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

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Things that hurt you: slowly brightening star, HZ drift, stellar variability, CO₂ drawdown, etc. Many nominal HZ planets will only be clement for 10⁷–10⁸ yr, not billions.astrobiology.nasa.gov<ref>{{cite web|title=astrobiology.nasa.gov|url=https://astrobiology.nasa.gov/nai/annual-reports/2015/vpl-uw/stellar-effects-on-planetary-habitability-and-the-limits-of-the-habitable-zone/index.html|publisher=astrobiology.nasa.gov|access-date=2025-12-21}}</ref>

Pessimistic guess:

: 

Now N = N₀ × f_climate = 10⁹ × 0.1 = 10⁸.

===== Issues: =====
* Some fraction of small planets are volatile-rich mini-Neptunes or deep waterworlds.Centauri Dreams<ref>{{cite web|title=Centauri Dreams|url=https://www.centauri-dreams.org/2018/08/20/the-prevalence-of-water-worlds/|publisher=Centauri Dreams|date=2018-08-20|access-date=2025-12-21}}</ref>
* Others may be very dry or lose water early (especially around M dwarfs).PubMed<ref>{{cite web|title=PubMed|url=https://pubmed.ncbi.nlm.nih.gov/25629240/|publisher=pubmed.ncbi.nlm.nih.gov|access-date=2025-12-21}}</ref>
* Models of HZ exoplanets show a broad distribution of water mass fractions, with substantial tails toward both very wet and very dry.A&A<ref>{{cite web|title=A&A|url=https://www.aanda.org/articles/aa/full_html/2024/01/aa46988-23/aa46988-23.html|publisher=aanda.org|access-date=2025-12-21}}</ref>

Pessimistic, “Goldilocks volatiles” fraction:

: 

Now N = 10⁸ × 0.1 = 10⁷.

===== Plate tectonics (or some robust analogue) appears important to: =====
* recycle carbon, avoid runaway greenhouse/snowball, and keep nutrients moving.Wikipedia<ref>{{cite web|title=Wikipedia|url=https://en.wikipedia.org/wiki/Geodynamics_of_terrestrial_exoplanets|publisher=en.wikipedia.org|access-date=2025-12-21}}</ref>

We don’t know how often plate tectonics happens on Earth-mass planets; simulations disagree, and some work suggests stagnant-lid planets can be habitable but likely with narrower margins.A&A<ref>{{cite web|title=A&A|url=https://www.aanda.org/articles/aa/full_html/2017/09/aa30728-17/aa30728-17.html|publisher=aanda.org|access-date=2025-12-21}}</ref>

To be very pessimistic, assume Earth-like plate tectonics (or equally effective cycling) is rare:

: 

Now N = 10⁷ × 0.1 = 10⁶.

===== 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.