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I have been working on the setting for a hard speculative biology/evolution project that for now will simply be referred to as the Gemini Project, and the main issue I've been having is whether or not complex life comparable to that seen on Earth in the last ~100 million years could emerge and evolve naturally on moons orbiting a gas giant. Life on Earth has shown itself to be extremely resilient, and can exist in relatively extreme environments, so it would be reasonable to assume that it could exist beyond completely Earth-like conditions to some degree.


Leda and the Twins:

TZ343-B/Leda is the gas giant within the habitable zone that the twin moons orbit, and its mass, radius and gravity are 1.078M♃ / 343M⊕, 1R♃ / 11.210R⊕ & 2.725g / 26.723m/s²

TZ343-B-1/Castor and TZ343-B-2/Pollux are both approximately the same approximate mass, radius and gravity: 2M⊕, 1.202R⊕ & 1.385g / 13.582m/s². They both orbit Leda, and are tidally locked in their orbits; meaning that their orbital periods and diurnal cycles are practically one and the same.


Other Issues:

The other issues I am having are deciding the exact mass of the parent K-class star, TZ343-A/Tyndarzeus; the semi-major axis of Leda; and the semi-major axes/orbital periods/diurnal cycles of the moons, although Castor should have an orbital period/diurnal cycle somewhere between 20 and 48 hours long.

The setting also assumes that superhabitability is real/true, and conforms to it as best as I understand it.


Overall, could carbon-based life emerge and evolve under such conditions from what we understand?

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    $\begingroup$ I'm close to voting to close this question. We have one and only one data point to work with: Earth. Based on that one data point, the safe answer is "no, it's not plausible." The best scientific answer is, "maybe, we don't have enough data." The worldbuilding answer (remember that our goal is to help you build a fictional world) is, "sure, you can have that." What are my other problems? In what context to you ask if it's plausible, Real Life (which we don't work with) or Fiction (which we do)? What does "superhabitable" specifically mean? Does it matter if we say "no?" Who would notice? $\endgroup$
    – JBH
    Commented Oct 10, 2023 at 5:40
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    $\begingroup$ This might sound funny, but what's the real problem you're trying to solve? Whenever someone asks "is it plausible?" my knee-jerk reaction is to wonder why they're seeking our permission to build a fictional world using the idea. What's stopping you from using this idea and forging ahead? Are you worried that someone might find it implausible? Larry Niven was famously harassed by MIT students for his novel Ringworld because those clever monkeys discovered it's unstable. He loved the fact that they'd added to science via an undiscovered flaw in his book! And so he wrote another to fix it. $\endgroup$
    – JBH
    Commented Oct 10, 2023 at 6:38
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    $\begingroup$ The problem is that I'm trying to create a speculative biology project as rooted in known science as I possibly can, and the number of relevant scientific fields are simply too much for one person to understand extensively. I only want to make reasonable assumptions on things that science does not yet know, and I often get very annoyed when things don't have answers rooted in science/with evidence. $\endgroup$
    – Inanis343
    Commented Oct 10, 2023 at 7:20
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    $\begingroup$ As others have said the honest answer is we don't know. Nobody has seen life emerging or demonstrated it in a lab. There are a number of theoretical possibilities, but their probability is indeterminate. In my opinion one of the best current theories comes from biochemist Nick Lane en.wikipedia.org/wiki/The_Vital_Question he suggests that life may be "relatively" common, but that multicellular life might be extremely rare indeed based on energy considerations and the way EukaryotIc cells are thought to have formed. So by life do you mean bacteria or an advanced civilization of aliens? $\endgroup$
    – Slarty
    Commented Oct 10, 2023 at 8:45
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    $\begingroup$ @LordMalecith, getting annoyed when answers rooted in science/evidence don't exist is a wonderful reason to get an education, but a terrible reason to build a fictional world (our purpose per the help center) or write fiction. You're desire to write a documentary about hypothetical life without fist obtaining the necessary credentials isn't really our problem, is it? I'm sorry for being blunt, but Stack Exchange isn't intended to replace an education, and we get far too many questions from people who want to write hard-science fiction without first obtaining the necessary education. $\endgroup$
    – JBH
    Commented Oct 11, 2023 at 3:40

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Scientists have done a lot of thinking in recent decades about the possibility of more or less Earth like life, using liquid water, on very large moons of giant planets in other solar systems.

