It's so situnational that it can be justified to be so stupidly powerful. Unless you actually score a desert or terran planet that connects to at multiple enemy worlds or strongpoints it is just a mediocre support ability. It it pretty much one-time use and can be stopped (as others have mentioned) by reducing population on the enemy planet.
From an Asteroid it does 20x20 damage max, that's 400 damage.
From a Moon it's 20x30, 600 damage (--> this is where your lighter frigates start dying if you didn't research shields and HP)
From a Volcanic planet 20x70, 1400 damage (--> this is where lighter frigates and some cruisers die)
From an Ice planet 20x160, 3200 damage (--> if your enemy has a massive frigate fleet try this for instakill)
From a Desert planet 20x247, 4940 damage (--> this is where it gets dangerous for your structures and capships)
From a Terran planet 20x322, 6440 damage (--> run, Titan, RUN)
The damage seems to ignore armor multiplyers and instantly kill constructors starting with volcanic worlds, but as I said, actually getting a planet close to your enemy, getting the population up to the highest level and attacking them with it requires some serious effort and some luck on your side. I consider it more of a "break free" ability than an actual superweapon replacement. Not to mention you wont be using it twice if it fails.