Ultima Thule was an inhabited planet. This planet was the location of a dolamide purification plant. Ultima Thule was the last stop on the route taken by Valerian traders supplying weapons-grade dolamide to the Cardassians during the Occupation of Bajor.
The Valerian transport Sherval Das visited the Ultima Thule station in 2369 a week prior to docking at Deep Space 9, suggesting that they were carrying dolamide. (DS9: "Dramatis Personae")
Ultima Thule means "the extremest north" in Latin. According to the script, the pronunciation for Ultima Thule was "ULL-tih-muh THOOL".