How many intelligent extra-terrestrial civilizations exist in our Galaxy? Astronomer Frank Drake's equation has become the touchstone for answering such questions. Here's how the Drake Equation works.
If the Universe is so old, and stars so abundant, why aren't we overrun with intelligent aliens? Could they actually be hiding in our own Solar System? Dr. Jill Tarter of the SETI Institute explains why we must still search vigorously.