(covers information from several alternate timelines)
"You lie! On Ceti Alpha V, there was life! A fair chance –"
"THIS IS CETI ALPHA V!!!"
Ceti Alpha V was the fifth planet of the Ceti Alpha system, in Sector 25712 of the Alpha Quadrant. It was home to the non-sentient Ceti eels, and briefly hosted a colony made up by the crew of the SS Botany Bay.
The planet was described as "barely Minshara-class" in 2156 and "habitable, although a bit savage, somewhat inhospitable" in 2267. However, shortly thereafter its orbit shifted and it became decidedly more hostile. (TOS: "Space Seed"; ENT: "Twilight"; Star Trek II: The Wrath of Khan)
Ceti Alpha V 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. The nearest starbase was Starbase 12. (ENT: "Twilight"; TOS: "Space Seed"; 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 race from 2156 to 2165 – survivors of the devastating Xindi attacks on Earth and its colonies – as it was far from the Delphic Expanse. The colony's location was kept secret for a great deal of time, but a Yridian discovered it by tailing Doctor Phlox, thus allowing the Xindi to attack. This timeline was negated during the subsequent Battle of Ceti Alpha V. (ENT: "Twilight")
In 2267, after discovering the Botany Bay adrift in space in an unnamed sector, the ancient spacecraft was rigged for towing by crew from the Enterprise. Traveling at warp factor 2, the Enterprise, with the Botany Bay traveling alongside, entered the adjacent Sector 25712 on a heading toward the sector's command base - Starbase 12. This heading brought the starship near the Ceti Alpha star system. After a failed spacejacking by Khan Noonien Singh, he, his lover Marla McGivers, and his Augment followers were settled on this planet by Captain James T. Kirk. (TOS: "Space Seed")
Only six months later, Ceti Alpha VI exploded, shifting Ceti Alpha V's orbit and turning it into a desert-like world, dominated by craylon gas, sand and high-velocity winds. The planet thus became inimical to life, with the only remaining known indigenous species being the Ceti eel.
In 2285, the USS Reliant visited the planet as part of Project Genesis. Mistaking it for Ceti Alpha VI, Captain Terrell and XO Commander Pavel Chekov beamed down to the surface, only to be taken prisoner by Khan and his surviving followers. Khan then hijacked the Reliant and stranded its crew on the planet's surface. The crew were later recovered by the USS Enterprise, following Khan's death at the Battle of the Mutara Nebula. (Star Trek II: The Wrath of Khan)
Ceti Alpha V inhabitants
Starbase 12's sector was named in Star Trek: The Next Generation episode "Aquiel". This planet's quadrant of origin is inferred based on the position of this starbase as seen in the star chart appearing in the Star Trek: Discovery episode "Magic to Make the Sanest Man Go Mad".
In the script of ENT: "Twilight", a scene description referring to a view of Ceti Alpha V from space in 2156 characterized the planet as "a dry, inhospitable-looking class-M planet." In the same script, the surface view of Ceti Alpha V as of 2165 was described thus: "Dozens of makeshift Human dwellings are clustered in this small valley. We're on a harsh but habitable Class-M world... only a few occasional patches of vegetation are visible here and there."
The writers joked on the audio commentary for "Twilight" that, if it wasn't bad enough the Xindi killed off almost all of Humanity, a hundred years from "Twilight", Ceti Alpha V would have been destroyed as well.
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.
The spin-off novel To Reign in Hell: The Exile of Khan Noonien Singh explicitly addressed the above issue. Spock postulated that Ceti Alpha VI's destruction and the environmental impacts on Ceti Alpha V had another, shared cause (possibly a small black hole passing through the system), while apparently the miscounting resulted from the Reliant's crew counting inward, finding the outermost planet (and mistaking the remains of Ceti Alpha VI for part of an outer asteroid belt). The Vanguard novel Storming Heaven had Ceti Alpha VI being destroyed as a result of experimentations with Shedai technology by Dr. Marcus and Ming Xiong.
In Vonda N. McIntyre's novelization of Star Trek: The Wrath of Khan, the planet is called Alpha Ceti V. There was some dialogue between the Reliant's bridge crew speculating about the cause of the discrepancy between the planet count of the system reported by an old interstellar probe (20) and the current count (19) – one possible conclusion they drew was that the limitations of the probe's imaging systems had led to inaccurate data being reported. This dialogue occurred shortly before Terrell and Chekov decided to beam down to the planet.