That’s true — but, you could say that comment seeding is a (possible) causal variable embedded in “fi”. Or, you could just guess.
fi = The fraction of life bearing planets on which intelligent life emerges.
And I can see your point being that the formation of any (solar) system probably follows a pattern vis-à-vis Oort clouds and the like.
comment s/b comet (do not ever rely on spell check)