The planet whose society is split between the cloud city of Stratos and the Troglyte miners working its zenite mines.
Ardana is a Federation member world and the only known source of zenite, a mineral used to fight botanical plagues. Its society is divided: the elite live in Stratos above the clouds, while the Troglytes mine zenite on the surface.
In The Cloud Minders, the Enterprise needs zenite to save Merak II. McCoy discovers that zenite gas in the mines reduces the miners' intelligence. Kirk forces a compromise in which the Troglytes receive filter masks.