The Drake Equation Estimated the Scope of the SETI Project


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Drake Equation and Seager Equation : Image by Mike DeHaan

Calculating Probability of Aliens – A Different Approach

Dr. Sara Seager suggests a different approach. Rather than listening for radio signals, she proposes searching for biosignature gasses in the atmospheres of exoplanets.

The Earth’s atmosphere consists of about 20% oxygen; but without photosynthesis, that oxygen would react with other chemicals and leave the air. An alien astronomer should be able to detect the oxygen and deduce that a biological process maintains the oxygen level.

Dr. Seager’s equation is “N = N* * FQ * FHZ * FO * FL * FS“, where

  • N is the number of planets with detectable biosignature gasses. (Drake’s ‘N’ is quite different: the number of planets with detectable radio  transmissions).
  • N* is the number of stars in the sample astronomers can examine. [Dr. Seager’s working estimate is 30,000]
  • FQ is the fraction of quiet stars in the sample of stars. (Active stars emit so much radiation that life would be precarious); [0.2]
  • FHZ is the fraction of planets in the habitable zone of quiet stars; [0.15]
  • FO is the fraction of observable star systems (where we can detect an atmosphere as a planet transits across the star); [0.01 is ten times Dr. Seager’s estimate]
  • FL is the fraction of planets that actually have life if they have biosignature gasses; [1]
  • FS is the fraction of planets with detectable spectographic signatures (where we can detect an atmosphere as a planet transits across the star) [0.5]


My calculation shows “N = 4.5” based on the above numbers, although A New Equation… states that Dr. Seager is working with “N = 2”.

The image above compares the Drake Equation with the Seager Equation.

The Value of the Drake Equation

The Drake Equation, and others like it, are valuable scientific tools even when their parameters are uncertain.

The first benefit is to clarify the goal. Dr. Drake sought “alien civilizations that transmit detectable radio signals,” while Dr. Seager wants to detect the biosignature gasses of exoplanets. These goals are more precise than “seeking extraterrestrial life” – without first deciding “what is extraterrestrial life”.

Secondly, such equations list important components, which may support finding new factors or eliminating those not required.

Third, separate projects may focus on different components. SETI listens for radio signals. Meanwhile other astronomers will improve the estimate of ‘fp‘, the fraction of stars with planets, by discovering new exoplanets.

In short, the Drake Equation applies mathematics to increase the chances of successful discovery despite uncertainty.


Dvorsky, George. A New Equation Reveals Our Exact Odds of Finding Alien Life. (2013). IO9. Accessed June 25, 2013.

Seager, Prof. Sara. Biography. (2013). MIT. Accessed June 25, 2013.

Cain, Fraser and Gay, Dr. Pamela. The Drake Equation. (2007). Astronomy Cast.  PDF accessed June 25, 2013.

Christian, Eric and Safi-Harb, Samar. How Large is the Milky Way? (1998). NASA. Accessed June 25, 2013.

Ford, Steve. What is the Drake Equation? (1995). SETI League. Accessed June 25, 2013.

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