The more we learn about planets inside and outside our Solar System, the more difficult the question "Well, where the hell are aliens then?" seems to become.
As well as finding a slew of planets in the habitable zones around their stars and locating new classes of exoplanets that may be good candidates for life, we have discovered life on Earth enduring extreme environments we hadn't thought possible. So where is everybody?
Among early attempts to answer this question, known as the Fermi Paradox , one astrophysicist tried to investigate how long it would take for one (or many) civilizations to colonize the galaxy. In 1974, Michael Hart's dramatically-titled paper " An Explanation for Absence of Extraterrestrials on Earth " argues that if a civilization tried, it would be able to colonize the galaxy in a relatively short time span.
"Assume that we eventually send expeditions to each of the 100 nearest stars. (These are all within 20 light-years of the Sun.) Each of these colonies has the potential of eventually sending out their own expeditions, and their colonies in turn can colonize, and so forth," Hart wrote .
"If there were no pause between trips, the frontier of space exploration would then lie roughly on the surface of a sphere whose radius was increasing at a speed of 0.10c. At that rate, most of our Galaxy would be traversed within 650,000 years. If we assume that the time between voyages is of the same order as the length of a single voyage, then the time needed to span the Galaxy will be roughly doubled. We see that if there were other advanced civilizations in our Galaxy they would have had ample time to reach us, unless they commenced space exploration less than 2 million years ago."
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