In 2246, a pioneering group of several thousand set off from Earth, on a generational ship, bound for a system 14 light years away with an identified, habitable planet waiting for them. A dying Earth, and the only slightly more palatable prospect of a difficult life on an overcrowded Mars, had left their journey as one of the only remaining options.
On the way out of the Solar System, while passing through the extreme outer reaches of the Oort Cloud, and in an incredibly poor stroke of luck, their ship passed within 100,000 kilometers of a previously unidentified dark and icy exoplanet. The significant gravitational perturbation pushed them well off-course, and with limited excess fuel to re-adjust, eliminated their prospects of reaching their initial destination.
An emergency meeting was called, and the decision made to target a new system within their reduced range. It had five planets, and although there was no certainty any of them were habitable, it was their only chance to avoid effectively drifting until their reactors gave out.