if i just demolish a planet surface, so there is just like HALF of the planet, will it stay that way? see below :
at some point they will probably decide that the planet is toast and let it blow up. otherwise, hit it again with a small asteroid in the upper left quadrant, put engines on the backside and you can play pacman in the solar system.
The PacMan idea is actually good. Direct the semi-destroyed planet to an asteroid belt and let the PacnetMan (I know, horrible name ) eat the asteroids to get back to full health! And if there's no asteroid belt? Tough luck...