Titan

Titan was a moon of Saturn, the sixth planet of the Sol system. Its (Galifreyan-style) coordinates were "quadrant 62 WHI 12 12 9990 EX 41." (TV: The Invisible Enemy)

Astronomical data
Titan was smaller than Earth, but had a much denser atmosphere. The surface temperature was 200 degrees below 0. The upper atmosphere was rich in tholins, which would rain down on the surface. The surfaced looked like the barren landscapes of Earth, but with different colours; the black lakes were actually made of liquid methane and ethane, while the red rocks were made from ice. There were icecanoes, which spewed out water and ice. It reminded Jamie McCrimmon of the Land of Fiction.

Though it was believed to be incapable of supporting anything more complex than bacteria, Titan actually had advanced life, such as the massive Titan-sharks that swam through the lakes of methane. (PROSE: The Wheel of Ice)

History
In the 21st century, there were a few human colonies harvesting methane on Titan. When the children of Mnemosyne were blamed for the damage caused by the Blue Dolls, some of the teenagers decided to revolt and escape to Titan. Once they got there, tempers flared and accidents happened. When Sam Laws was badly injured, they were forced to call for help and return to Mnemosyne. (PROSE: The Wheel of Ice)

In the 47th century, citizens of Saturn who reached 100 years old were forced to go to Titan, where they ended up doing menial jobs and remotely operating all the public services and amenities on Saturn. (PROSE: Saturn)

In the year 5000, the Swarm infected Titan Base and attempted to turn it into its Hive, but the Fourth Doctor destroyed it by igniting the fuel tanks. (TV: The Invisible Enemy)

Titan was the homeworld of notorious criminal Dirk Slipstream. (PROSE: Night of the Humans)