The Drake Equation is an attempt to calculate the number of intelligent species in the universe (or galaxy) by evaluating certain “constants” that are thought to drive the development of life.

Here’s a quick rundown of the equation itself:

N = N* fp ne fl fi fc fL

N* represents the number of stars in the Galaxy
fp is the fraction of stars with planets
ne is the number of earthlike planets estimated per star with planets
fl is the fraction of earthlike planets where life begins
fi is the fraction of fl where intelligence evolves
fc is the fraction of fi that are detectable in some way (this is questionable, sometimes the equation is concerned with life we can find and that’s when this term becomes important.)
fL is fraction of the planet’s life between intelligenct life arising and dying.
When all of these variables are multiplied together when come up with: N, the number of detectable civilizations in the galaxy.

If you go to the website I got these definitions from, you can use a calculator to play with these values and set them as either likely or unlikely. One of the most important numbers is the lifespan of a civilization itself. If that is low, say a thousand years or so, then we are unlikely to find many intelligent alien lifeforms. If that is high, say a billion years or so, then it is much more likely. So if you are optimistic about human civilization then you’d expect there are a lot of others like us out there. If you are pessimistic, you may conclude that intelligent civilizations are brief cosmic flashes, slowly developing, then suddenly burning out in nuclear suicide.

I’m an optimist. So where are all the aliens?