According to the theoretical Drake Equation of 1961, mathematical calculations designed to quantify the odds of alien life indicate that there are potentially 10,000 intelligent civilizations in the Milky Way. Dr. Frank Drake, while working at the National Radio Astronomy Observatory, invented this theory as part of the agenda for the world's first SETI meeting. The Drake Equation has generated opposition due to the variables involved.
Drake's original calculations began with self-imposed limitations, such as confining it only to hypothetical extraterrestrials in the home galaxy and only ones that might be capable of interstellar travel. In his mathematical equation, he included all the conditions required to enable such civilizations to evolve. Many variables needed to be considered in this theory. Some of these uncertain factors included the rate of star formation in the galaxy, the fraction of stars that form planets, the number of Earth-like planets hospitable to life, the fraction of these planets on which any life actually emerges, the fraction of planets on which intelligent life may arise, the fraction of those planets with intelligent beings capable of interstellar communication and the length of time such a civilization survives. Astronomers can only make educated guesses about these variables, which is what led to the controversy about the accuracy of this theory. The odds of alien life vary depending upon the interpretation of the variables.