Arret

Arret

You may also be looking for the planet in the negative universe known by the same name.

Arret was a planet located near Sarpeid, hundreds of light years past the limits of Federation exploration in 2268. This planet was located in the Beta Quadrant. (TOS: "Return to Tomorrow", TNG: "Conspiracy")

Originally Earth-like (though much older), its atmosphere was ripped away by a cataclysmic war fought half a million years before the USS Enterprise arrived in orbit. Its surface had since been uninhabitable.

The non-canon novel Q-Strike implies that the Beta XII-A entity was responsible for provoking the war.

Eleven survivors of that great war created a shelter 112.37 miles below the surface. This vault was constructed of material harder and stronger than anything Commander Spock had measured at that time; its atmosphere (perhaps arranged especially for the landing party) was "a fraction richer in oxygen than usual".

Originally humanoid, the eleven surviving specimens 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")

The planet was noted on a star chart in 2364. (TNG: "Conspiracy")

The name "Arret" (Terra backwards) was used to describe a planet in a cut scene from TOS: "Return to Tomorrow"; it is also referenced in the Star Trek Concordance.

Arret was also mentioned as a small note on a very large star chart graphic created for TNG: "Conspiracy". It was the image behind the chair in the room where Dexter Remmick was killed.