The sixth planet of the Ceti Alpha system, which exploded six months after Khan was marooned next door and turned Ceti Alpha V into a desert.
Ceti Alpha VI was a planet of the Ceti Alpha system. It is never seen, but its fate drives the plot of Star Trek II: The Wrath of Khan.
In Space Seed, Kirk exiles Khan Noonien Singh and his followers to the fertile but untamed Ceti Alpha V. Six months later the neighbouring sixth planet exploded. The shock shifted the orbit of Ceti Alpha V and laid it waste, and many of Khan's people died, his wife among them.
Fifteen years on, the USS Reliant arrives looking for a lifeless world on which to test the Genesis device. Captain Terrell and Chekov believe they are surveying Ceti Alpha VI. They are in fact on the fifth planet, and they walk straight into Khan's hands.