Drake Equation

A probabilistic expression proposed by Frank Drake in 1961 that is an interesting, though highly speculative, attempt to determine the number of advanced intelligent civilizations that might now exist in the Milky Way galaxy and be communicating (via radio waves) across interstellar distances. A basic assumption in Drake’s formulation is the principle of mediocrity - namely, that conditions in the solar system and even on Earth are nothing particularly special but rather represent common conditions found elsewhere in the galaxy.

Just where do we look among the billions of stars in our galaxy for possible interstellar radio messages or signals from extraterrestrial civilizations? That was one of the main questions addressed by the attendees of the Green Bank Conference on Extraterrestrial Intelligent Life held in November 1961 at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. One of the most significant and widely used results from this conference is the Drake equation (named after Frank Drake), which represents the first credible attempt to quantify the search for extraterrestrial intelligence (SETI). This “equation” has also been called the Sagan-Drake equation and the Green Bank equation in the SETI literature. Although more nearly a subjective statement of probabilities than a true scientific equation, the Drake equation attempts to express the number (N) of advanced intelligent civilizations that might be communicating across interstellar distances at this time. As previously mentioned, a basic assumption inherent in this formulation is the principle of mediocrity. The Drake equation is generally expressed as

N = R* f_p n_e f_l f_i f_c L

Where

• N is the number of intelligent communicating civilizations in the galaxy at present
• R* is the average rate of star formation in our galaxy (stars/year)
• f_p is the fraction of stars that have planetary companions
• n_e is the number of planets per planet-bearing star that have suitable ecospheres (that is, the environmental conditions necessary to support the chemical evolution of life)
• f_l is the fraction of planets with suitable ecospheres on which life actually starts
• f_i is the fraction of planetary life starts that eventually evolve into intelligent life-forms
• f_c is the fraction of intelligent civilizations that attempt interstellar communication
• L is the average lifetime (in years) of technically advanced civilizations

An inspection of the Drake equation quickly reveals that the major terms cover many disciplines and vary in technical content from numbers that are somewhat quantifiable (such as R*) to those that are completely subjective (such as L).

If we assume that N is about 10 million (a very optimistic Drake equation output), then the average distance between intelligent, communicating civilizations in our galaxy is approximately 100 light-years. If N is 100,000, then these extraterrestrial civilizations on average would be about 1,000 light-years apart. But if there were only 1,000 such civilizations existing today, then they would typically be some 10,000 light-years apart. Consequently, even if the Milky Way galaxy does contain a few such civilizations, they may be just too far apart to achieve communication within the lifetimes of their respective civilizations.

Questions to Ponder

• What is the purpose of SETI?
• What is the Zoo hypothesis?