The Drake Equation

Drake's Equation is an often quoted formula that is used to guesstimate the number of extraterrestrial civilizations that might be in existence at any one time. It combines eight (often arbitrary) elements from star formation to stages in life development. From this is derived a supposed number that represents how many advanced civilizations might exist in our galaxy right now.

These elements are:

• Average rate of star formation in our galaxy (R*)


• Fraction of those stars that have planets (f_p)


• Average number of planets that potentially support life per star that has planets (n_e)


• Fraction of those planets that actually develop life at some point (f_l)


• Fraction of those planets that then develops intelligent life (f_i)


• Fraction of those civilizations that develop technology that can and does send detectable signals of their existence into space (f_c)


• And, the length of time such civilizations remain detectable via their transmissions (L)

The formula this looks something like N = R* x f_p x n_e x f_l x f_i x f_c x L.

Here's an example of how to use the formula based on estimates of values for its variables.

• R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)


• f_p = 0.5 (half of all stars formed will have planets)


• n_e = 2 (stars with planets will have 2 planets capable of supporting life)


• f_l = 1 (100% of these planets will develop life)


• f_i = 0.01 (1% of which will be intelligent life)


• f_c = 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 civilizations with which there is possibility for us to currently communicate. This can be useful to provide rough guests as to the chances of how successful we can expect to be in our hunt for E.T.'s with programs like SETI.  SETI has a lot of equipment pointed into space looking for signals.

However, the formula is so full of guests, it may not have any value at all. In fact, even the elements of the formula are themselves guests. For example, one might question why star formation rate has any impact on how many planets may form with life. Also, given more modern understandings of our galaxy, such as the Rare Earth Hypothesis, many other factors may be more important that those used the Drake Equation. Additionally, there is a problem that many stars have been discovered to have Hot Jupiters, which would destroy any terrestrial worlds, thus preventing the opportunity for life to develop in that star system. Even worse, the question is raised about our own arrogance in the assumption that all life would resemble us enough to communicate in the same forms as we do.

It appears now that the Drake Equation is actually pseudo-science. It is not based on any hypothesis. It is nothing more than a series of guests. It does not produce anything that is testable. In fact, it's author, Dr. Frank Drake, didn't originally intend for this formula to be used in the way that is has been. It was meant to be an organizational tool for the discussion about intelligent life in the Universe specifically for a gathering called The Green Bank Meeting in 1960, so named for its location at the Green Bank Telescope.

Right now, it seems the best way to know how common life is in our galaxy is to explore it. The Drake Equation is more of a mathematical toy than an actual useful formula.