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$N \approx f_l \times f_i \times f_c \times L$  The first interesting term to analyse is $f_l$. Given the presence of the right conditions, how \textbf{How  likely is life to emerge? the emergence of life}?  The problem is that if life was impossible nobody would know it and at the same time our existence on Earth can not be used to draw conclusions on how probable is life in the Universe. Even if this probability was extremely low and life existed only on one planet in the Universe we would necessarily be on that planet.  For the time being an interesting argument that is used to contrain $f_l$ is the rapidity of biogenesis on Earth. That is how long it took for life to emerge once the conditions at the surface of our planet were "stable" enough. The argument is the following: imagine the emergence of life as a lottery. The appearence of life corresponds to winning such lottery. Now if life is very unlikely, then to win the lottery one has to play many times. That is one requires a very long time before the conditions are just right. If on the other hand winning the lottery is relatively easy (many winning tickets, translating into environmental conditions that are not too restrictive) one needs to play just a few times before winning. It turns out that biogenesis on Earth was fairly rapid. Using a conservative upper limit of 600 million years allows to constrain the probability for biogenesis in terrestrial planets older than 1 billion years to be greater than $13\%$ \cite{Lineweaver_Davis_2002}. \\