One Equation - One or Many Worlds?

Nearly 60 years ago, Francis Drake, introduced an equation to estimate the number of communitive extraterrestrial civilizations in the galaxy. A graphic from the SETI institute, shows what is known as the Drake Equation:

So, how many worlds are out there besides Earth where there are civilizations with whom we could communicate? There are widely varying estimates for each of the seven factors, R*, fp, Ne, fl, fi, fc and L, so estimates of N range from near-zero to many million.

N = The number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable.

R_* = The rate of formation of stars suitable for the development of intelligent life.

f_p = The fraction of those stars with planetary systems.

n_e = The number of planets, per solar system, with an environment suitable for life.

f_l = The fraction of suitable planets on which life actually appears.

f_i = The fraction of life bearing planets on which intelligent life emerges.

f_c = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.

L = The length of time such civilizations release detectable signals into space.

Since the development of the equation, there have been improvements is estimating some of the factors. 

The combination of f_p  and n_e  produce the number of planets suitable for life per star system. Sixty years ago, the detection planets outside our solar system was just an idea and some believed our type of planet would be very rare. Now, with several methods exist for detecting exoplanets and over 4,000 have been detected.

Another factor that has increased in the last sixty years is fl. At one time it was reasonable to assume that there is a very narrow window for conditions suitable for life. Such belief was based on our own narrow experience. While we cannot yet detect life on other planets, as we explore our own planet, we find life everywhere and we find it in conditions we once thought too harsh to host life. The entry Extremophile lists such life forms. Just a sampling include Pyrolobus fumarii that live in submarine hydrothermal vents at temperatures exceeding 110 °C, Synechococcus that live in ice at temperatures below -20 °C, and Chroococcidiopsis that live in deserts under extreme desiccation. Perhaps not an extremeophile, the first animal not requiring oxygen was recently discovered.

L, the length of time civilizations release detectable signals into space, can also be considered the lifetime of a civilization. At  the time of the introduction of the Drake Equation, we were in a Cold War. Early in our technological development, we had the power to destroy our own civilization. Many thought we would not survive to the year 2000. We survived the Cold War but other threats to civilization exist. Carl Sagan theorized that our technological development is so recent in our species existence, that any other civilization we contact is likely much more advanced than our own.

5-19-2020 Update: recently a researcher proposed a lower value for the factor, fi.

Also see: Rogue Planets.

(Graphic and equation from the SITI Institute - permission not received)