I recently heard that before anyone had begun to build practical aircraft of any kind, it was thought by some that once flight was achieved you would be able to simply fly to another planet. The discovery that space was a vacuum put paid to this idea, as did realisation of just how large the solar system actually is.
The idea of being able to use conventional aircraft in space is interesting because it justifies a lot of the common tropes about how spaceships work in softer science fiction. The adherence to a aircraft-like flight model being the main one, along with spacecraft needing continuous thrust, and coming to a stop when thrust is no longer applied.
To this end I postulate a kind of air or 'ether' which while it has the properties of air is not affected by gravity, allowing it to be evenly distributed throughout space. This has several issue however.
The first is planetary motion. If space is filled with a medium that creates drag the surface of any orbiting body would be scoured clean, unable to support any kind of biosphere. But what if we assume that planets do not obit the sun, but are instead stationary relative to it, and only rotate.
I'm not sure that this would be enough, however. If such a universe existed, would planetary surfaces be habitable? Would you get a layer of air relatively stationary to the planet's surface? Would there be gigantic 'whirlpools' formed at each pole?