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%\item How common are planets?  %\item How common are planets similar to the Earth (Earth-like exoplanets)?  %\end{itemize}  The wealth of data provided by Kepler has revealed an astonishing fact: "When you wish upon a star, you are wishing upon a star with planets" (W. Borucki). There is in average one planet orbiting every star in the Universe \citep{2013ApJ...764..105S,2012Natur.481..167C}. Just in our Galaxy this means we have 100 billions planets. Since we have about 100 billion galaxies in the Universe, there are about 10,000,000,000,000,000,000,000 = $10^{22}$ planets out there.   %To put it in the words of W. Borucki, Principal Investigator of the KEPLER satellite: "When you wish upon a star, you %are wishing upon a star with planets".      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 10 billion habitable planets just in the Galaxy, with the closest possibly "just" 12 light years away.\\   %$n_e \approx$ 0.2