I would recommend starting the countdown timer much sooner. I understand the intention with the countdown timer, but you are able to just stay above the start zone for as long as you like until you find the perfect route to take. This takes away from the point of the timer in my opinion. Having the timer start as soon as you start the next level would make it so you are actually being pressured by the timer. You could balance it out by starting at 40 or 45 if need be. But that leads onto my next point, the difficulty.
The game is very lacking of any difficulty. The planets don't appear to rotate much faster and you are able to complete levels in around four seconds (I played to wave 70 so it's based off of that.) I would recommend adding new obstacles that you must pass such as some sort of slowdown that will cause you to slowly get through it will the asteroids around you continue onto their original course, some sort of spaceship that will fire lasers at you a few times, or something simple like lasers! I would also recommend lowering the timer to 15 seconds for the first x amount of waves. Another option is adding in some sort of stationary obstacles that rather stay stationary the whole time or follow the planets rotation (such as the volcano.) Going off the volcano, you guys should add more obstacles that are stationed on the planet! The volcano is very useful since it limits the landing space you have.
As for the artistic theme, I would recommend creating multiple backgrounds that go alongside the planets. As for the planets, I would love to see the planets be round! I LOVE the look you guys are going for but the planets just look like they're low quality more than anything. You could also add a bomb that will get closer and closer to you as the timer counts down and finally attaches to you and explodes when the timer hits 0!
As for game-modes, keep looking into different objective based game-modes! Such as rescuing survivors from the planets! You should also consider offering a sandbox mode where you can alter the rotation speed of the planets, speed of the asteroids, and speed of the spaceship!
Overall, this game has a lot of promise and is very polished! You guys are some incredible game developers and I'm excited for what the future holds for you! Please don't take my criticism harshly, it's all to help the game reach its full potential! Keep up the good work guys! :)