Zom was a low-gravity planet. It was the homeworld of twenty million members of a once violent criminal race. Beyond the city of Zombus, the agricultural lands (used to grow Krakwheat) were mechanised. The rest of the planet was swamp. The Brains Trust were developed to find a solution to Zom's problems. They concluded the cause of all crime to be emotion. In the controlled environment of the City of Zombus, emotion was removed. This meant that the people of Zom were fitted with cranial sockets at birth, creating a direct link to the brain for the Harmoniser. In extreme cases a Ultra-Harmoniser was used for a full brain wipe. There was much resistance to this. Emotion was later restored following the Fourth Doctor's visit when the City was attacked by the Barabara. (COMIC: City of the Damned)
At some point in its history, Zom was placed under the protection of the Galactic Heritage. (PROSE: The Tomorrow Windows)