Time vortex planet

Time vortex planet was an informal name for the planet where the Guardian of Forever was located. This planet had a Class M atmosphere.

The planet was littered with the ruins of a massive, forgotten city that stretched beyond the horizon in all directions around the Guardian. Based on initial observations, the ruins appeared to be at least one million years old, although the Guardian claimed to be at least five billion years old, predating the existence of Sol system.

The planet of the time vortex was discovered by the crew of the USS Enterprise (NCC-1701) in 2267 and was revisited in 2269 by Captain James T. Kirk and his science officer Spock, accompanied by a team of historians. 

Apocrypha
The novelization of "Yesteryear" (in Star Trek Log 1) names a city on the time vortex planet as "Oyya."

In the novel Imzadi, the time vortex planet is referred to in the 25th century as "Forever World", since giving it anything other than such a Guardian-referencing name was felt to have been, in the words of one character, "... like painting a mustache on the face of God."

Other Pocket Books novels and Star Trek Online have called the time vortex planet "Gateway," a reference to the Guardian of Forever's final spoken line in "The City on the Edge of Forever", "Let me be your gateway."