Is it far enough away that it could have been fired before the cease fire, or before you captured the planet?
I thought that was it so I waited for the next round and 2 of my planets was hit again.
It is the novalith cannon you can build in orbit at a planet. The one AI have built 4 of them. You get the mess about a massive power thing then you see the shot and follow it straight to me or were ever it is going.
I have a trade alliance, a ship vision, and a ceace fire with the AI player.
cheers