Yea, pretty good...
This was a nice use of the drawing API, well done.
I have a few points to make however.
Firstly, the gameplay didn't really expand beyond the basic actions. Sure you had different enemies and such but a few more differences would have been welcome, aswell as some more powerups. I can't think of anything off the top of my head... maybe a powerup that lets you break through those shield enemies or one that surrounds your planet in a shield of 'line', maybe increase the hits your planet can take or increase the damage the balls do, then you could throw in magnets and bumpers which fire the ball off faster and walls/tunnels etc yea that could be good...
Alright anyway, next point some music would be good (i didn't notice any but yea some good music maybe even a selection).
Also, level 7 is impossible if the last guy remaining is in the top corner cause his shield comes back before u can hit him, or he doesnt take it off if u do it the other way so yea that sucked.
A level editor could be added that would be pretty cool.
Multiplayer mode (over the internet would be even better i can help you with that if you need).
Ok i'm sure i can think of more to add but this will do. Btw, its still a great game don't get me wrong ;), but as a game developer i'm always looking for holes in games hehe :P