Astronomy has nailed down the first three terms. The fraction of stars with planets (fp) is close to 1.0 — nearly all stars have planets. The average number of habitable-zone planets per star (ne) is estimated at 0.2 to 0.5 for Sun-like stars. Multiply these out and there are tens of billions of potentially habitable worlds in our galaxy alone.

The biological factors are where guesswork takes over. Does life emerge wherever conditions allow (fl = 1), or is it a freak accident (fl = 0.0001)? Does intelligent life always evolve given enough time, or is it vanishingly rare?