First of all, well done for making a game! I think its cool to try and combine learning and gaming like this, though I personally already know the german numbers well ;D (maybe you could make a setting that lets users switch between different languages?).
I liked the fact that the planet numbers correspond to the order that they appear in from the sun outwards, so this game could be used to learn the planets as well!
But.. I think its difficult for somebody who doesn't already know german numbers, since they would have to guess what e.g. 'Sieben' means or fire on planets until they hit the right one. Maybe you can give some kind of hint by highlighting the active planet? Also I noticed that hitting asteroids makes the player lose a life but hitting planets does not, why?