Most of what follows is scientifically dubious. I'm looking to clarify one aspect and one aspect alone.
A small planet is being terraformed by an alien race who have been doing this for millennia. They proceed by wrapping the planetoid at ground level in non-permeable, initially flexible, transparent, indestructible unobtainium.
Then they inflate the balloon using gases they have brought with them in liquid form. The resulting balloon has a diameter approximately a mile more than the diameter of the planetoid. This is their standard method. They always choose small planetoids and make the balloon the same height above the ground. The balloon is pressurised and the density of the unobtainium is more than that of the atmosphere it surrounds. The pressure is such that the surface of the balloon becomes very taut like that of a rubber balloon about to burst.
Their purpose is to farm the entire surface of the planetoid. EDIT - To make farming easier they flatten any significant mountains then grind the resultant debris into topsoil and fertilise it with chemicals they bring along.
The planetoid does not have any substantial satellites - only small alien-made ones. It is circling around a Sol-like star at a Mars-like distance.
Given the conditions above, will the balloon remain concentric with the planet or will it tend to brush the ground at times?
My belief is that it will self-stabilise because atmospheric gradient* will ensure greater pressure at lower altitudes and so push the balloon away if it drifts too close to the ground. I also believe that it will end up spinning in the same direction as the planet and thus perhaps be flattened somewhat at the poles.
Am I right that it will self-stabilise or do the aliens have to have some extra mechanism to keep it from touching the surface? Might the solar wind be a destabilising factor? Would increasing the height above the ground provide a more significant pressure gradient?
If it isn't self-stabilising then suggestions for a minimal change to the system to make it stable would be appreciated.
*I've edited edited 'pressure' to 'gradient'. Apologies, it was a slip of the fingers. I did mention gradient later.
The balloon is made from non-permeable, initially flexible, transparent, indestructible unobtainium.