Planet under the Earth Empire's control. Its atmosphere was unbreathable by humans, because nitrogen isotope in its soil combined with ultra-violet rays to generate a poisonous mist. Solos was, however, rich in thaesium, a mineral coveted by the Empire. With Prof. Jaeger's help, the sadistic Marshal tried to alter its atmosphere. He also took pleasure in hunting and killing Solonian Mutants. From the Time Lords' records, and Prof. Sondergaard's research, the Third Doctor found out that the mutations were in fact caused by Solos' five-hundred year-long seasons. (The planet took two thousand years to orbit around its sun.) (NNN).