6 signs that aliens might exist

In "The Day the Earth Stood Still," a remake of the 1951 science-fiction classic, an alien named Klaatu (played by Keanu Reeves, right) visits Earth to save us humans from ourselves. The story is a work of science fiction, with the emphasis on fiction, says Seth Shostak, a senior astronomer at the SETI Institute and a technical adviser on the film. For example, to be able to detect a dangerous buildup of carbon dioxide in our atmosphere and come save us from global warming, an alien that could travel at light speed would have to reside no more than about 50 light-years away. "I doubt that there are any aliens that close," Shostak says. And even if there are, "they might not care about our problems."Scientific accuracy aside, Shostak says the film could hook a new generation on space science, just as the original film helped direct his career, which is dedicated to the search for E.T. As kids stumble out of the theater, they might ask, do aliens exist?Click the "Next" arrow above to explore the evidence — from the scientifically plausible to the incredible — suggesting that we may not be alone.