Reading Cocconi & Morrison (1959) again, I’m (as always) fascinated to see the seeds of the 1961 Drake Equation in the first paragraph of the seminal paper.

You do have to squint a little for some of them, I admit. The factors explained by the SETI Institute are:

  • R* = The rate of formation of stars suitable for the development of intelligent life.” The paper doesn’t bring up the rate of star formation, but does discuss suitability – “stars of the main sequence” “rather like the Sun”.
  • fp = The fraction of those stars with planetary systems.” This is explicitly mentioned at (1).
  • ne = The number of planets, per solar system, with an environment suitable for life.” Discusses the number of planets in the local “small set” that might support life.
  • fl = The fraction of suitable planets on which life actually appears.” This is explicitly mentioned at (2).
  • fi = The fraction of life bearing planets on which intelligent life emerges.” Takes a little squinting, but you can read this as implied by (3), along with the next factor.
  • fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.” Generally limiting myself to the first paragraph of the paper, but the second paragraph discusses why a society with “advanced scientific capabilities” would want to signal us.
  • L = The length of time such civilizations release detectable signals into space.” The question of lifetime as an unknown is explicitly raised, with the assumption being that they would patiently signal for very long periods. This is also expanded upon in the second paragraph.