Polymos was the home planet of the Nestene Consciousness. It was located at the edge of the Mutter’s Spiral galaxy. (Doctor Who and the Terror of the Autons)
Shub-Niggurath conquered the planet and colonised it with her offspring, the Nestene Consciousness. (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. (Synthespians™™)
The planet was destroyed during the Last Great Time War by the Dalek’s Time Destructor, the Eighth Doctor helpless to save it. (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. (The Dust of Ages)
BEHIND THE SCENES
A similar planet is mentioned briefly in 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.