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It all started in the mid '90s with the first discovery of new worlds around other stars. The term "Extrasolar planet" (or Exoplanet) became widely used to identify planets\footnote{A planet is a celestial body massive enough to be bounded by its self gravity (unlike a rock or an asteroid, that are kept together by electromagnetic forces), but not massive enough to produce energy through nuclear fusion (as stars do)} orbiting a star other than the Sun.   %I remember hearing about the discovery of 51 Pegasi b, a giant planet about 50 light years away, on a bus ride to school. I was blown away. That night I could not sleep, my mind racing towards the amazing implications of that discovery. Little I knew that was just the beginning.   As of today\footnote{6 February 2014, check \href{http://exoplanets.org/}{exoplanets.org} for the most recent figure}, plantary scientists have confirmed the existence of 765 exoplanets and have identified an additional 3466 exoplanet candidates that requires more investigation before they can join the planet club. The most remarkable discoveries came only in the last couple of years thanks to the \href{http://kepler.nasa.gov/}{KEPLER} space telescope. This amazing instrument has been patiently looking for the extremely tiny dimming induced by the passage of a planet in front of its host star, star.   %,  and helped to answer two fundamental questions: \begin{itemize}   \item %\begin{itemize}   %\item  How common are planets? \item %\item  How common are planets similar to the Earth (Earth-like exoplanets)? \end{itemize} %\end{itemize}   The wealth of data provided by Kepler has revealed an astonishing fact: there is 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 too much about the likelihood of extraterrestrial life. For example planests like Jupiter or Mercury are not expected to host life due to their extreme surfae conditions. But it could well be that this is the rule in the Universe and that Earth, with its mild average surface temperature was just a lucky shot of nature. Something that only happened, say, once.     How common are planets that can likely support biological life as we know it?   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 catalyst for biological life on Earth.   And just very recently we learned that Earth-like planets are indeed very common. Statistically speaking 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 of habitable planets just in the Galaxy and the closest to us potentially within 12 light years.\\ %$n_e \approx$ 0.2