Hadhazy, A., Scientists: If Aliens Arrive, Expect Robots, CBS News, Mar 10 2011 (Hat tip: irrationaltheorist)
"If an extraterrestrial spaceship ever lands on Earth, I bet you that it is 99.9999999 percent likely that what exits that ship will be synthetic in nature," said Michael Dyer, a professor of computer science at the University of California, Los Angeles (appropriately enough).
In civilizations advanced enough to travel between the stars, it is quite likely that machines have supplanted their biological creators, some scientists argue. Automatons -- unlike animals -- could withstand the hazards to living tissue and the strain on social fabrics posed by a long interstellar voyage.
Furthermore, nonliving beings would not have to worry all that much about the environmental conditions at their destination -- if the planet is hot or cold, bacteria-ridden or sterile, has oxygen in the air or is airless, machines would not care.
In short, don't expect cuddly, squishy E.T.s to come calling someday; instead, picture robots descending from the sky.