Transcript: Astronomers who are optimistic about the prospect of intelligent civilizations in the universe tend to take high values for the numbers in the Drake equation. For example, assuming that the fraction of stars with planets is close to a hundred percent, that there is at least one habitable planet per star, and that when a habitable planet exists not only does life develop but intelligent life develops eventually, then intelligent life with technology will naturally want to communicate through the universe. With optimistic values for the factors in the Drake equation, the end result is that the number of intelligent, communicable civilizations is roughly equal to L, the lifetime of a civilization. Thus, if civilizations live ten thousand years, there may be about ten thousand in the Milky Way galaxy. If they live a million years, there are a million civilizations in the Milky Way galaxy, and so on. Dr. Christopher D. Impey, Professor of Astronomy, University of Arizona