Well, we know microbes exist in the atmosphere, well into the stratosphere. They have to be hearty to survive up there. The idea of life evolving and living in orbit certanly does not seem far fetched in light of that, after all we have found life everywhere else on this planet. However, in order to travel to say Mars on it's own, a creature must not only be able to travel interstellar distances, but be able to know where it is going.
ONE
How can a species evolve a instinctual understanding of orbital mechanics as to arrive at it's destiations instead of overshooting?
TWO
How would it be able to propell itself through space?
THREE
How would it be able to sense the whole of the solar system, as to be able to see where it was going over vast distances? How would it be able to see in 3D and know distances to planets?