If we assume that our alien neighbors are trying to contact us, we should be looking for them. We are currently engaged in several programs that are now looking for the evidence of life elsewhere in the cosmos. Collectively, these programs are called SETI (the Search for Extra-Terrestrial Intelligence.) In this article, I'm going to concentrate on only one of those searches, the SERENDIP project (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) of the University of California, Berkeley. You will find links to others in the Internet resources section. Background 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. Seems like there should be lots of life out there! Can we make an initial estimate? The first to do so was the astronomer Frank Drake. He came up with a simple equation, now called the Drake Equation, that maps out the possibilities. The equation is quite easy to understand, so don't tune out, even if arithmetic isn't your strong suit! Here it is: N = R * f(p) * n(e) * f(l) * f(i) * f(c) * 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 star formation in the galaxy. "f(p)" is the fraction of stars that have planets. "n(e)" 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. "f(l)" is the fraction of those planets within the ecosphere on which life actually evolves. "f(i)" is the fraction of those planets on which intelligent life evolves. "f(c)" 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. Let's briefly look at each of these factors separately and try to put some reasonable numbers to them.
Although the rate of star formation was undoubtedly much higher when our galaxy formed, one can still see where stars are being born today. Take a look at the beautiful pictures of stellar nurseries taken by the Hubble Telescope in the Eagle nebula and the Orion nebula. Here, huge clouds of gas collapse to form stars. A good guess at the rate of this star formation is about 20 stars per year. R=20.

 The Eagle nebula image provided by the Hubble Space Telescope.

 "SERENDIP: The Search for Life"