Well they said it was too large to go around, also the entire food contamination part was kinda short imo. Overall the story felt rather rushed to me, as if they wanted to give us a sneak-peak of the next few episodes. As to how they knew, their food source got contaminated, so they probably sent out some raptors and the likes to scout. Infact, it shows Sharon scouting the planet and then returning with the "good news". Its probably they somehow knew there was a planet on the other side, but they did not know whether or not it had a food source. At any rate, we'll be seeing this planet again (something with the eye of Jupiter )