The Posadians were the inhabitants of the planet Posadise. Posadians were humanoid, with grey skin, antennae/horns on their heads, flat faces, three digits on each hand and digitigrade legs. They were led by a First Minister and tended to wear green clothing.
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)