Gyre

The Gyre (official name Object 556/C) was a flat planet composed of space junk that met due to the combined gravity of nearby systems. It was located in the Battani 045 system. A human expedition crash-landed on the Gyre several thousand years before 250,339, forgetting their history and becoming a primitive society, believing that the Gyre was planet Earth. The planet was directly in the path of the comet Schuler-Khan, the collision of which would potentially cause dozens of nearby systems to be wiped out, including the home world of the Sittuun. The Eleventh Doctor and Amy Pond visited the planet and discovered the primitive human society and the danger of the comet, eventually aiding the Sittuun with their plan to destroy the Gyre in an effort to save the billions living in the nearby systems. (NSA: Night of the Humans)