Nice, little game ;)
Here are things to improve: There is no GUI at all, no scoring system, no health indication, so player don't know what to do - one of the goals of the game design is for the player to present state of the game and how you could affect it. The other thing to consider is the visibility - I wasn't sure whether I could collide with planets, but it turns out I can - do you know why? Both planets and dangerous objects had the same colour tone and it wasn't possible for the player to differentiate planets from them. If your intent was to use them as background objects, you would need to work on their tone, to really not interfere with the plane, where objects to avoid are ;)
After taking too much damage (so game over), you can't replay it in any other way than reloading the page.
Other than this - you made a playable simple game in 3 days! Congrats! :D