So a good place to start getting an answer would be scientific papers on the subject.

Like this one:

https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf

And on pages 3 to 4 it discusses the possible mass range for a habitable planet, moon, or other type of world.

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕, Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al. 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al. 2012).

Thus it claims that the upper limit for a habitable world is roughly approximately about 2 times the mass of Earth, which is about the mass you have decided upon for your two moons Castor and Pollux.

So possibly you might want to consider reducing the masses of your moons a tiny little bit incase those calculations are correct and the upper mass limit for a habitable world is approximately 2 times the mass of Earth. On the other hand you did write that your story assumes superhabitability of large terrestrial type planets much larger than Earth.

You write:

Leda and the Twins:

TZ343-B/Leda is the gas giant within the habitable zone that the twin moons orbit, and its mass, radius and gravity are 1.078M♃ / 343M⊕, 1R♃ / 11.210R⊕ & 2.725g / 26.723m/s²

TZ343-B-1/Castor and TZ343-B-2/Pollux are both approximately the same approximate mass, radius and gravity: 2M⊕, 1.202R⊕ & 1.385g / 13.582m/s². They both orbit Leda, and are tidally locked in their orbits; meaning that their orbital periods and diurnal cycles are practically one and the same.

So Leda has 1.078 the mass of Jupiter in 1.0 times the radius and thus volume of Jupiter, and so Leda has 1.078 the average density of Jupiter.

Giant planets much more massive than Jupiter don't get much larger in volume. Instead their increased gravity compresses their matter more and more and they get only a little larger than Jupiter and can even get smaller than Jupiter. I don't know the formula, if any, for calculating the radii of gas giant planets more massive than Jupiter. I hope you have checked to see whether Leda has the correct radius for its mass.

Jupiter has a surface gravity 2.528 that of Earth, and Leda has a surface gravity 2.725 that of Earth, which is that of 1.077 that of Jupiter. According to this online escape velocity calculator:

https://www.omnicalculator.com/physics/escape-velocity

Leda should have an escape velocity of 61.88 kilometers per second, 5.53 that of Earth, while having a surface gravity 2.725 that of Earth. So clearly the surface gravities and the escape velocities of planets do not vary in the same proportion.

The data you give on the twin moons are:

TZ343-B-1/Castor and TZ343-B-2/Pollux are both approximately the same approximate mass, radius and gravity: 2M⊕, 1.202R⊕ & 1.385g / 13.582m/s². They both orbit Leda, and are tidally locked in their orbits; meaning that their orbital periods and diurnal cycles are practically one and the same.

So each of the moons has two times the mass of Earth within 1.202 the radius of Earth and thus 1.7366544 the volume. Thus each moon should have about 1.1516 the average density of Earth.

Of course all large worlds compress the matter inside them. I don't know how to calculate whether your moons have, or less, or the same, average density as they would have if they have twice the mass of Earth and are made of the same mixture of materials Earth is.

The moons Castor and Pollux have twice the mass of Earth and 1.202 times the radius, so their escape velocity should be 14.43 kilometers per second, 1.29 that of Earth. Since their surface gravity is 1.385 that of Earth, this is another example for world builders that surface gravity and escape velocity vary in different proportions and must be calculated separately using different formulas.

You wish to decide:

and the semi-major axes/orbital periods/diurnal cycles of the moons, although Castor should have an orbital period/diurnal cycle somewhere between 20 and 48 hours long.

Since you give the masses of Leda and castor, it is easy to calculate the semi-major axis to go with a specific period.

Using this orbital period calculator:

https://www.omnicalculator.com/physics/orbital-period

I found that an arbitrary semi-major axis of 1,000,000 kilometers gives an orbital period of 6.202 days or 148.84 hours. Trying a semi-major axis of 200,000 kilometers give 13.312 hours. 300,000 kilometers gives 24.456 hours.

