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

==== To get from “HZ + Earth-size” to “actually suitable for the evolution of complex or intelligent life,” you need to introduce extra filters. Schematically: ====

Nintel≈N∗×η⊕×fEarthlike×flong-term×flife×fintelN_{\text{intel}} \approx N_* \times \eta_\oplus \times f_{\text{Earthlike}} \times f_{\text{long-term}} \times f_{\text{life}} \times f_{\text{intel}}Nintel≈N∗×η⊕×fEarthlike×flong-term×flife×fintel
Where:
'' N_'' = number of stars (≈ 10¹¹ in the Milky Way).
* η⊕ = HZ, roughly Earth-size planets per star (≈ 0.1–0.5).
* f_Earthlike = fraction of those HZ rocky planets that have Earth-like bulk properties and early climate (mass ~0.5–2 M⊕, not mini-Neptunes, right volatile inventory, not tidally sterilized, not deep in the Venus Zone, etc.).
* f_long-term = fraction that keep temperate surface conditions for billions of years (stable orbit, magnetic field, plate tectonics or at least some carbon-silicate cycle, avoid runaway greenhouse or atmospheric collapse).
* f_life = probability that abiogenesis actually happens on such a world.
* f_intel = probability that life eventually produces technological intelligence.

Only the first two factors can be constrained even loosely by current exoplanet data:
* f_Earthlike: - We know many “Earth-size HZ” planets are probably mini-Neptunes or super-Venuses (too much gas, too much flux). arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/1406.5587|publisher=arxiv.org|access-date=2025-12-21}}</ref> - Exoplanet atmosphere surveys are starting to show substantial diversity among “Earth-sized in HZ” candidates. - A plausible guess (not a measurement) might be f_Earthlike ≈ 0.1–0.5.
* f_long-term: - Earth has maintained liquid water for ~4 Gyr; Venus likely had temperate conditions at most for a much shorter interval; Mars cooled and lost atmosphere early. - For M dwarfs, long pre-main-sequence luminosity and strong flaring may strip atmospheres from HZ planets altogether. Space<ref>{{cite web|title=Space|url=https://www.space.com/venus-exoplanets-find-habitable-worlds|publisher=space.com|access-date=2025-12-21}}</ref> - A guess might be f_long-term ≈ 0.1–0.5, but we don’t have an empirical distribution yet.
* f_life and f_intel are essentially unconstrained. They could each be anywhere from “almost always” to “almost never” and we have exactly one data point (Earth).

===== Take: =====
'' N_'' = 10¹¹
* η⊕ = 0.3
* f_Earthlike = 0.3 (30% of HZ rocky planets have truly Earth-like bulk and early climate)
* f_long-term = 0.3 (30% of those stay temperate for billions of years)
* f_life = 0.1 (life arises on 10% of such planets)
* f_intel = 0.01 (1% of life-bearing worlds evolve technological intelligence)

Then:

Nintel≈1011×0.3×0.3×0.3×0.1×0.01≈2.7×106N_{\text{intel}} \approx 10^{11} \times 0.3 \times 0.3 \times 0.3 \times 0.1 \times 0.01 \approx 2.7 \times 10^6Nintel≈1011×0.3×0.3×0.3×0.1×0.01≈2.7×106
So under that particular set of assumptions, you’d get millions of intelligent-civilization-bearing planets in the Milky Way.

But:
* If either f_life or f_intel is 10⁻⁶ instead of 10⁻², this collapses to ≪1.
* If both are near 1, you get ≫10⁸ such planets.

The real uncertainty is almost entirely in biology and evolution, not in η⊕ anymore.