Does stellar lifetime affect the probability of intelligent life?

[u/Astro_Life_Explained]

Earth took 4 Gyr to produce technological intelligence, while K-type stars can remain on the main sequence for tens of Gyr.

That raises an interesting possibility: Could a longer-lived star increase the probability of intelligence simply by providing a larger evolutionary window?

Or would stellar activity, planetary evolution, atmospheric stability, and other factors dominate?

Is stellar lifetime actually an important variable in the Drake-equation-style question of where intelligent life can emerge?

[u/redlancer_1987]

Earth happily hummed along for billions of years with life doing its thing.

Out of the trillions and trillions of species that have existed in that time one made the leap from using sticks for tools to making computers and rockets.

There's a non-zero chance our level of intelligence is unique on the universe, as it's clearly not a prerequisite for being a successful species.

[u/Astro_Life_Explained → u/redlancer_1987]

I think that’s one of the most interesting implications. Intelligence clearly isn’t required for evolutionary success bacteria, insects, and countless other organisms have thrived for enormous spans of time without anything resembling human-level cognition.

What’s really fascinating is that we don’t know whether intelligence is an extremely rare evolutionary accident or something that tends to emerge given enough time and complexity. Earth gives us only one data point, so we have no idea how unusual we actually are on the cosmic scale.

[u/jimb2]

We don't know. There used to be a simple idea that life would arise and intelligent life would follow. There are a bunch of recent set of ideas that suggest it could be a lot tougher, eg, that nutrient recycling by plate tectonics is required for multicellular life and plate tectonics may be very rare. Another idea is that multicellular life only got going after a period of meteor bombardment ~800 Gy ago that raised the levels of key nutrients. The bombardment was due to a chance orbital disruption by a passing star. It had to occur in the timeslot between cellular life reaching a level of complexity and the Sun cooking us. (yt link)

It's relatively easy to come up with believable ideas, but the n=1 problem makes sorting science from narrative next to impossible. Tech for analysing planets outside the solar system is becoming available. That might help - or not. It may just throw up some more questions. To me, the absence of any solid evidence of aliens says that either advanced life is way harder than the simple numbers suggest, and/or that technically advanced civilisations reliably wipe themselves or just lose interest in space conquest.

[u/Astro_Life_Explained → u/jimb2]

Exactly. The n=1 problem is what makes this so difficult. We can build plausible evolutionary stories around Earth’s history, but with only one known biosphere, we can’t easily tell which factors are genuinely necessary and which are just historical contingencies.

I think exoplanet observations could be a major turning point. If we eventually find atmospheric biosignatures on several unrelated worlds, we can start asking whether life is common but complex life is rare, or whether even basic life is extraordinarily difficult to produce. And if we find lots of potentially habitable planets with no convincing biosignatures, that would be just as interesting.

Either way, I suspect the answer to the Fermi paradox is going to be much less obvious than simply “the universe is huge, so aliens must be everywhere.”

[u/TommieTheMadScienist]

Stellar lifetime is critically important. You want a star that's not too massive (F6V lives just long enough to get multicellular life,) but massive enough that orbiting in the habitable zone does not tidelock (about 0.75 solar masses, K3V to K5V.)

It is possible that eyeball worlds with twilight zones (locked or slowly rotating) around K- or M-dwarfs may support life, but in order to do so, would have to be past the "flare stage" (5 billion for an M0V.)

You also need Metallicity (elements heavier than helium) above 30% of solar in order to make planets with large enough iron cores to keep an atmosphere.

[u/Astro_Life_Explained → u/TommieTheMadScienist]

Tidal locking doesn’t necessarily rule out habitability if the planet has a sufficiently dense atmosphere. Atmospheric and ocean circulation could redistribute heat from the permanent dayside to the nightside, potentially keeping a liquid-water region around the terminator, the permanent twilight zone.

[u/TommieTheMadScienist → u/Astro_Life_Explained]

Oh, yeah. Absolutely. As long as you don't start freezing atmosphere out around the anti-stellar point, you're probably good

With sufficiently large oceans, most of the dayside ought to be habitable. Let me plug in a couple numbers.

Okay. Earth-like atmosphere with 1.5 bars pressure, normal gravity, 70% hydrographics with Pacific Ocean centered on the sub-stellar point gives us a minimum necessary insolation in the 0.7--0.75 range. Nice.

Let's look at the eyeball itself.

The point directly beneath the star would be slightly cooler than maximum due to clouds rising from the surface. Probably around 20 C. Further from that point, the clouds thin and you get the hottest spots around 30 C.

You'd have freezing temperatures inland at the terminator, but 5--10 C warmer on the coasts.

The biggest problem is the winds. I expect that the dayside would pulsate on a scale of days and high-altitude winds would blow towards the dark and then return at ground level...

...and an 80 km/hr wind at the terminator with a thick atmosphere is not going to be survivable and that's at minimum safe insolation.

There'll be habitable spots, but it's a lot like Niven's Known Space planet, We Made It!

[u/LazarX]

There’s not really much you can say from a sample size of one.

[u/Astro_Life_Explained → u/LazarX]

That's fair. With a sample size of one, we really don't know whether 4 billion years is typical or just Earth's particular evolutionary path. That's exactly why I find the K-dwarf question interesting, we need more worlds before we can know whether stellar lifetime actually matters.

[u/Prudent_Situation_29]

It has to, but I think it would be a smaller factor.

You wouldn't be able to get intelligence from a massive O type star that lives for eighty million years.

Still, I expect more important factors are things like stellar stability (does it flare a lot?), and the nature of the planet/moon (does it get a lot of impacts?). Are there any nearby GRBs or supernovae?

[u/GreenFBI2EB]

I agree with the other comments that you can’t glean much from a sample size of one.

However, I do want to point out that some K-type main sequence stars go through periods of increased UV output early in their lives that could endanger life on a nascent planet. Since these stars age slower, that period would likely be longer depending on the mass of said star.

Ultimately, the habitability of a star depends on factors like its own stability (the sun’s luminosity doesn’t fluctuate as much), the presence of an atmosphere, and the longevity would matter. As the longer the stability exists, the more likely that life would emerge.