The Robots' planet was a planet about twice the size of Earth, the homeworld of a race of Robots encountered by the crew of the USS Enterprise in 2266. The planet was originally inhabited by a humanoid culture, who a million years earlier had built the Robots and then left them to fend for themselves. The Robots learned to maintain themselves, but after a millions years their power unit began to malfunction so they took remote control of the Enterprise and brought it into land on their planet, where they hoped to take the ship's engines apart and use the components to refuel their power supply.
After coming to understand the Robots' predicament, Captain Kirk offered to help them with their energy problems, but the Robots insisted on taking the required power rods by force, prompting Kirk to respond in kind and eventually leading to the destruction of the Robots' energy facility, causing the Robots to shut down through lack of power. (TOS comic: "Life Form Nonexistent")