Erratoon was a former prison planetoid that was forgotten about in the galactic recession. Because of this, the descendants of the prisoners still lived the same life the former prisoners did, carrying out the same tasks as their imprisoned forebears.
Eratoon was surrounded by a geodesic sphere, which stopped Harmonious 14 Zink from fixing his position in space, when he and Zara were dragged there via his Time Ring. During Zink's scan he noticed that the planetoid was densely packed with elesium ore.
A segment of the Key to Time was hidden as a lake on the surface, and Zara took it. As part of this incident, a rocket made by Seventh Doctor's companion Ace was launched by Zara, to break the sphere, hoping that the inhabitants would die, which they didn't. The hole that was created became a spaceport for Eratoon. (AUDIO: The Prisoner's Dilemma)