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

=== Assistant: I’ll do exactly what you asked: start from a very pessimistic total number of Earth-ish HZ planets over the Milky Way’s… ===
I’ll do exactly what you asked: start from a very pessimistic total number of Earth-ish HZ planets over the Milky Way’s history, then multiply by a stack of “bad-luck” filters with deliberately low probabilities.

I’ll keep the math explicit.

==== From earlier, using: ====
* total suitable stars ever formed in the Galaxy ~ a few × 10¹¹
* extremely conservative η⊕ ≈ 0.01 (only 1 Earth-size HZ planet per 100 stars)

you get an order-of-magnitude:

: 

That’s already a pessimistic baseline.

==== ### ====

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.

==== Here we’re in almost totally unconstrained territory; I’ll choose deliberately small numbers, but not so small that our own existence becomes paradoxical. ====

===== This is what people sometimes call f_life. We know life on Earth appeared relatively early (≲ few hundred Myr after oceans formed), which some argue is weak evidence that abiogenesis on good planets is not astronomically rare.PubMed Central<ref>{{cite web|title=PubMed Central|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC7474626/|publisher=PubMed Central|access-date=2025-12-21}}</ref> =====

Still, you asked for pessimistic, so:

: 

Now N = 10⁵ × 0.01 = 10³ life-bearing worlds.

===== Call this f_intel: going from microbes to something roughly human-level (or better) over multi-Gyr. =====

We have no data other than Earth, where:
* complex multicellularity took ~3 Gyr to appear,
* technological intelligence appeared only once (as far as we know).

So I’ll again pick a small number:

: 

Now N = 10³ × 0.01 = 10.

==== Using: ====
* N₀ = 10⁹ “Earth-ish HZ” planets ever in the Milky Way (already pessimistic), and
* Six pessimistic filters

fclimate=0.1,  fvolatiles=0.1,  fplate=0.1,  fsurvive=0.1,  flife=0.01,  fintel=0.01f_{\text{climate}} = 0.1,\;
f_{\text{volatiles}} = 0.1,\;
f_{\text{plate}} = 0.1,\;
f_{\text{survive}} = 0.1,\;
f_{\text{life}} = 0.01,\;
f_{\text{intel}} = 0.01fclimate=0.1,fvolatiles=0.1,fplate=0.1,fsurvive=0.1,flife=0.01,fintel=0.01
you get:

Nintel, ever≈109×(0.1)4×(0.01)2≈10N_{\text{intel, ever}} \approx 10^9 \times (0.1)^4 \times (0.01)^2 \approx 10Nintel, ever≈109×(0.1)4×(0.01)2≈10
: 

A few important notes:
* Every single factor I chose is intentionally on the pessimistic side of what current literature and simple reasoning would support.
* If any of these fractions is higher by even an order of magnitude, N jumps rapidly (e.g., making f_life or f_intel = 0.1 instead of 0.01 boosts N by ×10 each).
* If you pushed one or two factors much lower (e.g., f_life = 10⁻⁴), you quickly drive N ≪ 1, which conflicts with the fact that at least one such world (ours) clearly exists—anthropic selection then forces you to revisit the assumptions.

So, under a stack of quite harsh assumptions that still respect basic empirical constraints, a reasonable “pessimistic but not absurd” answer is:

>