This planet's existence remained unknown at least 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.
"The Assigners" wanted to help Earth survive its early technological age. To this end, around 3000 BC, they took certain Humans to serve as the forebears of a pool of genetically-perfect Earth operatives. As Gary Seven, one of their Class 1 Supervisors, said on Earth in 1968: "A truly advanced planet wouldn't use force. They wouldn't come here in alien forms. The best of all possible methods would be to take Human beings to their world [and] train them for generations until they're needed here."
These agents were Human but flawless: not even the Vulcan nerve pinch could stop them. They were transported back to Earth as needed by means of an incredibly powerful transporter beam which, in the one case where it hit the Enterprise, shook the vessel violently and locked her circuits wide open (fusing those having to do with recording). At least one agent was accompanied on the interstellar beaming by a humanoid who preferred to appear to be a domestic cat (or vice versa).
Gary Seven followed two agents dispatched to Earth, 201 and 347, on the day in 1968 when there occurred a major assassination, an Asian coup d'etat, and the launch into orbit of a US nuclear weapons platform. Seven came because the agents, dead in an automobile crash, had been incommunicado. He took over their assignment – which was to effect a misfire and near-holocaust to frighten the USA and the other major powers out of their foolish "balance of power" strategy – and completed it by the skin of his teeth. (TOS: "Assignment: Earth")
An original story concept for "Prime Factors" (an episode of Star Trek: Voyager) involved U.S.S. Voyager finding this planet in the Delta Quadrant. (Prime Factors)