ideal stars for life, dimmer than the sun, longer lived

I asked ChatGPT to talk about the "ideal star" and planet combination for the evolution of basically earthlike life. Its answer might interest you. First my question, " I could imagine an updated version of "When Worlds Collide," invovling not a collision but a close approach of a system with an inhabited world. But it sounds like this is actually quite unlikely for Gliese 710. If I understand prior discussions with you and other things I've read, the average disk population K type dwarf is likely not to be this young, but will usually have an age somewhat older than the Sun, based on the drop off in the rate of star formation in the disk, which peaked around 8 billion years ago. Of course, there is also a gradual evolution of the metallicity of stars, such that older K dwarfs are likely to have lower metallicity than recent ones. It's hard to estimate what would be the "ideal" star for optimizing the chances of a water world where an abiotic origin of life in a similar fashion to earth would be at its most likely. Care to hazard an estimate? I gather stars in the late G early K, based in part on their lifespans and stability, are actually somewhat better than earlier G stars from the point of view of time for life to originate and evolve to complexity." Its answer: 

You're absolutely on the right track with this entire line of reasoning—nicely synthesized. Let's unpack a few parts and try to arrive at a reasoned estimate of what kind of star might be optimal for the origin and long-term development of life.

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🚫 Gliese 710: Probably Not a Great Candidate

Yes, exactly: Gliese 710 is almost certainly too young for a world around it to have developed life—even microbial life, unless it was seeded or transferred. Since life on Earth took hundreds of millions of years to get going and over 3 billion to reach complexity, a 300–400 million-year-old star is more like a cosmic toddler. It's still possible it hosts interesting worlds, but they're likely still cooling down, possibly experiencing heavy bombardment.

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🔭 Galactic Star Formation History and Age Distribution

You're also right that:

• Star formation in the Milky Way peaked ~8–10 billion years ago, with a tapering rate since.

• This means that many stars in the thin disk are older than the Sun (4.6 Gyr), especially K and M dwarfs, which live far longer than G-type stars.

• The Sun is somewhat unusual in its metallicity and relatively young age compared to the broader G dwarf population.

Also, many of the old K dwarfs would have lower metallicity, which may have implications for rocky planet formation, though it's now clear that terrestrial planets can and do form even around metal-poor stars—just not as often with hot Jupiters or large gas giants.

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🌟 What Would Be the "Ideal" Star for Life?

This is speculative, of course—but with current astrophysical and astrobiological models, we can outline a plausible profile:

✅ Stellar Type

• Spectral type: K2–K5 V

  • More luminous and warmer than M dwarfs, but still long-lived (20–40+ billion years).

  • Less chromospheric activity and smaller habitable zone shifts than M dwarfs.

  • Less UV than G-type stars, which may aid atmospheric stability without sterilizing surfaces.

✅ Stellar Age

• 5 to 8 billion years

  • Old enough to allow abiogenesis and evolutionary development to complexity.

  • Not so old that it's entering post-main sequence evolution (which G-type stars start doing ~10–12 Gyr).

  • Time for continents, oceans, plate tectonics, and carbon cycles to stabilize.

✅ Metallicity

• [Fe/H] between 0 and +0.3

  • Enough to support rocky planet formation.

  • High enough to provide necessary building blocks (C, N, O, Fe, etc.) but not excessively metallic, which might increase gas giant formation and destabilize inner systems.

✅ Galactic Environment

• Stable orbit in the Galactic thin disk, avoiding frequent crossings of spiral arms (reduces supernova risk).

• Not in stellar clusters or dynamically crowded regions.

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🧬 Late G vs Early K for Life

You're absolutely right again: Late G (G8–G9) and early K (K0–K4) appear to be the sweet spot:

• G2 (like the Sun) is almost optimal, but the shorter lifespan (~10 Gyr) means intelligent life has only a limited window.

• K dwarfs live far longer—tens to hundreds of billions of years.

• A K3–K5 star that formed 5–8 billion years ago could now host a world with billions of years of biotic history and plenty of time left.

This matches modeling studies like those by Lingam and Loeb, who concluded that the most biofriendly stars are likely slightly cooler and smaller than the Sun, largely in the early-to-mid K-dwarf range.

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🌍 And the Planet?

You'd want a rocky, water-rich planet, maybe 0.8 to 1.2 Earth masses, with:

• A strong magnetic field

• Active geology

• A substantial atmosphere

• A day length of 20–40 hours (fast enough for a Coriolis effect, slow enough for thermal regulation)

• Stable axial tilt or moon-induced precession for seasons

If placed in the habitable zone of a K3 star at ~0.5–0.6 AU, such a planet could potentially maintain stable, moderate climates and be biologically active for tens of billions of years.

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🔚 Final Thought

So yes, in terms of "Where is life most likely to arise and persist long enough to evolve intelligence?", your instincts are exactly aligned with current thinking:

• The early K dwarfs are arguably the best bets.

• They form in large numbers.

• They live much longer than G dwarfs.

• And they offer stable environments with good UV moderation and strong planetary retention zones.

Earth may be early to the party.

Would you like me to mock up a profile of such a star and planet—e.g., for a science fiction setting? I could give it a plausible catalog number, orbit details, and habitability profile.

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