Kolossen is the fourth planet in the Edel system in the Rosette Nebula, administered as the United Provinces of Kolossen. It is a fairly standard hydrogen-helium gas giant with a potent gravity well and moderate radiation. Due to the radiation, its 11 moons have not seen a great deal of colonisation, though they contain military installations and research stations. It has a history of human settlement from the 2700s and boasts a population of 138.6 million people in a dozen twinned pairs of orbital habitats protected from lethal radiation by both powerful energy shields and hard radiation shielding. Its largest and most populous city is Wallerstein, a pair of habitats with a total population of 21.4 million people. The world capital has its seat at Rothstein-2, the surviving cylinder of the city of Rothstein; Rothstein-1 was destroyed during the Battle of Kolossen in 3285, and its population of 1.4 million people were killed.

Founded in 2705 as the Crown Colony of Kolossen, it began as a private venture between brothers Éamon, Michael, and Patrick McGrady, Isaac Rothschild, and Trey Waller. An initial settlement group of 21,000 people were intended to be housed in three pairs of orbital habitats. Cost overruns saw only two pairs constructed initially. Over the centuries, the population grew and more habitats were constructed to accommodate. It was incorporated in 3275 as the Colony of Kolossen in the new Republic. In 3279 it was admitted to the union as a member world.

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