The Drake Equation.  This is cool.  The Drake Equation provides an approximate number of planets, N, with intelligent life (communicative civilizations).  It is , of course, only as accurate as the estimates of the variables.  There’s nothing magic about this equation…take a look at the definition of the factors and you will see it is all pretty logical.

N = R* f_p n_e f_l f_i f_c L

where,

 N = The number of communicative civilizations 

R* = The rate of formation of suitable stars (stars such as our Sun).  In the Milky Way, 100 billion stars.

f_p = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun.)  Say …¼, 1 in 4.

n_e = The number of Earth-like worlds per planetary system.  Maybe… ½,  1 earth-like planet in every 2 planetary systems.

f_l = The fraction of those Earth-like planets where life actually develops.  Maybe …. ⅛, 1 in 8.

f_i = The fraction of life sites where intelligence develops.  Say …. ¼, 1 in 4.

f_c = The fraction of intelligence planets which are  communicative (those on which electromagnetic communications technology develops).  Many scientists will argue that this is a large fraction ….maybe ½,  1 in 2.

L = The “lifetime” of communicating civilizations as related to the lifetime of the solar system they are a apart of.  Using  Earth as an example this would be  maybe 40 years out of 4 billion, or 1 over 100 million. (That is to say, Earth has been capable of communicating extra-terrestrial for 40 years.)

Given the numbers in green above, N would equal 2,500 communicative civilizations for the Milky Way.  Of course, any other considered estimates would be just as valid, given our present knowledge of the Milky Way.  However, this exercise allows us to more readily appreciate the sensitivity for the existence of life in our galaxy.

Note Well!  This gives the odds of finding a civilization which has developed far enough to communicate with us.  On the other hand maybe we just want to know the odds of finding a caveman civilization.  Variables f_c and L would be re-defined to reflect the estimated odds of that case .