The famous Drake Equation gives the number of intelligent civilizations able to communicate within our own galaxy.

N=R*fs*fp*ne*fl*fi*fc*L

R is the average rate of star formation in the galaxy, equal to about 20 stars per year (i.e. recent HST observations of M16 showing star formation sites).

f_s is the fraction of stars that are suitable suns for planetary systems, approximately 0.1 (most stars belong to spectral type M and are probably too cool to support habitable earthlike planets, whereas a few, such as O and B stars, are too short-lived). This leaves spectral types F, G and K as being the best candidate types suitable to support habitable earthlike planets. The circumstellar disk around Beta Pictoris provides significant evidence of planetary formation.

f_p is the number of suitable suns with planetary systems, now thought to be around 1/2 (e.g. 110 planetary candidates have been discovered as of to date), a value that has made Astronomers optimistic about the possibility of intelligent life elsewhere in the galaxy.

f_l is the fraction of such planets on which life actually originates (given enough time primative organic substances will be synthesized into DNA and RNA, e.g. the recent detection of the amino acid Glycene in the ISM by radio observations).

f_i represents the fraction of such planets on which, after the origin of life, some form of intelligence arises (evolution probably occurs wherever life arises, and intelligence has survival value.

f_c is the fraction of such intelligent species that develop the ability and desire to communicate with other civilizations (e.g. they construct radio telescopes or laser systems).

L is the mean lifetime (in years) of a communicative civilization, and because it depends on the NATURE of the race of intelligent beings, it is the most uncertain factor in Drake's Equation.

Enter your own values for the Drake Equation factors.

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