So, how about a sort of "Doomsday Device" artifact for planets. The purpose would be that if your planet was successfully invaded, it would immediately wipe out all structures on the planet. It could be an artifact that lasts for a week like an alarm sentry, or it could be an actual planetary structure. Size 1 with another smaller bonus accompanying it. Thought?

If I understand your idea correctly, you would be kinda screwing yourself over by using this. If it has an immediate effect, there would be nothing left on the planet if you immediately reconquer...

Does have some advantages in a war but realistically speaking that's the only time it'll ever be used.

i could imagine being a jerk and installing this on a planet you just stole from someone 100 ranks above you. especially if they had paid for buildings on their planet.