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

=== Assistant: There are several layers to this, so I’ll go in steps: ===
There are several layers to this, so I’ll go in steps:

==== From Kepler and Gaia, the current ballpark for η⊕ (average number of roughly Earth-size planets in the habitable zone per star) is: ====
* For Sun-like FGK stars (Teff ≈ 4800–6300 K), Bryson et al. 2020: η⊕ ≈ 0.37–0.60 (conservative HZ) and 0.58–0.88 (optimistic HZ). arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/2010.14812|publisher=arxiv.org|access-date=2025-12-21}}</ref>
* For M dwarfs, updated analyses give roughly 0.24–0.5 Earth-size HZ planets per star, depending on exact radius and HZ definition. ResearchGate<ref>{{cite web|title=ResearchGate|url=https://www.researchgate.net/publication/270593735_The_Occurrence_of_Potentially_Habitable_Planets_Orbiting_M_Dwarfs_Estimated_from_the_Full_Kepler_Dataset_and_an_Empirical_Measurement_of_the_Detection_Sensitivity|publisher=researchgate.net|access-date=2025-12-21}}</ref>

Roughly: η⊕ is of order a few tenths per main-sequence star.

==== Useful anchor numbers: ====
* Milky Way stars: ~10¹¹ (100 billion), with a plausible range 10¹¹–4 × 10¹¹. Wikipedia<ref>{{cite web|title=Wikipedia|url=https://en.wikipedia.org/wiki/Milky_Way|publisher=en.wikipedia.org|access-date=2025-12-21}}</ref>
* Fraction of stars by type: - M dwarfs ≈ 70–85% stsci.edu<ref>{{cite web|title=stsci.edu|url=https://www.stsci.edu/contents/newsletters/2020-volume-37-issue-01/how-well-do-we-understand-m-dwarfs|publisher=stsci.edu|access-date=2025-12-21}}</ref> - Sun-like FGK stars are a minority, maybe ~10–20%; rest are earlier types, white dwarfs, etc.

Take a simple toy mix:
* 70% M dwarfs with η⊕ ≈ 0.3
* 10% FGK stars with η⊕ ≈ 0.5
* Ignore the rest (either short-lived or poorly constrained).

Then the mean HZ rocky planets per star is roughly:
* ⟨η⊕⟩ ≈ 0.7×0.3 + 0.1×0.5 ≈ 0.21 + 0.05 ≈ 0.26 per star

For 10¹¹ stars, that gives:
* N(HZ, rocky) ≈ few × 10¹⁰ ≈ 2×10¹⁰–10¹¹

So order-of-magnitude: tens of billions of roughly Earth-sized planets in habitable zones in the Milky Way, under current occurrence-rate estimates.

This is “geometric HZ + Earth-like radius” only: no climate, no chemistry, no biology.

==== Short answer: η⊕ by itself does not distinguish “Earth-like climate” from Venus-like or Mars-like. It is defined in terms of: ====
* Radius (e.g. 0.5–1.5 R⊕)
* Incident stellar flux / orbital period (within some HZ bounding flux range)

You can choose different inner and outer HZ boundaries (e.g. “recent Venus” inner edge, “early Mars” outer edge ADS<ref>{{cite web|title=ADS|url=https://ui.adsabs.harvard.edu/abs/2014ApJ...787L..29K/abstract|publisher=ui.adsabs.harvard.edu|access-date=2025-12-21}}</ref>), but that only says “in the range where an Earth-like atmosphere could in principle support surface liquid water,” not what the planet actually did with that flux.

Hence:
* A Venus analog (similar size, runaway greenhouse) at the inner HZ edge can easily be counted as an “HZ Earth-size” planet if you use an optimistic HZ boundary.
* A Mars analog near the outer edge can also be counted, even if it lost most of its atmosphere and surface water.

Because of this, η⊕ is better read as “Earth-radius planets in the right sunlight zone”, not “actual Earth-like habitability.”

==== There is a parallel concept to η⊕: η_Venus, the occurrence rate of Venus analogs. ====

Kane, Kopparapu & Domagal-Goldman (2014) defined a “Venus Zone” (VZ): the region closer to a star than the HZ where a terrestrial planet is likely to undergo a runaway greenhouse and become Venus-like. arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/1409.2886|publisher=arxiv.org|access-date=2025-12-21}}</ref>

They found:
* η_Venus ≈ 0.32 for M dwarfs and 0.45 for GK dwarfs (terrestrial planets in the Venus Zone). arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/1409.2886|publisher=arxiv.org|access-date=2025-12-21}}</ref>

So, very roughly:
* Venus analogs are at least as common as Earth analogs, possibly more so.
* There is significant overlap between the outer Venus Zone and the inner HZ, so some planets classified as “in the HZ” by flux may actually be on Venus-like trajectories. arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/2204.10919|publisher=arxiv.org|access-date=2025-12-21}}</ref>

There isn’t yet a comparably standard “Mars Zone,” and Mars-analog occurrence is less studied; outer-edge HZ planets are harder to detect and characterize.

==== 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.

==== 1. “Based on the number of stars in the galaxy (and their types), estimate the number of planets suitable for evolution of life as occurred on Earth.” - Purely from exoplanet occurrence: tens of billions of Earth-size, HZ planets in the Milky Way seems reasonable. - After applying extra filters for Earth-like geophysics and long-term climate, you probably drop to something like 10⁸–10¹⁰ candidates, but that is very model-dependent. - Going all the way to “suitable for intelligent life” demands assumptions about f_life and f_intel; no robust empirical estimate exists. ====
# “How are planets like Venus or Mars accounted for? Do η⊕ figures distinguish between Mars-, Venus-, and Earth-like planets?” - Not really. η⊕ is defined by size + insolation, not by actual climate. - Venus-like and Mars-like planets can both be counted in η⊕ depending on the chosen HZ boundaries. - Separate work defines a Venus Zone and an η_Venus, which suggests Venus analogs are at least as common as Earth analogs.
# “If not, how could an estimate be made of how many planets might actually be suitable for biological intelligence to evolve?” - Start from η⊕ and then explicitly introduce additional factors: - filter out Venus-zone planets and mini-Neptunes (f_Earthlike), - require multi-Gyr climate stability (f_long-term), - then multiply by uncertain biological probabilities (f_life, f_intel). - This is essentially a Drake-equation-style treatment, applied at the planet level rather than the galaxy level.

So: η⊕ tells you “how many Earth-sized targets in the right sunlight range exist.” To get to “how many worlds are actually good for evolving intelligence,” you must layer on several additional, highly uncertain filters that current data cannot yet pin down.