deletions | additions       diff --git a/Life Emergence.tex b/Life Emergence.tex  index c73f0b3..e20e79c 100644  --- a/Life Emergence.tex  +++ b/Life Emergence.tex 
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How can we estimate 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 life is in the Universe; if Earth were the only life-hosting planet in the cosmos, we would necessarily be living there.  However, finding just another place outside Earth where life can be supported changes everything. Evidence of life (even fossil) on Mars or on a moon 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 used to constrain $f_l$ is the rapidity of biogenesis on Earth. It is the following: imagine a lottery with life as first prize. If the emergence of life is a \textbf{very unlikely} outcome (requiring very specific conditions), then to win \textbf{one has to play many times} to get the winning ticket. If on the other hand winning the lottery is relatively easy (either many (many  winning tickets, or meaning that  many different combinations of environmental conditions 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 geologic timescales. Using a conservative upper limit of 600 million years for conditions to be "stable enough" for life to emerge, the probability of biogenesis on terrestrial (Earth-like) planets is constrained to those older than 1 billion years, greater than $13\%$ \cite{Lineweaver_Davis_2002}. That is, about 1 in 10 Earth-like planets in the habitable zone should develop life.  \begin{quote}