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One of the most important factors in the equation is $f_l$, the fraction of Earth-like planets in the habitable zone that develop life. It is basically a measure of the likelihood of \textbf{life emergence}.  Here I won't go into the (interesting) debate about defining what life is; I will use a somewhat restrictive definition of life: "life as we know it" which loosely means anything similar to what we've seen on Earth. One has to start somewhere.  How can we estimate this number? the likelihood of life emergence?  If life was impossible nobody would know about it. Similarly, the fact that we see life on Earth can not be used to draw conclusions on how widespread is life in the Universe\footnote{If Earth was the only life-hosting planet in the cosmos, we would necessarily have to be living there}. However, finding just another place outside the Earth where life can be supported changes everything. Finding traces of life (even fossil) on Mars or on one of the moons of Saturn or Jupiter would demonstrate that the emergence of life (biogenesis) does not require a very narrow, unlikely set of conditions.  I strongly believe we'll see proof of the existence of life in other regions of the solar system very soon. Until that moment an interesting argument that is used to constrain $f_l$ 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 a lottery with life as first prize. If the emergence of life is a very unlikely outcome (requiring very specific conditions), then to win the lottery one has to play many times, just because the winning ticket is one out of very many. If on the other hand winning the lottery is relatively easy (many winning tickets, or if you want many different combinations of the environmental conditions can lead to life) one needs to play just a few times before winning. It turns out that biogenesis on Earth was fairly rapid compared to geological times. Using a conservative upper limit of 600 million years required by life to emerge once the conditions were "stable enough", constrains the probability of biogenesis in terrestrial planets older than 1 billion years to be greater than $13\%$ \cite{Lineweaver_Davis_2002}. That is to say about 1 in 10 Earth-like planets in the habitable zone should develop life. $f_l \ge 0.13$ \\