Loved the art style and color choice! I really enjoyed the smash powerup. It felt so good to go on a rampage against the asteroids for a few seconds.
Some minor feedback, that you're welcome to ignore :), would be to add WASD controls (not sure if possible with PICO8) and maybe make the shields stack if the player gets more than one (not sure if they do, just seemed like they didn't).
And I really loved the asteroid impact carving out parts of the planet. Along with the bit of screen shake, made for some great juiciness!