deletions | additions       diff --git a/Let_s_recap_we_are__.tex b/Let_s_recap_we_are__.tex  index 8980d26..fb6e975 100644  --- a/Let_s_recap_we_are__.tex  +++ b/Let_s_recap_we_are__.tex 
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Let's recap: we are thinking creatures living on a planet orbiting a pretty common star in a pretty common galaxy. Our home planet has been around for about 4.5 billion years, while the Universe has about 13.7 billion years. We \href{https://www.authorea.com/10997}{just learned} that there are some other 1~000~000~000~000~000~000~000 = $10^{21}$ planets potentially similar to the Earth in the cosmos, a number larger that the amount of \href{http://www.npr.org/blogs/krulwich/2012/09/17/161096233/which-is-greater-the-number-of-sand-grains-on-earth-or-stars-in-the-sky}{ grains of sand} found in every beach and every desert on Earth.\\  \textbf{ Are we alone?}\\  To answer this question the scientist Frank Drake came up with his famous equation in 1961, have a look at \href{https://www.authorea.com/11048}{previous post} for more details. This equation allows to calculate the number $N$ of communicative civilizations we might be able to contact andwe were able to rewrite it  in this simple form: its 2015 form reads:  \begin{equation}\nonumber  N \approx 2\, f_l \, f_i \, f_c \, L  \end{equation}\label{Drake_simplified}