this brought up a few questions, that puzzled us all.
1. Can you banish a Ghost? he says they exist in both the material and etherial plane, so you should be able to. I think Banishment is intended for summoned creatures and demons, but I can't find anything definate.
2. If you Can banish said ghost, what plane should it go to? He says it could be either. I agree but think you can't banish something to the plane you are currently on.
We both agree that 20% chance of sending it to a wrong plane would apply.