Polymos was the home planet of the Nestene Consciousness, located at the edge of the Mutter's Spiral galaxy. (PROSE: Doctor Who and the Terror of the Autons)
History
Shub-Niggurath conquered the planet and colonised it with her offspring, the Nestene Consciousness. (PROSE: Millennial Rites)
The Nestene completely infused the planet, destroying all other life on it. It was far from a red star and had surprisingly little tectonic activity. Far in the future, it was invaded by the Time Lords attempting to stop the plastic asteroids from leaving. (PROSE: Synthespians™)
The planet was destroyed during the Last Great Time War by the Dalek's Time Destructor, the Eighth Doctor helpless to save it. (PROSE: Natural Regression, A Brief History of Time Lords) The Darksmith Collective salvaged plastics from Polymos' wreckage in the 28th century to build a robot called the Agent. (PROSE: The Dust of Ages)
Behind the scenes
A similar planet is mentioned briefly in WC: Revenge of the Nestene. However, it is there referred to as "Nestenia" and no indication is given that it and Polymos are the same world.