1960 AD
GREEN BANK, WEST VIRGINIA
Frank Drake, the man who conducted the first modern search for radio signals from other civilizations, developed a way of thinking about how many civilizations there could be in our galaxy. It’s now known as the Drake Equation: N = R x Fs x Fp x Ne x Fl x Fi x Fc x L.
What he did was to break up the big problem into a number of little ones. In the equation, N is the number of advanced civilizations existing in our Milky Way Galaxy right now.
R is the average rate at which stars form. There are about 400 billion stars in the galaxy, which have been around for about 10 billion years. So, 400 billion stars divided by 10 billion years is 40 stars per year. In other words, something like 40 stars are born every year in the galaxy.
Fs is the fraction of stars like the Sun. Stars that are much bigger than the Sun burn out more quickly, and wouldn’t last long enough for life to form. Fortunately, the Sun is a very common type of star. There are many such stars in the galaxy that are not much hotter or colder than our own Sun — just about the right size. So a reasonable estimate for this term is one-tenth.
Fp is the fraction of those good stars that have planets. We don’t know for sure, but there is some indirect evidence that planets are a natural result of the formation of stars. It looks like the junk that is left over when a star forms tends to condense into planets. So a reasonable guess here is that perhaps one-tenth of the stars that are good can have planets.
Ne is the number of planets around the good stars where life could exist. In our own solar system, we only know of one planet where life exists — Earth. However, there are other places where, theoretically, it might exist, or might have existed in the past, such as Mars. Still, to be conservative, a reasonable guess is that there could be one Earth-like planet in a system, where life could exist.
Fl is then the fraction of those Earth-like planets that life actually does arise on. Here the question becomes very controversial. No one knows how easily life can form if you have a nice planet near a good Sun. Some scientists think Earth is the only place with life. Others think that once you have a reasonable planet near a decent star, it’s almost automatic that the chemicals that are widespread in the universe will form more and more complicated molecules, and before long some of those molecules will start reproducing themselves — life is born. So some scientists say that this fraction is approximately zero, and others say that it’s close to one. We just don’t know.
Fi is the fraction of those planets with intelligent life. This is even more uncertain. We have no idea whether — once life starts on a planet — it may become intelligent. It could be that, if we humans had not come along, no other species on Earth would have become intelligent. Or, it could be that, if we hadn’t arisen, some other animal, such as the chimpanzees, or the bears, or the dinosaurs, might have developed intelligence. Again, the answer could be anything from zero to one.
Fc is the fraction of those planets with intelligent life that are capable of communicating across space. It could be that there are planets of intelligent dolphins that don’t have the ability to use radio, in which case, we can never detect them. Or it may be that any species smart enough to be considered intelligent will sooner or later discover the laws of electricity and magnetism that make radio communication possible.
L is the lifetime of the communicating civilization. In other words, how long does a civilization last once it starts being able to communicate across the distance between the stars?
The bottom line is that some scientists are pessimists and think life rarely gets started, and, if it gets started, it rarely develops intelligence. If they are right, there may be no intelligent civilizations in our galaxy today. A few think there may not even be any other civilization in the whole universe. However, optimists think that life could be widespread in the universe. There could be thousands, or even millions, of civilizations in the Milky Way today.
So the Drake equation is a fascinating way of analyzing the question of whether we’re alone in the universe. But there are too many unknowns. The only way we can know for sure is to look. This is why some scientists believe in SETI (the search for extraterrestrial intelligence). If we look for astronomical signs of another civilization, we may find that we have neighbors in our galaxy.