Trying to calculate the probability that extraterrestrial life exists in the universe is actually quite complicated. The universe isn't a static environment. Stars are born, they live and they die. Some stars form in association with planets. Others don't. Only some of those planets have the right conditions to support life.
Life is a tricky variable in its own right. Some planets might support complex organic molecules -- proteins and nucleic acids -- and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. Finally, even organisms that have adapted extremely well to their environments don't last forever. As both the dinosaurs and the Roman Empire illustrate here on Earth, all dynasties come to an end, be it cataclysmic or otherwise.
Frank Drake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy -- and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:
N = RfpneflfifcL
In this equation, N is the number of detectable civilizations in our galaxy. The other variables are described below:
- R is the rate of star formation in the galaxy
- fp is the fraction of stars that form planets
- ne is the number of planets hospitable to life (i.e., Earth-like planets)
- fl is the fraction of these planets on which life actually emerges
- fi is the fraction of these planets on which intelligent life arises
- fc is the fraction of these planets with intelligent beings capable of interstellar communication
- L is the length of time such a civilization remains detectable
The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year [source: Cain]. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.
Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets -- those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that ne equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:
N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000
which gives us a value of 12,000 civilizations in our galaxy.
Drake's original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way [source: Garber]. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy [source: Lemarchand]. That's a lot of ETs!
No wonder astronomers were so optimistic when they started searching diligently for extraterrestrial life in the 1960s. On the next page, we'll look at how they've conducted this search and what it has turned up.