- The planet's name originates from the episode script and is not heard in spoken dialogue itself.
Around 498,000 BC (reference stardate -5,000/00), the people of the highly advanced civilization on this world had thought they were as powerful as gods, but ended up destroying themselves in a world-devastating civil war. (FASA RPG module: The Orions: Book of Deep Knowledge) The inhabitants of the planet were engulfed in a devastating conflict that destroyed the surface of their world, leaving only three survivors. (TOS episode: "Return to Tomorrow") This war was created by energies sent out from the entity known as (*) during its battle with the Q Continuum. (TNG novel: Q-Zone)
Later, the Arret civilization would be the earliest known interstellar civilization for which coherent records existed, and its demise was observed by the Rigellians. The three survivors were discovered and interviewed by Federation scientists shortly before their deaths, and told of their original humanoid forms and far-flung settlements, suggesting that the races of the Federation might be their descendants (though this was considered unlikely, based on archaeological evidence to the contrary). (FASA RPG modules: The Orions: Book of Common Knowledge, The Orions: Book of Deep Knowledge)
Settlers from the world were forced to abandon it, due to the destruction brought about by their brethren. They were chosen to live on a new planet that was later designated as Gamma Trianguli VI millions of years before the world was encountered by Captain James T. Kirk. These inhabitants were only a small fraction of their race, but vowed never to allow the potential to destroy their newly adopted home as they did to Arret.
In the antimatter universe, an Arret (reverse spelling of "Terra") is the planet of Karla Five and Karl Four. (TAS episode: "The Counter-Clock Incident"). [Alan Dean Foster's adaptation, in Star Trek Log Seven, retconned much of this episode into an alien illusion.]
In 2268, the Federation Starfleet starship USS Enterprise visited Arret and found the life essences of Sargon and his two companions, the last surviving Arretians on the planet. (TOS episode: "Return to Tomorrow")
On stardate 7###.8 (circa 2270), the USS Trexelprise encountered an ancient transport ship near Arret. The Trexelprise rescued the ship from destruction and its crew, which was trapped in a holographic program. (ST video game: Trexels)