The Drake Equation, Counting the Stars

The current state of UFOlogy notwithstanding, it’s an awe inspiring thing to look into the night sky and ponder the question that haunts so many…is there life elsewhere in the universe?

It has been a question that has weighed on my mind for many years, and one that has occupied the intellect of some of our civilizations most celebrated geniuses.  To say that I stand on the shoulders of giants would be a monumental understatement, but nonetheless I offer you commentary based on the intelligence of those who came before me.

The genius I refer to is the Drake Equation (also known as the Greenbank Equation or the Greenbank Formula).  The equation bears the name of its originator, Frank Drake, one time Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz.  The anecdotally fulfilling story of how and when Drake came up with the formula is an aside to the real value that the formula offers to those who search for signs of such extraterrestrial life, though it is worth telling.

In 1960, Drake conducted the first search for radio signals from extraterrestrial civilizations at the National Radio Astronomy Observatory in Green Bank, West Virginia. Soon thereafter, the National Academy of Sciences asked Drake to convene a meeting on detecting extraterrestrial intelligence. The meeting was held at the Green Bank facility in 1961. The equation arose out of his preparations for the meeting:

As I planned the meeting, I realized a few days 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. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.” –Frank Drake[1]

The equation allows one to work out how many intelligent civilizations are likely to evolve within the Milky Way galaxy.  It uses a number of values that are, shall we say best guesses at most, such as the number of stars that are created in a given period, or the number of habitable planets there are at any given time.

The Drake equation states that:

N= R* x Fp x Ne x Fl x Fi x Fc x L


  N = the number of civilizations in our galaxy with which communication might be possible;


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

Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were:

  • R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
  • fp = 0.5 (half of all stars formed will have planets)
  • ne = 2 (stars with planets will have 2 planets capable of developing life)
  • fl = 1 (100% of these planets will develop life)
  • fi = 0.01 (1% of which will be intelligent life)
  • fc = 0.01 (1% of which will be able to communicate)
  • L = 10,000 years (which will last 10,000 years)

Drake’s values give N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.

Little consensus can be found for the values above, while R* is based on a considerable amount of astronomical data and is the least argued value, others, such as ne and fl are hotly debated.  As shown above, Drake came to the conclusion that there should be 10 intelligent civilizations with the capability of communicating within the Milky Way galaxy (via radio waves etc).  The one factor that the Drake Equation does not address is when these civilizations might exist, which is the chief criticism of the formula.

Critics of the formula cite that many of the values are soaked in anthropic bias, or in common terms, they are based on information that is true about Earth, but may not be true of other planets.  Over the years various other people have worked the Drake Equation using different values, whether based on scientific data (i.e. geology, astronomy and anthropology) or even just intuition, ultimately causing great divergence among their individual values for N, providing for between 0.000065 to 20,000 communicative civilizations.

Ultimately the critics are correct, while the equation may provide an invigorating thought experiment; it does not provide any real answer to the question that preceded it.  Since most of its values are little more than guesses, the resulting expression is worthless.

Or as noted by science fiction author Michael Crichton:

“The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. As a result, the Drake equation can have any value from “billions and billions” to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless…”

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