Multiple realities
(covers information from several alternate timelines)

The Ceti Alpha system was a planetary system in Sector 25712. This system was located a considerable distance from the Delphic Expanse and the planet Denobula as well as three days away from Regula I at an unspecified warp speed. Its primary, Ceti Alpha, was orbited by at least six planets until 2267. Ceti Alpha V was formerly M-class until it was devastated by the destruction wrought by Ceti Alpha VI when it exploded. The nearest starbase was Starbase 12. (ENT: "Twilight"; TOS: "Space Seed"; Star Trek II: The Wrath of Khan)

Ceti Alpha V was a "habitable, although a bit savage, somewhat inhospitable" planet chosen by Captain Kirk to resettle Khan Noonien Singh and the crew of the SS Botany Bay after their attempted takeover of the USS Enterprise in 2267. (TOS: "Space Seed")

Six months later, Ceti Alpha VI exploded, shifting Ceti Alpha V's orbit and rendering the latter planet incapable of supporting complex life. This was discovered when the USS Reliant visited the system in 2285. (Star Trek II: The Wrath of Khan)

In an alternate timeline, Ceti Alpha V was the refuge for the last 6,000 members of the Human species in the 2150s and 2160s, survivors of the devastating Xindi attacks on Earth and its colonies, as it was far from the Delphic Expanse. The Enterprise guided a convoy of refugees there. (ENT: "Twilight")

According to Star Trek: Star Charts ("United Federation of Planets II") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Ceti Alpha system was located in the Beta Quadrant.