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I am trying to design an alien ecosystem around a K type star. My idea was to have purple plants and high wind atmosphere so originally I wanted to make it tidally locked. However after watching some Artifexian videos I was wondering if I could achieve this in other ways. For example, if the air density on the planet would be around 3.078 kg/m³, much higher than on Earth (1.225kg/m³) and have uneven heating via long day and night cycles (let's say about 24 hours each) could that result in high winds on the surface? Also, if the planet is rather close to its star, would it become tidally locked anyways? According to my calculations the habitable zone is somewhere between 0.61 AU and 0.87 AU.

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  • $\begingroup$ I can be mistaken, but doesn't tidal locking means the same side of the planet is looking at the sun? So you have a crispy hot side, a freezing one and no days or nights, which is quite incompatible with a 48h day-night cycle. $\endgroup$ Jul 9, 2023 at 21:34
  • $\begingroup$ We are talking about 2 different scenarios here. One scenario in which the day/night cycle is very slow and one where there is no day and night cycle at all. $\endgroup$ Jul 9, 2023 at 21:38
  • $\begingroup$ What I mean is that you tell that your world have long days. Why do you ask whether it could be tidally-locked then? $\endgroup$ Jul 9, 2023 at 21:41
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    $\begingroup$ I want the planet to orbit a red dwarf star. Due to the weeker light and heat that it radiates the habitable zone is sowhat close to the star. Considering this aspect I was wondering if the planet would always end up tidaly locked or if I would have more freddoms in design. $\endgroup$ Jul 9, 2023 at 22:33
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    $\begingroup$ @RaduDanCoroian For future reference, please be aware that we allow one and only one question per post - which in this case would have removed some confusion. In the mean time, on a tidally-locked world you're likely to only have significant winds at the terminator and no where else. If that's not what you want, tidal locking is not what you want. As for will it become tidally locked? Yes, eventually. If you don't want it tidally-locked, deal with it as a younger planet. $\endgroup$
    – JBH
    Jul 10, 2023 at 2:52

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With such a rotation speed, or even slower, you could have it where the high daily temperature is higher than Earth norm, while the daily low is lower than Earth norm. Hotter temp equals more evaporation, which rushes towards the dark side of the planet to equalize the pressure and temp. This would result in nightly storms in most locations on the planet. Your plants would likely need to close their leaves during the main heat of the day and receive most of their solar energy during the long mornings and evenings.

Next, we have the issue of coronal mass ejections (solar flares) with the planet being as close as it is. A slowly rotation planet means a slowly rotating core. This causes a weak geomagnetic field and little protection against high energy particles. The result is the destruction of ozone and the stripping of atmospheric gasses from the planet. Fortunately, there is a solution which solves multiple issues. A dense near-orbit moon will increase the rotation speed of both the core and the clouds while also increasing tidal height and subsequent evaporation.

Earth’s moon has a negligible effect on wind. Obviously, a denser or larger moon with a closer orbit would increase that effect. The increased speed of the core rotation would generate a stronger magnetic field. As the moon is closer to your planet, it would need to be travelling at a higher speed. This would result in more frequent high and low tides than on Earth (which has high/low tides roughly every six hours). If we guestimate a tidal frequency of every four hours, your planet has water nearly constantly surging and retreating, and at a much higher height. On Earth typical coastal tidal range can reach over 11 meters, but on your planet, you could have it reach twice that height or more. This would result in a lot of flooding near coastal regions. As the water retreats, the shallow water, tidal pools, and puddles evaporate in the hot sun. More wind is created.

Average day cycle: The sky is clear as the Sun rises. Waterlogged plants are fully open to receive light. As the sunlight intensity increases, the leaves of the plants curl closed. This allows sunlight to reach the wet ground and evaporate any standing water. The sky becomes cloudy as the evaporated water moves towards the dark side of the planet. When midday passes, the plant leaves open back up, relying on stored water to release heat and moisture. The sky is now partially cloudy, as most of the afternoon evaporation is due tidal water. As the sun sets, massive thunder clouds move in to pour rain back down on the plants, which store it for the following day. Thick layers of decaying plant matter on the ground help prevent the cold night air from harming the plant roots. This material also acts like a sponge, absorbing the water and preventing flooding. While the top layers might dry out during the heat of the day, dig down a handful of centimeters and the thick humus will be damp to the touch. When the sun has fully set, a thick fog rises from the warm ground to fill the cold air. The cold stiff breeze causes a second mock rain as the condensing water drips from the foliage. In colder climates, thick frost covers every available surface. The sky clears as the air becomes too cold to hold much moisture.

A possible backstory could be that the moon originated from outside the system, perhaps as a remanent core from an Earth-type planet after its star went nova and burned off the crust. The nickel/iron core was ejected from the system and drifted through space until it was captured by the gravity of the local star, and eventually the planet. The planet was initially rotating slower than modern times, but the moon has increased the speed over the last few million years. The plants had originally colonized areas closer to the poles, where the sunlight and radiation was less intense. The increased geomagnetic field strength and rotational speed increased their habitability zones to the rest of the planet. The moon has still not become tidally locked with the planet but will in another few million years.

