Draw a circle around the various planets of the same color to join them so they vanish. The more planets you join, the more crystals will appear and as you collect them you'll earn more points and fill the level bar at the top (when it's filled, you go to the next level). If by mistake you join different planets, a black hole will be created, and to eliminate it you'll have to draw a circle around it.
Click and drag to draw a circle around many planets of the same color: more planets is better. Then pass the mouse over the little colored diamonds to earn points and fill the progress bar visible at the top.