deletions | additions       diff --git a/We_are_thinking_creatures_living__.tex b/We_are_thinking_creatures_living__.tex  index 4d8ca47..e64d8e0 100644  --- a/We_are_thinking_creatures_living__.tex  +++ b/We_are_thinking_creatures_living__.tex 
...
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 is about 13.7 billion years old. We \href{https://www.authorea.com/10997}{just learned} that there are about 1~000~000~000~000~000~000~000 = $10^{21}$ planets potentially similar to the Earth in the cosmos, a number larger than 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 on every beach and every desert on Earth. \textbf{ Are we alone?}  To answer this question in 1961 scientist Frank Drake formulated his famous equation, which I discussed in the \href{https://www.authorea.com/11048}{previous post} of this series. The Drake equation calculates the number $N$ of communicative civilizations in our Galaxy. In its 2015 form it reads:  \begin{quote}  $N \approx 2\, f_l \, f_i \, f_c \, L$ \end{quote}