Lakertya was the homeworld of the Lakertyans. It was located in the Lakertyan System, and the Sixth Doctor regarded its natives as peaceful. (AUDIO: The Brink of Death) Lakertya was in the same solar system as Carsus. (PROSE: Spiral Scratch)
The Sixth Doctor tried to visit Lakertya with Melanie Bush, but found a deadly source of radiation that kept him from going to the planet. He was sent a psychic signal by his future self to go back as part of a preemptive effort to defeat the Valeyard through self-sacrifice by encountering the radiation, a kind fatal to Time Lords but not humans.
En route to Lakertya, he was attacked by the Rani, whose weaponry used the offending radiation. Upon being exposed to it, the Doctor soon regenerated, and his TARDIS was forced to land on Lakertya after the Rani placed it in a tractor beam. (AUDIO: The Brink of Death, TV: Time and the Rani)
The planet was conquered by the Rani and her Tetraps from their planet Tetrapyriarbus. She built a base of operations in a ravine and she littered the area with traps. Fresh from his regeneration, the Seventh Doctor repelled the Tetraps, who turned on the Rani. (TV: Time and the Rani)