Editing Openai/6946e998-79f0-8007-9a65-19828450838d (section)

===== Here we fold together: =====
* catastrophic late giant impacts,
* extreme atmospheric erosion episodes,
* nearby GRBs / SNe that permanently destroy habitability (as opposed to cause temporary extinctions), etc.

Life on Earth rode out multiple mass extinctions, so this factor is probably not tiny. Being pessimistic but not absurd:

: 

Now N = 10⁶ × 0.1 = 10⁵ physically excellent, long-lived, geologically active, undestroyed Earth-class planets over Galactic history.

Up to here, all filters are at 10% — quite harsh but still compatible with what little we know.