Planet (The Sour Note)

An Earth-like planet had three yellow suns and a green sky. At least part of its surface was mountainous. Its inhabitants may have lived underground.

When the Second Doctor arrived on the planet, Ben Jackson went out for a run, while the Doctor and Polly followed after him. They encountered a giant insectoid robot which appeared to be guarding an entrance to an underground settlement. Concerned that the robot might had swallowed Ben, the Doctor used his recorder to lure it to the TARDIS so he could examine it with his instruments. However, they found that Ben was actually inside the TARDIS, and they left the planet. (PROSE: The Sour Note)