Arret was the formerly inhabited planet in an unnamed star system. In the ancient past, this was the homeworld of a warp-capable humanoid species that went on to colonize the Milky Way Galaxy. It was located "hundreds of light years past where any Earth ship ha[d] ever explored" in 2268, or "over three weeks" distance to Starfleet via subspace radio. (TOS: "Return to Tomorrow")
Originally Earth-like, though much older, the atmosphere of Arret was ripped away by a cataclysmic war fought 500,000 years before the USS Enterprise arrived in orbit. The ship's sensors detected no life on the planet's surface.
Eleven survivors of that great war had created a shelter 112.37 miles below the surface. This vault was constructed of a material harder and stronger than anything Commander Spock had measured at that time; its atmosphere was "a fraction richer in oxygen than usual".
Originally humanoid, the eleven surviving members of Sargon's species placed the essence of their minds into spheroid receptacles. In 2268, only two spheres besides Sargon's still contained a living mind: that of Thalassa, Sargon's wife, among the five spheres kept on the lower tier, and that of Henoch, a member of the conflict's "other side", on the upper. (TOS: "Return to Tomorrow")
In 2293, the location of Arret in the Milky Way Galaxy was labeled in the star chart "The Explored Galaxy", which was on display in James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
Background information Edit
The name "Arret" (Terra backwards) was used in the final draft of the script for "Return to Tomorrow" (dated 15 November 1967). Here it described: "The Enterprise is approaching a planet (Arret) about the size of Earth – with the notable difference that this planet is scarred, devastated and seemingly, very dead."
In a deleted line from the episode, Spock stated in his analysis of planet that it was a "Class "M" planet. Oblate spheroid, ratio 1 to 296. Mean density 5.53. Mass .9." Furthermore, he added that the planet was "about two-and-a-half billion years older [than Earth]", while the dialog simply stated that it was "much older than Earth." Additionally, in dialog it was stated that the great cataclysm had happened to the planet "about a half million years ago", while the script made no notation of when it had happened. Finally, in more deleted lines, Kirk asked Spock if perhaps it was a "Nuclear catastrophe of some sort?", but Spock disagreed, stating that it was "Something more powerful, I would say..."