How SETI Works


If a signal is detected, there are a series of steps that follow to confirm that the signal is extraterrestrial:

  1. The radio telescope is moved off the target (off-axis) -- the signal should go away, and it should return when the telescope is pointed back to the target. This confirms that the signal is coming from the telescope's field of view.
  2. Known Earth or near-Earth sources, such as satellites, must be ruled out as originators of the signal.
  3. Known natural extraterrestrial sources, such as pulsars and quasars, must be ruled out.
  4. The signal must be confirmed by another radio telescope, preferably one on a different continent.

Once a signal has been confirmed, there are very specific steps that must be followed in the release of this information (see SETI Institute: Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence for details). The movie "Contact" has a good depiction of the detection of an ET signal and subsequent events.

What are the possibilities that we will find ET signals? To address this issue, astronomer Frank Drake introduced an equation to calculate the number of ET civilizations in the galaxy in 1961. The equation, now referred to as the Drake Equation, considers astronomical, biological and sociological factors in its estimates:


  • N - Number of communicative civilizations
  • R* - Average rate of formation of stars over the lifetime of the galaxy (10 to 40 per year)
  • fp - Fraction of those stars with planets (0 < fp<1, estimated at 0.5 or 50 percent)
  • ne - Average number of earth-type planets per planetary system (0 < ne<1, estimated at 0.5 or 50 percent)
  • fl - Fraction of those planets where life develops (0 < fl<1, estimated at 1 or 100 percent)
  • fi - Fraction of life that develops intelligence (0 < fi<1, estimated at 0.1 or 10 percent)
  • fc - Fraction of planets where intelligent life develops technology such as radio (0 < fc<1, estimated at 0.1or 10 percent)
  • L - Lifetime of the communicative civilization in years (estimates are highly variable, from hundreds to thousands of years, approximately 500 years for example purposes)

The fractions in the Drake equation have non-zero values between zero and 1. The first three terms on the right side of the equation are the astronomical terms. The next two are the biological terms. The final two are the sociological terms.

The Drake equation has been a guideline in SETI research. The value of N has been calculated to be anywhere from thousands to billions of civilizations in the galaxy, depending upon estimates for the other values.

If we use the estimates listed above, and decide R* equals 40 , then the drake equation becomes:

As you can see, the results of the Drake equation are highly dependent upon the values that you use, and values of N have been calculated at anywhere from 1 to in the thousands. Some aspects of SETI and general astronomical research have been devoted to gathering data for reliable estimates of the terms in the Drake equation, such as the number of extrasolar planets. See the Links section for more details on the Drake Equation.