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.
The Valerian transport Sherval Das visited Ultima Thule 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 Star Trek: Star Charts, on page 46, Ultima Thule was located in the Ultima Thule system, in the Alpha Quadrant. This was a binary star system. Primary was a Class A star. Magnitude of this star was +3, which was 10 times brighter than Sol. Stellar companion was a Class F star.