The Posadians took advantage of the weather conditions of their planet to keep their cities clean. Sandstorms would sweep through their cities, picking up garbage and adding them to the rings of the system. After centuries of this, an alien species came to their planet. The Posadians were unable to identify their tools and didn't understand them and assumed they were invaders. They planned to attack them with missiles, but with the help of the Eleventh Doctor they found out that they only came to clean up the planet. (COMIC: A Mess of Trouble)