What if a planet with living beings was gradually loosing its atmosphere?
One of the species could develope a carapace or a shell that would protect it from UV radiation and also, a thinning atmosphere. Its evolution could produce a creature capable of sustaining itself, even under total vacuum, thanks to the neighbouring star energy. Then, its evolution could allow it to move under vacuum, using some kind of propulsion mean (Ok, I was thinking like a successive mumber of farts...or a thin star sail like an insect wing); it would feed itself from rocks. And then, it would become a spacefaring species, but not intelligent the way we think it, just evolving in space as its natural element like water is for a fish.

Raoul