Zetar (or more specifically, Zetar II) was a planet located somewhere in the Alpha or Beta Quadrants, within the local space of the galaxy known to the Federation. This was the second planet in orbit of the Zetar star system in Quadrant 3. (ST reference: Star Trek Maps)
By the 23rd century, it was common knowledge that Zetar was a dead world, with all life there having been exterminated by a natural disaster hundreds of years earlier. Captain James T. Kirk attested to this fact when faced with a group of energy lifeforms who claimed to be the non-corporeal continuance of the Zetarian civilization in 2269. (TOS episode & Star Trek 6 novelization: The Lights of Zetar; ST - Who's Who in Star Trek comic: "Issue 1")
- The episode itself does not reference the time of the world's death, but the figure placing it hundreds of years prior to the 2260s is sourced from the Who's Who reference comic.