Third planet

The Planet of Evil, also called third planet, was an artificial planet used as a base by the Shadow in attempt to obtain the sixth segment of the Key to Time.

The Shadow positioned the third planet between Zeos and Atrios so neither could be seen from the other. It was destroyed by Romana I and the Fourth Doctor when the missiles fired by the Marshal of Atrios's Command Capsule were rerouted into it. (TV: The Armageddon Factor)