The Planet of Light was a planet inhabited by a humanoid race. It had three suns which provided perpetual light. However, once every three hundred years, all three suns would be eclipsed, resulting in a brief period of darkness in which the inhabitants would go insane with fear and destroy their civilisation.
The Second Doctor, Polly and Ben visited the Planet of Light shortly before an eclipse. At an observatory, they met Igor, who had learned the cause of the madness which caused the people to destroy themselves. Since the people would not believe his warnings, he had prepared for this by placing their children in a deep sleep underground. He asked the Doctor to return one day and teach the survivors what had happened and how to prepare for it. From orbit, the Doctor and his companions watched as the planet burned, determined that it would be the last time it would do so. (COMIC: World Without Night)