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where $N$ is the number of civilizations in the Milky Way Galaxy with whom we could make contact.  $R$ is the rate of star formation, which tells how many stars are born every year in our Galaxy.  $n_e$ is the average number of habitable planets in any planetary system  All $f$ termn are factors $\le 1$, corresponding to \begin{itemize}   \item $f_p$ is the fraction of stars that have planets   \item $f_l$ is the fraction of planets where life actually begins   \item $f_i$ is the fraction of life-forms that develop intelligence   \item $f_i$ is the fraction of intelligent civilization that decide to communicate   \end{itemize}   Finally $L$ is the longevity of a communicative civilization (in years). In the case of humankind L is, so far, just a few decades.