-D off guard. (Star Trek novel: Engines of Destiny)