Tau Alpha C
Tau Alpha C was an inhabited planet and the reported home of the The Traveler. The  library computer was familiar with its inhabitants.

It was located "at least" 1,000 light years from the Sol system. Its existence remained unknown well into the mid-23rd century. Up to that point the planet had not appeared on contemporary Federation star charts. This was due to active concealment of the fact of the existence of the planet by its inhabitants. ()

Considered to be very distant by Starfleet, at warp 9.5, it would take 123 days to travel from Starbase 133 to Tau Alpha C. A subspace message would take days to travel back and forth between both locations. This location was listed on Federation star charts. () 

In an alternate universe created after Doctor Beverly Crusher became trapped within a warp bubble, Wesley tried to contact Tau Alpha C for assistance when individuals began disappearing from the Enterprise-D, with only Dr. Crusher noticing. Ultimately, that entire universe, including its Tau Alpha C, disappeared. ()

Appendices

Background information 

According to StarTrek.com, Tau Alpha C was the name of a star system.  Under naming conventions for stars (ex. Alpha Centauri C), the name would suggest Tau Alpha C was the third star in a trinary star system.

According to the , Tau Alpha C was a "very distant planet". Whereas, in , the Traveler was instead said to be from Tau Ceti, a star system located approximately 11 light years from Earth.

An original story concept for  involved the  finding this planet (i.e. "Gary Seven's homeworld") in the Delta Quadrant.

Apocrypha
According to The Rise and Fall of Khan Noonien Singh, Volume One, the Assigners' planet was a cloaked planet of the  in system , and was 50,000 light years from Earth.

In the Star Trek: Department of Temporal Investigations novel Watching the Clock, the planet was described as having orange skies,  trees, and cities made of crystal spires. It was home to "vast, extraordinary beings, with...great eyes."

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