The sheer talent on display here is undeniable! The graphics? Spectacular! (And adorable!) The sound and music? Fantastic! The game design? Top-notch! And it's a fairly original idea to boot! The accuracy though leaves a bit to be desired... While you did stick to the 8 x 8 pixel grid, the 4 colours and the resolution, you pretty much eschewed every other limitation of the Game Boy...
2 background layers, which I could have forgiven if it were only the stars and they were a bit more sparse, we could have pretended the stars were sprites rendered behind the BG layer, and you could have even moved each star at a different rate were that the case! But you put a big'ol planet there and it becomes pretty obvious that you're using a 2nd BG layer, the GB only had one of those...
The sprites also use all 4 shades of grey, which is a bit more forgivable all things considered, but the GB sprites were limited to 3 colours each, the same way NES sprites were (though it is possible to simulate a higher colour depth by rendering 2 sprites on top of one another and having each sprite use a different palette, hence why I feel this is more forgivable, though it is a bit unreasonable given the sprite count limitations)
And the sound and music, as much as I love it, sounds more like a PC engine than a Game Boy...
I really hope I'm not being too harsh here, I really did love your game, and I'll definitely be replaying it! You deserve all 5 stars on every other aspect! Good job!