N = R* * fp *ne * fl * fi * fc * L
Straight outta Wikipedia:
- R* = the average rate of star formation per year in our galaxy
- fp = the fraction of those stars that have planets
- ne = the average number of planets that can potentially support life per star that has planets
- fℓ = the fraction of the above that actually go on to develop life at some point
- fi = the fraction of the above that actually go on to develop intelligent life
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L = the length of time for which such civilizations release detectable signals into space
But the story doesn't end there. Notice that term three includes the phrase "that can potentially support life." Our best guess is that this requires liquid water and temperatures between 0 and 100C. Our system does this but the other systems don't look like our system:
"... according to Johnson ... our solar system is extremely rare. 'It's just a weirdo,' he says."
Of course, these other systems are around cooler stars, and so the liquid-water zone will be closer to the planet, and we know at least some planets are in the right zone. So there's hope yet for alien life, but it's worth noting that we live in a weird solar system (on galactic terms). How necessary is the weirdness? Is it possible that being too close to a cooler star could doom the prospects of life for some reason, even if liquid water persists? (I'm thinking radiation damage may be greater closer in?)
At any rate, we're closer to getting more parameters fixed but I'm not sure how much closer to solving the Drake equation we actually are. The bootom line is that our solar system appears exceptional -- but is it unique? Still don't know.