Third planet

From Tardis Wiki, the free Doctor Who reference

The Third planet was a man-made planet used as a base by the Shadow in his mission to gain the sixth segment of the Key to Time.

He positioned it between the planets Zeos and Atrios so neither could be seen from the other.

It was destroyed when the Doctor rerouted missiles fired by the Marshal of Atrios's Command Capsule into it. (DW: The Armageddon Factor)