There are 400 billion stars in our galaxy, and astronomers estimate that each has 1.6 planets on average, bringing the total number of planets within the range of 660 billion.  The number of potentially habitable worlds differs according to the source, but most say the Milky Way may have over 60 billion habitable planets.  Based on those huge numbers alone, it seems improbable that we’re alone in space…

Therein lies Fermi’s Paradox:  If other forms of intelligent life exist, why haven’t we found any evidence yet?  Here are 10 possible solutions to the Fermi Paradox (click for a larger, more legible version):

Fermi Paradox Solutions

Source:  From Quarks to Quasars (Jaime Trosper)