don't know if have been posted before but here goes.
"Is there intelligent life elsewhere in the universe? Can we begin to come up with an estimate of how many communicating civilisations there are in our galaxy? The Drake Equation attempts to do this":
N = R* x fp x ne x fl x fi x fc x L
where:
N = the number of communicating civilisations in the Milky Way galaxy
R* = rate of formation of 'suitable' stars like the sun
fp = the fraction of those stars with planets
ne = the number of Earth-like planets per planetary system
fl = fraction of those Earth-like planets where life actually develops
fi = the fraction of those planets with life where intelligent life evolves
fc = the fraction of planets where technological civilisations develop
L = the 'lifetime' of communicating civilisation
The current estimated solution for N is 10000 and we are one of them
http://www.activemind.com/Mysterious..._equation.html