It's funny, how many conflicting, shaky arguments there are for that sort of thing. Enrico Fermi was a famous was a very famous physicist who invented a style of solving complicated problems with estimation which are now called "Fermi problems." The premise is that you break your impossible question up into the product of as many easier-to-estimate problems as possible, and let probability (assuming you overestimate some parameters and underestimate others) take over. We used them in class to estimate things like "how many piano tuners are in the city of Chicago?" -- things we could look up easily enough... and our estimates were surprisingly close. In general (depending on how many parameters the problem has, of course), if you can estimate each of the parameters to within a factor of 10 or so, then your solution will be accurate to within a factor of 4. At least, that's what we were told, and it seemed to work that way.It just goes to show that the likely hood of any alien race visiting our planet is slim to none.