I personally love capturing gravity drills since they give free resources.
Well, that affects all ships, even the AI. To some sense that planet would be good for defending if you set up a bunch of sniper and spider turrets.
Although you could capture the next planet and reinforce that one, this way you can have both the grav drill and have your ships move at normal speeds.
Then again, if you insist on destroying it then superking's idea is the way to go.