Guardian of Forever's planet
The Guardian of Forever's planet was a formerly inhabited planet located in the Alpha or Beta Quadrant. It was the home for the ancient Guardian of Forever.

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 the Sol system. 

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

During the Temporal Wars, the various factions attempted to use the Guardian as a means to travel through time and negatively affect their enemies. Because of this attempted misuse of his abilities the Guardian moved himself far away from his original home. By the late 32nd century, he was located on the planet Dannus V, which was located near the Gamma Quadrant. ()

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Background information 
The planet was not named in the episodes. It was called "the planet of the time vortex" in "Yesteryear" dialogue, and "the time planet" in the script notes. StarTrek.com called it the "Time Planet". 

According to Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the planet was called the Time Vortex Planet (Gateway). It was a quarantined planet in the late 24th century. TVP (Gateway) system was located in the Alpha Quadrant, near space formerly occupied by the Tkon Empire. The primary was a M-class star.

In the originally aired version of "The City on the Edge of Forever", the planet, as viewed from space, was represented with stock footage. However, the same footage was used in many other TOS episodes. For the  "remastered" version of "The City on the Edge of Forever", a new, computer-generated version of the planet was created to replace the stock footage.

Apocrypha 
In the reference works, such as Star Trek Maps (p. 30) and The Worlds of the Federation (p. 136), the name "Time Planet" was also used. According to The Worlds of Federation, it was the first planet in the UFC 465537 system.

The novelization of "Yesteryear" (in Star Trek Log 1) calls it the "Time Planet" and names a city on the planet as "".

In the novel , the 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."

The novel Yesterday's Son 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."

In the short story Guardians, the planet is called the "Rock of Ages".

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Unnamed planets