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$N \approx f_l \times f_i \times f_c \times L$  $f_l$ \textbf{How likely is the emergence of life}?   If life was impossible nobody would know it. Similarly the fact that we exist 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.   The cool thing is that, statistically speaking, finding just another place 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 change completely the picture, as it would demonstrate that the emergence of life (biogenesis) does not require extremely unlikely conditions. \\  For the time being 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 biogenesis 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, 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 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. \\  $f_l \ge 0.13$  %Having one data point, the Earth, doesn't give any statistical information: the solar system could well be an extremely %unlikely, or even unique, place in the Universe. As soon as you ask the question "Are we alone?" it means you are alive %and conscious, i.e. you are in a sweet-spot, and you can not make any claim about how common that might be. Beside the fact that one such place exists.   %Given what we know it could well be that the emergence of biological life was an incredibly fortuitous event that only %happened once, here on Earth. But note that finding just another instance of life (existing or fossil) on another %planet or satellite of our solar system would change completely the picture. That would demonstrate that life can %easily emerge when the conditions are approximately right, and it does not require some very special conditions. That's why the exploration of Mars or the moons of Saturn/Jupiter are of paramount importance.