deletions | additions       diff --git a/Let_s_recap_we_are__.tex b/Let_s_recap_we_are__.tex  index c0668c9..34fc239 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. 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} to see what it looks like. for more details.  This equation allows to calculate the number of communicative civilizations we might be able to contact. contact and we were able to rewrite it in this simple form:  \begin{equation}\nonumber  \boxed{N \approx 2\, f_l \, f_i \, f_c \, L}  \end{equation}\label{Drake_simplified}