...Or use fancy materials that trigger a destructive cascade reaction within the planet when subjected to enough initial heat or pressure from the impact. Like dilithium in the planet in the ENT Romulan War novels, or (in a reverse of that, sort of) a suitably "catalyzing" exotic substance aboard the ramming ship in the novel Great Starship Race by Diane Carey.
A planet that reactive probably would have exploded from natural causes long ago. You wouldn't need a starship to trigger it, a simple asteroid impact will do. Of course that leaves open the question of how such a planet would form in the first place since planets are formed from accreted impactors or asteroids and meteors.