deletions | additions       diff --git a/Astrobiology.tex b/Astrobiology.tex  index cd4b861..3fea4ec 100644  --- a/Astrobiology.tex  +++ b/Astrobiology.tex 
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%cradles for life as we know it are common in the universe or Next step is characterizing these worlds, and finding %trace of life.   $f_l$ \textbf{How likely is the emergence of life}?   If life was impossible nobody would know about it. Similarly the fact that we exist on Earth can not be used to draw conclusions on how probable is life in the Universe. Think about it:  Even if this probability was extremely low and life existed only on one planet in the Universe we would Universe, who's asking the question has  necessarily to  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 demonstrate that the emergence of life (biogenesis) does not require a very narrow, unlikely set of 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 this as a lottery with first prize being the appearence of life. Now if the emergence of life is 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 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. $f_l \ge 0.13$ \\