It's a question humanity has pondered for centuries. People have throughout time reported seeing mysterious objects flying in the sky, sightings of little green men, and even being abducted. But is this just the stuff of imagination? Does life exist in outer space? Or are we really all alone?
Probably yes, but we don’t yet have proof of life beyond EarthShow moreShow less
People taking this view consider that in a Universe where there are enormous numbers of worlds like ours it is likely that life has arisen many times. But until we have evidence we don’t know for sure
Drake equation  attempts to compute the probability of alien life civilisation based on a number of factors.
According to Frank Drake the equation gives a probability . According to this equation is extremely unlikely that Earth is only place in Galaxy where a civilisation has been developed.
Further data on exoplanets allowed to refine some of Drake's equation parameters, 
Some of the parameter values of Drake equation are simply guesses, or can have a very high variance. Therefore the range of values of its solution are so large that can not be of practical usage