Continuity mistake: When Cooper draws on the board and talks about landing on water, his drawing changes between shots. (01:04:25)
Continuity mistake: Cooper is saying goodbye to Murphy. Her necklace jumps in and out of her T-shirt throughout the scene. (00:38:25)
Continuity mistake: When Murphy pleads with Cooper to stay shortly before he leaves, as she shows him the message "STAY" on the notebook, in the first shot she looks down, but in the next one she looks at Cooper.
Continuity mistake: At the beginning of the movie when Cooper and the kids arrive to the lake after they took the control of the drone, Murph steps out of the truck and leaves the door open, but in the next shot the door is closed and it's sure that in the entire scene there's no sound of a car door getting closed.
Answer: I am not exactly sure what kind of propulsion system is used, however; you must consider that although in our galaxy even the closest planets to Earth take years to travel to, in this alternate universe, the planets may be arranged in a closer formation.