10 Huge Revelations From Star Trek: Khan

Among the many things that Khan blames James Kirk for is the hellish nature of his new home. While Ceti Alpha V was described as 'habitable, although a bit savage, somewhat inhospitable' by Spock in 2261, it still offered Khan and his people hope. 

Six months after their arrival, Ceti Alpha VI exploded, changing the orbit of Ceti Alpha V. This laid waste to the planet's surface, forcing the survivors underground. The explosion of the planet was foreseen, but was still expected to take centuries. Ursula, the chief of medicine for the colony, was aware of that planet's instability, but was initially unconcerned about an explosion that she would never live to see. 

While the question of how the Reliant failed to notice a missing planet in The Wrath Of Khan doesn't receive an answer, the cause for said planet is revealed. The El Boreans, another group that crash lands on Ceti Alpha V, are responsible for VI's destruction. Their ship was damaged in transit, requiring them to find an emergency landing zone. 

The type of engine they used could devastate matter, though they couldn't jettison it for fear of becoming stranded in deep space. Left with little options, they allowed their propulsion system to destroy VI, which they knew was uninhabited. Their ship crash lands on V, which in turn leads to several years of tension and secrets between them and Khan's group.