# Current best-known Drake Equation values?

Frank Drake's equation is used to estimate $$N$$ the number of civilizations we might be able to communicate with in our galaxy:

$$N = R_\star f_p n_e f_l f_i f_c L$$

where:

$$R_\star$$ is the average rate of star formation in the galaxy

$$f_p$$ is the fraction of stars with planets

$$n_e$$ is the average number of goldilocks planets per planetary system

$$f_l$$ is the fraction of goldilocks planets that actually develop life

$$f_i$$ is the fraction of life bearing planets that develop intelligent life

$$f_c$$ is the fraction of civilizations that broadcast their existence into space

and $$L$$ is the length of time for which a civilization broadcasts their existence

## Question

Have the many exoplanets we've discovered in the last several decades improved our estimates of $$f_p$$, and $$n_e$$? Are these values different than what was guessed about prior to the discovery of the first exoplanets? What is our current best estimate for $$R_\star$$ for the Milky Way?

• Have you consulted wikipedia on this? It seems there is a long discussion section about current values. – AtmosphericPrisonEscape Jan 3 at 19:43
• @AtmosphericPrisonEscape Yeah, I'd like to see a little more detail about $f_p$ and $n_c$, particularly. Also it seems like the wikipedia entry may be a bit dated. A lot of the stuff is from 2011. – Connor Garcia Jan 3 at 19:59
• We collected a bunch of estimates in arxiv.org/abs/1806.02404 (see the supplementary files), showing their distributions, but for a deeper discussion see Vakoch, D. A., & Dowd, M. F. (Eds.). (2015). The Drake equation: estimating the prevalence of extraterrestrial life through the ages (Vol. 8). Cambridge University Press. – Anders Sandberg Jan 3 at 20:42

Absolutely! The quantity $$f_p n_e$$ is a quantity of much interest in investigations of extrasolar planets, which effectively equates to the fraction of stars that have a habitable planet in their star's habitable zone. Plenty of studies have examined this specifically for Earth-sized extrasolar planets and place the value of $$f_p n_e$$ between ~$$\frac{1}{6}$$ (i.e. one in six stars have an earth-sized planet in a relatively close orbit to the host star) and ~$$\frac{2}{5}$$ [1][2]. A 2013 comprehensive study by Petigura is a great paper to get familiar with the specifics and assumptions of how the Kepler surveys are conducted and the quantities of interest calculated [3].
Unfortunately the answer is: it varies. Roughly, we think that the Milky Way puts out about 3 solar masses worth of new star every year [4]. However, these don't have to be in one star. Since we think most stars are probably ~1 M$$_\odot$$, this works out to ~3 stars/year. However, some estimates go less, and some estimates (based on chemical decay of elements like aluminum) go as high as 7 stars/year [5].