To be continued.

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  • $\begingroup$ To you I want to ask the same question as i did to the Questioner: When you have objects with up to two earth masses pass each other in such a "close" distance (< 1.000.000 km), will they not end up disrupting their obits? It you bring that system into existence, I give it less than 100 years until the inner moon gets either ejected or torn apart by tidal forces; the other stuff (life on gas giant moons) remains legit $\endgroup$ Commented Jan 2 at 6:59
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As others have said, it is hard to answer since we do not have any reference other than Earth. If an exoplanet is not like Earth, there is no good reason to believe it would emerge any, let alone Earth like, life.

Ultimately, i think you are asking the wrong question. Presumably you want to write a story / spec evo project, like will you just not do it if we tell you "Nah not realistic" ? With every single piece of media there is suspension of disbelieve. There was a great meme on this lemme find it;

enter image description here

I presume your goal is to have some kick ass spec evolution going on, so it is fine to just assume life developed. Similar to how it is ok that 99,9999% of so called hard sci fi novels ignore the fact Fusion reactors produce radiation and are probably garbage for thrusters.

So my answer would be, yeah just go for it. These details are not important, you want a certain environment to explore the evolution of life, just do it.

I think the much better question to ask is wth is going on in your planetary system. Two Earth sized moons around a Jupiter wannabe ? Not if physics has anything to say about it. Moons form through accretion, or get captured. Accretion is well documented to have a distinct relation between the mass a planetary disk can form, compared to the mass of the parent planet. That mass ratio is about 13000:1 if memory serves me correctly. So for every 13000 kg of Gas giant, you get 1 kg of moon. This would imply your Gas giant has a mass of around 13000 Earths, or 4 Jupiter's. Which is more of a little baby Star.

I can believe one Earth sized moon revolving around Jupiter after being captured, but then there are no other moons. If you took Earth and placed it around Jupiter, all other moons would be gone very quickly.

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    $\begingroup$ Re drake equation, it was concerned about sentient life that is willing and able to communicate with life on other planets. Compare Earth for sentient life - 4B years vs 10000 at most, and for "able to communicate" 100 at most. So for "just life" drake equation does not give a correct estimation. And frankly I really think that if both moons are earthlike, life would emerge on them over a span of 1B years, and evolve separately. $\endgroup$
    – Vesper
    Commented Oct 10, 2023 at 18:52
  • $\begingroup$ @Vesper thanks for clearing that up. I included the meme because i thought it fit the situation. But yeah it does not entirly apply. Still funny imo $\endgroup$
    – ErikHall
    Commented Oct 10, 2023 at 20:10
  • $\begingroup$ At 4 Jupiter masses the gas giant isn't even halfway to becoming a baby star. Lithium fusion starts at 13.8 Jupiter masses and deuterium fusion at 65 if I remember correctly. Stars are significantly bigger. Interestingly gas giants don't actually get much bigger than Jupiter, because more mass just compresses them more strongly. So even without getting a brown dwarf, Earth size moons are in the cards. $\endgroup$ Commented Oct 23, 2023 at 17:11
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I agree with some of the other comments and answer, no one knows where life can form in the universe so just go with it. On that note though, there are some conditions you can add to make your moons suitable to life to the best of our knowledge.

Habitable zone. there is a habitable zone both of the galaxy and of the solar system. This means that you're moons should have water. Early life should also create an ozone.

An active core that causes tectonics and creates a magnetic field. Tectonic activity supposedly increases the chances of life (hydrothermal vents, heat, churning of minerals to the surface) and magnetic field would protect life from dangerous space stuff.

Two problems I can think of two the best of my knowledge since I'm not an astronomer or something: Radiation - To the best of my knowledge Jupiter emits a lot of harmul radiation which is harmful to life. not sure if this is a problem with all gas giants or if a magnetic field would protect from that.

Meteor impact - again, to the best of my knowledge, one of the reasons the inner solar system is relatively calm meteor-wise is because jupiter's high gravity sucks them in. so if your moons are being bombarded by meteors all day that might be an issue. Maybe add a larger gas giant farther out in the solar system?

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