Our sun is only a single star in a collection of over 400 billion we call the Milky Way galaxy. The Milky Way is only 1 of billions of galaxies in the universe. The astronomer Frank Drake was the first who attempted to estimate how much life is in the universe. He came up with a simple equation, now called the Drake Equation, that maps out the possibilities: N = R x fp x ne x fl x fi x fc x L. "N" here represents the number of communicating civilizations in our Milky Way galaxy. This number depends on several factors. "R" is the rate of "suitable" star formation in the galaxy. "fp" is the fraction of stars that have planets. "ne" is the number of these planets around any star within the suitable ecosphere of the star. An "ecosphere" is a shell that surrounds a star within which the conditions are suitable for life to form. Too close and it's too hot; too far and it's too cold. "fl" is the fraction of those planets within the ecosphere on which life actually evolves. "fi" is the fraction of those planets on which intelligent life evolves. "fc" is the fraction of those planets where intelligent life develops a technology and attempts communication. The last factor, "L," is the length of time that an intelligent, communicating civilization lasts.
One result of the Drake Equation gives N = L. In other words, the number of intelligent communicating civilizations in the galaxy equals the number of years such a civilization lasts. The figure about which we know the least bears a great significance in our calculations. Most scientists hope that if a civilization can overcome its initial tendency to destroy itself with its own technology, then that civilization is likely to last for a very long time. Let's hope those scientists are right. In any case, there should be at least 50 (the number of years we've been around communicating) and if a communicative civilization lasts for millions of years, there may possibly be millions of civilizations we can look for.