Planet brains are a delicacy. They are really a subset of Space Brains because Planets are located in Space.
I had thought they could be space zombie doors, but we all know that space zombie doors are related to reptilians and not to grays (like space zombies). These are gray, and therefore must be related to the grays and ergo space zombies.
The reptilians are known for their door weaponry and leveraged hinge attacks as a common preemptive strike tactic.
This is common knowledge and was covered in your field manual so I am surprised I have to repeat this.