Pluto

Pluto was a celestial body in the Sol system between Neptune and Cassius, which was at various points in history considered to be either a planet (TV: The Sun Makers) or a dwarf planet. (AUDIO: The Anachronauts) K9 called it the ninth planet of the solar system. (TV: The Sun Makers) It had a moon called Charon. (PROSE: Pluto, GodEngine)

Classification
While it was considered a planet in the 20th century, it was classified as a dwarf planet by Steven Taylor's time. (AUDIO: The Anachronauts) However, some still called it a planet in later eras. (TV: The Sun Makers)

Gillian Roberts apparently had no idea that Pluto was a planet. She thought that it was a cartoon dog. (PROSE: Time and Relative)

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 hosted the Lowell Depot, which was attached to the Sol Transit System, and over three cities. The settlements of Yamaha, Dentsu and Nagorno-Karabakh were Japanese colonies. (PROSE: Transit)

During the Dalek invasion of 2400, Jeff Stone went to Pluto where he discovered how to bend the Sun's rays which first blinded, then disarmed Dalek invaders. (PROSE: Break-through!)

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)

When asked about her holiday experiences, Donna Noble listed Pluto among the locations which she had visited. (COMIC: The Widow's Curse)

The Company
After the Earth's resources were exhausted, desperate humans made a deal with the Company to settle Mars. After Mars' resources were depleted, the Company moved the humans to Pluto, (TV: The Sun Makers) Eris, Haumea, and their sister planetoids. (PROSE: Weapons Grade Snake Oil) Pluto was thus terraformed with an atmosphere, and six giant cities called Megropolises were founded, each one lighted by its own artificial sun in the form of a in-station fusion satellite.

Megropolis One was the capitol city and main administrative centre, where representatives of the Company led by the Collector administered the operation through human underlings. (TV: The Sun Makers) They led from a massive ziggurat of glass. (PROSE: Weapons Grade Snake Oil) Other cities included Megropolis Three, which housed miners, and Megropolis Four. All Megropolises had extensive industrial capacity (TV: The Sun Makers) with factories the size of small cities. (PROSE: Weapons Grade Snake Oil)

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.

Pluto grew to house 300 million humans. (TV: The Sun Makers) Many of these were cymbionts linked to Company-owned machines and ruthlessly exploited for their perceived lesser status. (PROSE: Weapons Grade Snake Oil) Humans in the colony were made subservient through an air-based drug called PCM. They were also taxed extensively, with grave punishments dealt out for any evasion or inability to pay. Criminal offences were curbed through screening with an extensive surveillance system, and caught criminals were deported to an unfailingly-fatal detention centre in Megropolis One. A more severe form of punishment was public execution by steamer.

However, PCM was not distributed in the undercity, so a freer society of outlaws existed there. With the help of the Fourth Doctor, they managed to sabotage the PCM dispersion system in Megropolis One, leading to a successful uprising against the Company. (TV: The Sun Makers) Sojourner Hooper-Agogô emerged as a leader of this uprising, and, with the help of Mick Spaceman's bloody rebellion on Eris and her own army of fellow cymbionts, she drove up the Company's security costs to the point that the colony was declared unprofitable. In their absence, Hooper-Agogô became the first president of the newly-founded Plutonic Republic Of Technosapien Enhanced Cultures. (PROSE: Weapons Grade Snake Oil)

Behind the scenes

 * As a matter of fact, Pluto is also — in fact, one that K9 was almost named after before Disney refused to give permission.
 * Pluto is also noteworthy for recently being declassified as a planet. Although most stories since this reclassification occurred have attempted to stay true to this belief, some stories made before this occurred set in the future featured references to it as a planet.