Solos

Solos is a planet, most likely in the Nebula of Cyclops. Solos is similar in size to Earth, but it takes 2,000 years to travel around its sun. Though it isn't tilted on its axis, Solos does experience seasons, since it moves closer and farther from the sun. These season last around 500 years and the temperatures can be become very extreme.

Solos was shown to be covered in barren heaths and caves. There is also a unusual nitrogen isotope found on the planet. This isotope turns into a gas when it reacts with the sun's UV rays. The native Solonians are immune to the gas, but it can kill a human in an hour and can also be dangerous for a Gallifreyans. Solos is also a source of Thesium, which is why the humans tried to colonize the planet. This Thesium is also used by the native Solonians for their transformation. (DW: The Mutants)