Find all the juniors and take them to the safety of your spacecraft. The inhabitants of the planet will be after both you and them, so try to avoid them. A junior can be saved by walking or flying over it, after which it starts following you.
You can have several juniors with you at the same time, but beware: with many juniors following you, they are also easier to catch. When all the juniors have been rescued, you should return to your ship and take off.
Look out for hidden areas, as they might contain keys to bonus levels!