Solos was a planet, most likely in the Nebula of Cyclops. (TV: The Brain of Morbius) Solos was similar in size to Earth, but it took 2000 years to orbit its sun. Though it wasn't tilted on its axis, Solos did experience seasons, since it moved closer to and farther from its sun. These seasons lasted around 500 years and the temperatures could become very extreme.
Solos was covered in barren heaths and caves. There was also an unusual nitrogen isotope found on the planet which turned into a gas when it reacted with the sun's UV rays. The native Solonians were immune to the gas, but it could kill a human in an hour and could also be dangerous for Gallifreyans. Humans were only able to survive the gas by means of an oxymask.
Solos was also rich in thaesium, which was why the humans colonised it. During its colonisation, it had an Earth Empire Skybase in orbit, called Skybase One. The skybase was used as a platform to manipulate the atmosphere into one suitable for humans; as a side-effect of this process, nocturnal firestorms began occurring in the sky. (TV: The Mutants)