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\textbf{Previous \href{https://www.authorea.com/users/2/articles/11048/_show_article}{Drake Equation} \href{https://www.authorea.com/users/2/articles/11048/_show_article}{"Drake Equation"}  -- Next "\href{https://www.authorea.com/users/2/articles/25815/_show_article}{Fermi Paradox}"} \\ 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}