Really solid game. Gameplay is simple but challenging. Maybe a little unfair. I'm looking at you level 10 :D That thing was just crazy. Often it was possible to really time your shots but getting the balls to the top right in level 10 was pure luck :D
Music is relaxing and fits this type of game very well. Visuals are ok, but there is improvement. So maybe you make the planets more interesting ;)
As mentioned below, you shoudl add another game mechanic (don't ask me what exactly) if you plan to make more levels to spice them up a little, because the 10 levels felt the same.
I don't know if you want to, but you could add the possibility to decide in which direction to shoot. This will not make much of difference for portrait mode. So just an idea ;)