deletions | additions       diff --git a/Intro.tex b/Intro.tex  index 75ba9b2..0f6ac5b 100644  --- a/Intro.tex  +++ b/Intro.tex 
...
  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 catalyst for biological life on Earth (see \href{http://io9.com/5728932/why-do-we-look-for-water-when-searching-for-extraterrestrial-life}{this article} for a nice discussion on the topic). \\  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. Meaning Galaxy, with  the closest could be within being "just"  12 light years.\\ years away.\\  %$n_e \approx$ 0.2