While we speculate on what biological forms any extraterrestrials may take, several experts believe that we are more likely to encounter advanced forms that can reach out to us. Maybe even artificial intelligence. When any intelligent species reaches the point of space travel, it shouldn't be too long -on a cosmic scale- before they can build machines that are smart enough to both replicate and improve themselves. And travel through space.
“If we build a machine with the intellectual capability of one human, then within 5 years, its successor is more intelligent than all humanity combined,” says Seth Shostak, SETI chief astronomer. “Once any society invents the technology that could put them in touch with the cosmos, they are at most only a few hundred years away from changing their own paradigm of sentience to artificial intelligence,” he says.
ET machines would be infinitely more intelligent and durable than the biological intelligence that created them. Intelligent machines would be immortal, and would not need to exist in the carbon-friendly “Goldilocks Zones” current SETI searches focus on. An AI could self-direct its own evolution, each "upgrade" would be created with the sum total of its predecessor’s knowledge preloaded.