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There is on average one planet orbiting every star in the Universe \citep{2013ApJ...764..105S, 2012Natur.481..167C}. If this comes as an exciting news, you might wanna have a look at the \href{https://www.authorea.com/10997/}{previous post} in this series. Our Galaxy (the Milky Way) is an immense disk of gas and stars with a diameter of about 100 000 light years, hosting about 100 billions stars and therefore also about 100 billions planets. Take a deep breath.  Now, it turns out the Milky Way it's just one of the 100 billion galaxies that populate our Universe, a colossal expanding stretch of spacetime with an age of 13.7 billions years.  Therefore the math is simple: There are about 10,000,000,000,000,000,000,000 = $10^{22}$ planets out there.   This number is extremely large, apparently larger than the\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}{ number of grains of sand} found in every beach and every desert on Earth.  But how many of these planets host life? And in particular,\textbf{ how many planets host intelligent life we might be able to communicate with}?   In order to estimate the number of technological civilizations that might exist among the stars, in 1961 Frank Drake proposed the following simple equation:   %$N = R \times f_p \times n_e \times f_l \times f_i \times f_c \times L$  diff --git a/layout.md b/layout.md  index 3fcbf89..dedb4a5 100644  --- a/layout.md  +++ b/layout.md 
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Equation.tex  figures/Drake3/Drake_Annotated.jpg  Updating Drake Eq.tex  Astrobiology.tex