I'm guessing when you owned the planet it wasn't a Core?
That's why those pesky pirates didn't need a Transport to take it from you, I presume (if they can build transports, I have no idea). And didn't have a Transport to take it from the Mimot. Clever Mimots AI for changing it's status BTW if that's what happened.
There's been some discussion about the word Colony being used to describe non-Core planets. Apparantly it's regarded by Stardock as basically an Outpost (guarded by a small group of beings and a dog/spacedog appropriate to the galaxy it lives in, they just mine the resources and send them to their Core World all the live-long day), some players on here were thinking "Planet ZergyRushieIV's as big as Earth, how can it be taken by One. Scout. Ship. Okay, the people on board may have guns, but there's only two/three maximum onboard!?" and so were, understandably given their logic, not pleased with this development.