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But how many of these planets host life? And in particular,\textbf{ how many planets host intelligent life we might be able to communicate with}?   In order to estimate the number of technological civilizations that might exist among the stars, in 1961 Frank Drake proposed the following simple equation   $N %$N  = R \times f_p \times n_e \times f_l \times f_i \times f_c \times L$It is a product of factors giving the number $N$ of civilizations in the Milky Way Galaxy with whom we could make contact. The terms in the equation are:  \begin{itemize}  %\item $N$ is the number of civilizations in the Milky Way Galaxy with whom we could make contact.  \item $R$ is the rate of star formation, which tells how many stars are born every year in our Galaxy.  \item $n_e$ is the average number of habitable planets in any planetary system  \end{itemize}  All the $f$ terms are factors $\le 1$:  \begin{itemize}  \item $f_p$ is the fraction of stars that have planets   \item $f_l$ is the fraction of planets hosting life  \item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_c$ is the fraction of intelligent life-forms that decide to communicate  \end{itemize}  Finally $L$ is the longevity of a communicative civilization (in years). Humankind, for example, has been "communicative" only for a few decades and we do not know for how long we will be.