Sciency Words A to Z: The Drake Equation

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, D is for:


In 1961, American astronomer Frank Drake proved that alien life exists.  He didn’t do this with a telescope or by analyzing a Martian meteorite. No, Frank Drake proved it with math, pure and simple.  Or at least that’s the impression some people seem to get when they first hear about the Drake equation.

The Drake equation was first presented in 1961 at a conference held at the Green Bank Telescope in West Virginia. Only ten people were in the audience when Drake gave his presentation (one of those ten people, by the way, was a young Carl Sagan).  And the topic to be discussed at this conference: a new and highly controversial idea called SETI.

In this article from Universe Today, Drake is quoted explaining what inspired his equation:

As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life.  And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy.

After reading All These Worlds Are Yours by Jon Willis, I’ve come to think of the Drake equation as a to-do list for astrobiologists.

N = R* · fp · ne · fl · fi · fc · L
  • Figure out how many stars are born in our galaxy per year (R*).
  • Figure out how many of those stars have planets (fp).
  • Figure out how many of those planets could support life (ne).
  • Figure out how many planets that could support life actually do (fl).
  • Figure out how often life evolves into intelligent life (fi).
  • Figure out how often intelligent life develops radio communications that we could detect (fc).
  • Figure out how long the average intelligent civilization keeps its radio equipment working (L).

Like I said, it’s a to-do list.  It’s presented in the form of an equation because… well, you know… scientists.

At this point, we have a pretty good feel for the first two variables in the Drake equation.  As stated in this article from Astronomy Magazine, 1.5 to 3 new stars are born per year in our galaxy, and each star has at least one planet, on average.  Current and upcoming missions should start to pin down real numbers for the number of planets that could potentially support life.

Beyond that, those questions do get progressively harder, but astrobiologists are steadily working their way down their to-do list—or rather, they’re working their way through the equation, starting from the left and heading to the right.  Answers are coming, slowly but surely.

Next time on Sciency Words A to Z, when astrobiologists talk about Earth-like planets, what exactly does that mean?