Schiaparelliorbust wrote: ↑January 9th, 2021, 5:06 am
How far away from the suns is the planet going to be?
As the planet gets farther away from the sun(s), the light energy from the sun(s) will spread out more before reaching the planet, so the planet will get less light energy. Specifically, the amount of light energy that the planet receives will be inversely proportional to the square of its distance from the sun(s). (In our case of the planet orbiting a pair of binary stars, we don't have to worry so much about accounting for the fact that most of the time, one star will be closer to the planet than the other because the distance between the two stars must be relatively small compared to the distance between the stars and the planet anyway in order for the planet to have a stable orbit.) If we assume that the G-type star's bolometric magnitude is equal to that of Earth's Sun and that the K-type star's bolometric magnitude is four-ninths of that of Earth's Sun, then the total magnitude of the two stars would be 1.44 times that of Earth's Sun, so in order to receive the same amount of energy as Earth, an Earth-sized planet orbiting this binary star system would need to be sqrt(1.44)=1.2 times as far away from the binary stars as Earth is from the Sun. (This does make some simplifications, such as ignoring the fact that the planet may absorb a different fraction of the light energy at different frequencies and failing to account for dimming when one of the binary stars passes in front of the other, but it's at least approximately correct and conveys the correct concept.) If we want the alien planet to be slightly warmer than Earth, then we should place it a little closer to the binary stars than that, and if we want it to be a little cooler than Earth, then we should place it a little farther from the binary stars than that.
Also, when I was checking yesterday that the binary stars can be far enough apart from each other to not merge and close enough together so that the planet can be far way enough to have a stable orbit without being too far away to have liquid water (and they indeed can), I learned about the concept of a Roche lobe, which is a region around a star in a binary system such that if a star tries to expand past this region, some of its mass will spill into the other star. I'm wondering whether it's possible to squeeze at least a few more million years of evolution time out of the star system by positioning the stars so that when the G-type star starts to expand near the end of its time in the main sequence, some of its mass will be transferred to the K-type star. It's possible that the decreased mass and pressure of the G-type star will cause it to burn through its remaining hydrogen more slowly, increasing the amount of time that life on the planet has to evolve. However, I'm not a stellar physicist, so for all I know, it's possible that the decreased pressure from the outer layers will cause the star to expand into a red giant sooner, that the star's decreased mass will cause its Roche lobe to shrink in a manner that will result in a runaway effect that won't necessarily be beneficial for our purposes, or that the result of the interaction will be dominated by some factor that I don't even know about.
You should be aware that if an animal only has a balloon and no way to steer itself, then it won't be able to find food, and it will be easy prey for any predator with controlled flight unless it has some other defense. However, I think that a flying animal that uses its balloon in order to not have to spend as much energy producing lift would be more practical. Also, since photosynthetic plants need access to sunlight, it's possible that they could take up this strategy. Not being able to steer towards food wouldn't be an issue because plants get their energy from the Sun and store it using chemicals that they can get from the air, and not being able to steer in order to avoid getting eaten shouldn't be an issue because most plants can't move at all. However, this would present difficulties for the plants in terms of not being able to get nutrients from the soil, and reproduction would also be tricky, both because the floating plants would likely be dispersed too far way from each other for effective sharing of gametes and because the young offspring wouldn't be able to float right away. Also, evolution from land plants requires a set of several large changes that must happen together in order for this to work, making it seem unlikely. However, it's feasible that a sea plant dwelling on the ocean surface could evolve a gas-filled sack near its top in order to prevent waves from flipping it photosynthetic side down. As more of its body comes above water, it will likely be less vulnerable to herbivory by marine herbivores, which will likely serve as an evolutionary pressure that would lead its gas sack to become larger and/or contain a lighter mixture of gases. This could eventually reach the point where the plant starts to float. However, because evolution is gradual, this will likely begin with particular large waves launching the plants a few feet in the air and the plants coming back down to the sea surface. Because the plants won't be in the air for more than a minute or so at a time, they could conceivably survive the ordeal, allowing these plants to evolve to be able to tolerable being in the air better instead of every plant launched into the air dying. (The fact that the plants would likely have had to evolve measures to prevent themselves from drying out as a greater proportion of their bodies came above water would also help.) Pressure from aquatic herbivores and being in the air for longer may lead plants to develop the ability to take in all of the chemicals that they need from the air instead of relying on nutrient-absorbing parts below the waterline. Alternatively, maybe the plants will evolve a feedback mechanism to empty their gas sacks somewhat if they're low on some element or nutrient that they have to get from the water and fill their gas sacks back up once they have enough of it. Also, juvenile floating plants could spend the early part of their lives entirely on the ocean surface. Then again, this is pure speculation, so maybe it won't happen at all or stop before the plants spend long durations of time completely airborne.
Schiaparelliorbust wrote: ↑January 9th, 2021, 5:06 am
We'll do this after you finish your atmospheric changes to allow for the 40% dinitrogen. What should I look up exactly?
We should check whether any of the components will absorb or reflect a significant amount of light in the visible or near-infrared portions of the electromagnetic specturm. We already know that dinitrogen, carbon dioxide, and water vapor won't absorb or reflect too much visible light because they are in Earth's atmosphere in significant quantities. However, this alone doesn't tell us that they don't absorb or reflect near-infrared light. I know that liquid water is very good at absorbing near-infrared light (which may be another pressure leading to the evolution of floating plants if they use near-infrared light for photosynthesis), but I don't think that they same is true for water vapor. I also know that carbon dioxide and other greenhouse gases work by absorbing infrared light, but I think that they absorb mid-infrared or far-infrared light and not near-infrared light (although I will check this). I remembered that sulfate aerosols in Earth's atmosphere can reflect sunlight, cooling the Earth, but I'm not sure which wavelengths this applies to, so we should check that (and also, if necessary, how much we can minimize this effect while still keeping sulfur-oxygen compounds in the atmosphere) before potentially cutting sulfur-oxygen compounds from the atmosphere.
Also, here's my proposed atmosphere with 40% dinitrogen:
40% dinitrogen
20% ammonia
17% carbon dioxide
7% sulfur oxides
6% carbon monoxide
3% hydrogen cyanide
2% hydrogen sulfide
1.5% methane
1.1% water vapor
0.6% ethane
0.6% sulfur oxyacids and oxyanions
0.6% isocyanic acid
0.6% trace gases
I didn't perfectly scale up everything else for several reasons (e.g. not wanting to increase the amount of methane and ethane lest the atmosphere explode once dioxygen is introduced), but as always, I'm willing to change the atmosphere in response to others' suggestions.