deletions | additions       diff --git a/Earth-like planets.tex b/Earth-like planets.tex  index 2843c8d..3a7d9be 100644  --- a/Earth-like planets.tex  +++ b/Earth-like planets.tex 
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This is fascinating, but still it does not tell much about the likelihood of extraterrestrial life. Planets like Jupiter or Mercury are not expected to host life due to their extreme surface conditions. This could well be the rule in the Universe, with Earth being just a lucky shot of nature. The question is then how common are planets that can likely support biological life.  A possible requirement is having a rocky planet in the so called Goldilocks or habitable zone, a range of distances from the host star where surface temperatures are just about right for liquid water to be present. This might well be a restrictive definition of habitability, as life forms might thrive in very different environments from what we are used to. But one has to start somewhere, and liquid water seems to have played a decisive role as a \href{http://www.universetoday.com/82316/astronomy-without-a-telescope-why-water/}{catalyst for biological life on Earth}. \\  And here is where it gets very interesting, as just recently we learned that Earth-like planets are indeed very common. Statistically speaking at least 1 in 5 planets around Sun-like stars could potentially support life \cite{Petigura_Howard_Marcy_2013}. And Sun-like stars are extremely common, resulting in about \textbf{20 billion habitable planets just in the Galaxy}, with the closest possibly "just" 12 light years away.\\   Given the stellar density in the solar neighborhood ($\sim0.14$ pc$^{-3}$, where 1 pc equals 3.26 light years) and $\sim$80 years of TV broadcasting this implies human-made electromagnetic signals have reached $\sim$2100 $\sim$1700  planets potentially suitable for life. The rumor is out we are here (and have some terrible tv channels). %$n_e \approx$ 0.2  \textbf{Next post: \href{https://www.authorea.com/11048}{The Drake Equation}}