Sarpeidon (also known as Beta Niobe I) was the first and only planet in the Beta Niobe system. This inhabited planet was classified as M-class. The planet was the homeworld for a pre-spaceflight humanoid species, the Sarpeidon natives. The planet was destroyed in the Beta Niobe supernova of 2269. (TOS: "All Our Yesterdays")
Many events of Sarpeidon's past paralleled events in Earth history, including an extended period of global glaciation, an ethnic or cultural group named Anglos, and paranoia due to fears of witchcraft. The planet was once ruled by a tyrant named Zor Kahn.
In 2269, hours before the sun went nova, the USS Enterprise arrived to warn the native species of their impending doom. However, they found that prior to the planet's destruction, all of its inhabitants had evacuated back in time, using a time portal known as the Atavachron, into various eras of the planet's past. (TOS: "All Our Yesterdays")
For the 2007 remastered version of "All Our Yesterdays", Sarpeidon was given a CGI-makeover, now a unique planet rather than a reuse of stock footage.
In "All Our Yesterdays", Spock stated that Sarpeidon was "millions of light years away" from Vulcan. TAS: "The Counter-Clock Incident" and ""The Explored Galaxy" star chart established however the planet and star system were still located inside the Milky Way Galaxy.
In the novel No Time Like the Past, Spock speculates that the Sarpeidons pursued time travel rather than space travel as a means of escape as the lack of other planets in their solar system meant that they had no real incentive to develop space travel out of a lack of anywhere to go in their immediate vicinity. During the novel, a time-displaced Seven of Nine meets a projection of Nepwa, the original inventor of the atavachron, who reveals that he left his people for the distant past out of disgust at what his work had come to.