It's called transpermiation. It's an interesting theory but usually used as a cop out. Evolutists like to use it against creationists to claim it looks like there is a god because there was an intelligence that helped created life on earth but it was aliens rather than god. Also, many people who don't believe in a god or evolution use this as a plausible explanation for life on earth without having to accept one of the two primary origin theories (Scientist Fredrick Hoyle for example). I think this theory will never be respected as long as there are so many UFO chaser and conspiracy nuts out there.