The Third Zone, also known as the Nine Planets, was an area of the Mutter's Spiral twelve days from Earth by hyperspace travel. Its inhabitants were known as Third Zoners.

In the Third Zone, Androgums were used for menial labour and the currency used was the narg. Although humans had not reached this part of the galaxy, they were known in the Third Zone as "Tellurians". The Third Zoners were familiar with transmat technology, and were adept at technologically augmenting other species, turning them into mega geniuses. Dastari sought to elevate the Androgums, as the civilisations of the Nine Planets were becoming older and effete. They also conducted experiments in time travel using the Kartz-Reimer module which measured 0.4 on the Bocca Scale, the Second Doctor explaining that anything higher could threaten the whole fabric of time. The Sixth Doctor even hypothesised that the Sontarans thought the Third Zoners were getting too powerful and would ally themselves with the Rutans.

Circa 1985, the several governments of the Third Zone pulled together to create Space Station Camera, where some of the greatest intellectuals in the universe gathered, including Kartz and Reimer. The station came under attack by the Sontarans who attempted to frame the Time Lords. (TV: The Two Doctors, PROSE: The Two Doctors)

Gustous R Thripsted authored the non-fictional Genetic Politics Beyond the Third Zone, which included entries on species not part of the Third Zone, including the Time Lords, (PROSE: Christmas on a Rational Planet) Faction Paradox, (PROSE: Alien Bodies) and the Voord. (PROSE: Interference - Book Two)

The Eleven later found the Kandyman in a Third Zone slave labour camp and repaired him. (AUDIO: Sweet Salvation)

The Congress of the Nine Planets sent fleets of ships to Ranskoor Av Kolos to stop Tzim-Sha after he combined all the knowledge of the Stenza with the abilities of the Ux to create a weapon capable of trapping entire planets in stasis. (TV: The Battle of Ranskoor Av Kolos)

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