Spaceballs: The Air Shield

I’ve been trying to figure this one out for awhile now: what is the deal with the air shield around planet Druidia in Mel Brooks’ Spaceballs?

Based on circumstantial evidence in the movie, planet Druidia seems to be similar to Earth in size and density. If this is true, and if the ‘air’ that the Druidians breath is reasonably similar to air on Earth, then it is unlikely that the air shield functions to artificially increase the mass of the atmosphere. Most likely, the air shield was built late in the planet’s evolution, so hydrogen escape would already have taken place and the presence of an air shield in the high atmosphere would have little effect on composition.

Spaceballs is explicit about the security function of the air shield. Obviously, a Spaceball air shortage was sufficient to create a planet-encompassing protection against theft. It can be inferred from the manner of opening the air shield that the shield itself is not an energy barrier, but is constructed from some solid material. Even at a nominal thickness of 10 or so meters, a planet-encompassing shield would require approximately the same amount of material as the construction of a “Spaceball One”-sized ship (and construction of the ship would certainly be no more difficult than encircling an entire planet). Penetrating the air shield was as simple as extracting the combination from King Rolland; the Druidians had no apparent defense mechanism beyond the existence of a passive shield. If they had opted for the construction of a large spaceship, however, they would have been able to actively guard and defend their air supply. Given the evidence, I must conclude one of the following:

1) Although the amount of required material for the shield and ship is approximately equal, the Druidians had an overabundance of shield-making material (or conversely, they had an extreme shortage of ship-building material).
2) The Druidians were a peace-loving society of pacifists who did not wish to act aggressively, even toward an aggressor.
3) The Druidians had sufficient knowledge and ability to construct a homogeneous planetary shield, but they lacked the technological prowess to construct an active, complex spaceship system.