\section{How common are planets?}   We now know that, in average, there is one planet orbiting every star in the Universe.   Just in our Galaxy this means we have 100 billions planets. Since we have about 100 billion galaxies in the Universe, there are 10,000,000,000,000,000,000,000 $\approx 10^{22}$ 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" \citep{2013ApJ...764..105S,2012Natur.481..167C} 