Ceres

Ceres was a dwarf planet in the Sol system, located in the asteroid belt between Mars and Jupiter. (AUDIO: The Bounty of Ceres)

At some point, Ceres was home to the Cerulians until their civilisation was destroyed by an incoming asteroid. (TV: Sirens of Ceres)

Ceres was later visited by the First Doctor, Vicki and Steven. By this time, the dwarf planet was devoid of any native life forms. A Cobalt Corporation mining base was located there. (AUDIO: The Bounty of Ceres)

In Steven's time, the political situation on the dwarf planet was akin to that of East and West Berlin in the 20th century. Steven and his fellow students weren't allowed to talk about Ceres, so they instead alluded to it through the 5D representations of East Berlin they built in class. (AUDIO: The Anachronauts)

Behind the scenes
Although not mentioned, in Resolution it was implied the Thirteenth Doctor and her companions helped its discovery by Giuseppe Piazzi.