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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  ...