Pluto

Pluto was a planet in the Sol system between Neptune and Cassius. It had a moon called Charon.

History
In 2050, Professor Alistair Gryffen revealed that Pluto was home to voracious, omnivorous microbes that devoured anything in their path, the worst bacteria in all the cosmos. They had been spliced with ordinary yeast cells to create a monster. (TV: Black Hunger)

Pluto supported a 30,000 year old civilisation of crystalline humanoids until they were wiped out by a flood of ionising radiation in 2157. (PROSE: GodEngine). In the 22nd century it also hosted the Lowell Depot, which was attached to the Sol Transit System. (PROSE: Transit)

After the Earth's resources were exhausted, the humans, in dire need of help, made a deal with the Company to settle Mars. After that planet was depleted, the Usurians moved the humans to Pluto. To this end the planet was terraformed with an atmosphere, and a city which the Doctor found quite impressive, Megropolis One. Later five more Megropolises were constructed. Each Megropolis was lighted by one of six artificial suns, in the form of fusion satellites. At the end, the planet housed 300 million humans.

Megropolis One was the main administrative centre, where the Collector administered the operation through human underlings from his palace. Other cities were Megropolis Three, which housed miners, and Megropolis Four. All Megropolises had extensive industrial capacity.

As a side-effect of the perpetual day caused by the artificial suns, many inhabitants were afraid of the dark. The monetary unit was the talmar.

Humans in the colony were ruthlessly exploited. They were drugged through the air with a substance called PCM, making them subservient. Furthermore they were taxed extensively, with tax evasion or simply being unable to pay the high fees a grave crime. Criminal offences were curbed through screening with an extensive surveillance system, and caught criminals were deported to a detention centre in Megropolis One, where they usually did not survive very long. A more severe form of punishment was execution by steamer, often public.

A freer society of outlaws existed in the undercity though, where PCM was not distributed and people were able to think more freely. They eventually, with the help of the Fourth Doctor, managed to sabotage the PCM dispersion system in Megracity one, which led to a successful uprising against the company. Once the company retreated, declaring their branch to be no longer profitable, the humans intended to re-settle the Earth, which the Doctor thought to have been regenerated. (TV: The Sun Makers)

The First Doctor moved The Cold to the far-future of Pluto. (PROSE: Time and Relative)

On Charon, a living shadow was captured by the combined telekinesis of a septet of extraterrestrial beings who had been placed in stasis within the shadow's lair. A scientific expedition to the planet disturbed this and ruined the whole system with the overzealous Professor Magellan murdering all the aliens when they took one of his men to replace their own and keep the shadow under mental lock and key. Luckily the Doctor and the team combined their will power to purge the shadow of all aggression and hatred, neutralising its threat forever. (PROSE: Pluto)

Behind the scenes

 * At the time that The Sun Makers was produced in the late 1970s, Pluto was officially classified as the solar system's ninth planet. Its subsequent demotion in 2006 is one of the most controversial decisions in modern astronomical history. Barring any on-screen reference, it's also possible to conjecture that in the Whoniverse the demotion never occurred, or was later rescinded at some point before the year 5000, where K9 (who references Pluto as a planet in The Sun Makers) originates.