The objective of Planetary Defense is to score as many points as possible by destroying asteroids and flying saucers. The player controls a ship that can rotate left and right, fire shots straight forward, and thrust forward. As the ship moves, momentum is not conserved – the ship eventually comes to a stop again when not thrusting.

Each stage starts with a few large asteroids drifting in random directions on the screen. Objects wrap around screen edges – for instance, an asteroid that drifts off the top edge of the screen reappears at the bottom and continues moving in the same direction. As the player shoots asteroids, they break into smaller asteroids that frequently move faster and are more difficult to hit.