However Dr. Marcus demanded them to go and check it out. Here is my beef. Ceti Alpha 5 was living M-class planet with millions if not billions lifeforms and all dies and vanish within 20 years except Khan and his merry men. I find it hard to believe. Life dosent end so quickly. Ceti Alpha in my opinion shouldn't never been consider as suitable planet for Genesis project cause despite changing the orbit and affect of explosion laying waste. There should still be plenty evidence of life still on Ceti Alpha 5(Khan planet).
You're missing an important detail: they thought
the planet was the Ceti Alpha VI, a lifeless planet in the same system. They didn't realize it was Ceti Alpha V until they found the Botany Bay
. That's why Marcus wanted them to check out the planet and see if it could be used for the Genesis experiment. Terrell hoped that whatever life they detected could be moved to another planet's ecosystem so that it wouldn't be affected by the Genesis wave if the device were used.