Last month, The New York Times published an article describing a secret government program investigating reports by military pilots of unidentified flying objects they encountered in the course of their daily duties. The media was awash with stories of flying saucers and extraterrestrial encounters, with scientists downplaying the likelihood of alien visitation and UFO enthusiasts exclaiming their excitement.
While I sit very firmly on the side that believes these reports more likely have an unremarkable and terrestrial explanation, whether alien life exists is a very real and credible scientific question. What is the possibility that life — and even intelligent life — exists around a star other than our own? And how can we find out?
In 1961, Frank Drake wrote down what is now called the Drake Equation, which is frequently used to help guide thinking about extraterrestrial life. The equation multiplies a string of probabilities — such as the fraction of stars with planets, the fractions of planets that are habitable, and the percentage of times intelligent life forms — to give an estimate of the likelihood that we are not alone in the universe.
When Drake wrote down his equation, very little was known about any of these probabilities. They were all pretty much guesses. However, using the best guesses of the day, he estimated there were 10 planets in the Milky Way galaxy emitting radio waves and thereby detectable in principle by Earth-based radio telescopes.