They learned of the Imperial presence just as an Imperial patrol came upon their landed ship, destroying it with proton grenades and dispatching all available forces to track the Rebels down.
After have lost the contact with Luke Skywalker and Leia Organa, GeneralJan Dodonna refused to try to send reinforcement ships force the blockade and rescue the two rebel scouts. So Han Solo decided to go to Jarnollen with his own ship.
Luke Skywalker and Leia Organa spent much of the night and dawn the next morning evading an Imperial AT-AR, before being rescued by Han Solo and the Millennium Falcon. During the escape in the Jarnollen system, the Millennium Falcon was damaged and the rebels forced to go to the nearest planet, Ord Mantell.