The Drake Equation

The famous Drake Equation gives the number of intelligent civilizations able to communicate within our own galaxy. Theoretical or otherwise, its makes an awful lot of sense. As was once said in the film 'Contact', "If there is nobody else out there, its an awful waste of space".


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).

fs 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 small, whereas a few, such as O and B stars, are too short-lived). The circumstellar disk around Beta Pictoris is evidence of planetary formation.

fp is the number of suitable suns with planetary systems, now thought to be around 1/2 (i.e. 51 Pegasus discovery), a value that has made scientists optimistic about the possibility of intelligent life elsewhere in the galaxy

ne is the mean number of planets that are located within the Continuously Habitable Zone (assuming liquid water is necessary for life)

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

fi 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

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

L is the mean lifetime (in years) of a communicative civilization (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.

since this first appeared planetary discoveries are 'falling out of the sky' - I wonder how accurate this equation is now we know about so many planets.