N = R * fp * NE * fl * fi * fc * L
The terms are defined as follows:
| N | The number of "detectable" civilizations. |
|---|---|
| R | The number of stars in the galaxy (approximately 400 billion). |
| fp | The fraction of those stars that have planets. Astronomers are beginning to find planets around stars, and the rate of stars with planets seems to be 3%. |
| NE | The number of those planets with earth-like environments. The assumption is that earth-like environments are required for "life". |
| fl | The fraction of the earth-like planets which have "life". |
| fi | The fraction of planets having life which have intelligent life. |
| fc | The fraction of planets having intelligent life where said life wishes to communicate with other civilizations. |
| L | The longevity of intelligent civilizations.
Intelligent civilizations might be short lived, because we see in our own case grave danger that might kill the civilization we are constructing. Or intelligence might be a stabalizing influence leading to a long-lived civilization. In any case as stars are born, age, and vanish, the civilizations that flourish on the suitable planets might not exist at the same time period as other civilizations on other planets. Obviously the longer lived a civilization is, the greater the chance it will exist at the same time as other civilizations. |