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Le Atmosphere; So, the number you give is possible but ill advised. You can use this website to estimate the atmospheric properties. Using fairly standard numbers (0.7 N2, 0.25 O2 and 0.05 CO2) at 3 bar and 30 Degrees we get a density of about 3.5 kg/m³.

However, if your goal fast winds a dense atmosphere is not the metric to look at. And in fact, making your atmosphere this dense will only decrease the wind speed. Because, like if the atmosphere is physical heavier it might be harder to move it.

Wind; Wind, and storms, are generally driven by Oceans. Large bodies of water evaporate water. This vapor accumulates in the atmosphere and is kept from condensing by the constant influx of energy from new vapor. The oceans keep clouds saturated with vapor essentially. When these clouds eventually fly over land, this energy source is adios and the energy is released in the form of Rain or storms.

From this, we can see that the main driving factor for Storms and wind are the oceans. Having a tidally locked planet is just about the worst option as one side of your planet will have a water concentration measured in the g/km³ while the other side will be in permafrost. So, if there even is an ocean, it wont be big. And the temperature differences in the twilight zone wont really substitute a global ocean system.

If your goal is to maximize winds and storms, you need to look at what makes them. And like... make more of that.

In this case we want lots of wind and storms. As such we want big oceans. they dont have to be deep, as a matter of fact we want shallow bikini lagoons. Not just to maximize the number of hot woman trapped in inexplicable situations that could be resolved by any number of basic precautionary measures.

But also to maximize evaporation. Shallow water has a lower energy capacity (because there is less water) and will release it faster when the sun sets.

What also helps is a big moon that creates strong tides as that too contributes to evaporation and winds.

Vegetation helps a LOT to. Go to this site for some truly cool footage. You will notice that clouds are generated by Rainforests in South America like there is no tomorrow. For good reason, plants trap a lot of water and give it of when the sun hits them. So the more tropical regions there are, the more vapor there is in the air which drives even more storms.

Lastly, geography matters too. You can engineer mountain changes and similar features to deliberately concentrate a lot of vapor in one area. Which will lead to a lot of wind.

You will notice, the atmosphere did not come up once in here. Is it important ? Yeah, sure. But i would argue these factors above are not only more visually appealing but also just more important. We have to assume the atmosphere is a certain way anyways. So it is really not a variable i would take into account so extensively this early. The main factor you have to look out for here is the Specific Heat Capacity, but like for most mixtures you can realistically use it is going to be around 1000 J/kg-K. So very earth like.

Tidal locking; Eventually, every planet would become tidally locked. Just put a big boy moon in the sky and dont worry about it.

Plants; On a closing note, quick tip. Evolution has no reason to it. My headcanon is that the reason why plants on earth are (generally) green is because like idk at least 3 billion years ago there was bob the green algae and alice the red algae but oh no alice just got rushed by a random rock, hence why all plants are green.

Obviously it is a bit more complex, the spirit is true. You can just have purple plants with the justification "Well, * Claps *, thats just what evolved first". So much of what we find in nature is the most half assed solution to a problem imaginable and it just stuck for millions of years. So dont stress to much about justifying those colors.

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  • $\begingroup$ I had been under the impression plants are green because green is the center of the spectrum of light that hits the Earth. Both infrared and ultra violet light can be harmful to plants, so dead center would be how you absorb the most light for photosynthesis with minimal cell damage. $\endgroup$ Jul 9, 2023 at 23:57
  • $\begingroup$ @TitaniumTurtle Last i checked the Green color is more related to heat management. In truth there really is no single reason why some plants are green. We need to remember that there are red plants. As i said, the true reasons will be very complex. To the point where i would argue it is ok to just pick a color. $\endgroup$
    – ErikHall
    Jul 10, 2023 at 0:00
  • $\begingroup$ @TitaniumTurtle plants are green because they reflect green light, and absorb red/blue... Remember that the color of an object is based on the color that it reflects and not on what color that it absorbs $\endgroup$
    – Questor
    Jul 10, 2023 at 20:44
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    $\begingroup$ @ErikHall Based on an answer [here]( biology.stackexchange.com/a/56739) Your head cannon is not quite correct, but not inaccurate. Though the reason is because of a purple bacteria that absorbed green light. which led to only red/blue light being available for use by proto-algae. $\endgroup$
    – Questor
    Jul 10, 2023 at 20:58
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Let's consider the wind on the planets in our solar system:

  • Mercury: No significant atmosphere, no detectable wind.

  • Venus: ~100 m/s in the upper atmosphere, 1-2 m/s at ground level. Very thick atmosphere.

  • Earth: 25+ m/s average in the upper atmosphere, ~9 m/s average at ground level.

  • Mars: Up to 30 m/s, 1-4 m/s average at ground level. Thin atmosphere.

  • Jupiter: 100+ m/s average.

  • Saturn: 225 m/s average.

  • Uranus: 250 m/s average.

  • Neptune: 530 m/s average.

  • Pluto: 10 m/s average. Thin atmosphere.

The general trend we see here is that winds are paradoxically higher further away from the sun, and that they are higher in bodies with thicker atmospheres. Where we are able to measure wind at multiple altitudes, wind is slower nearer the ground.

I believe that the limiting factor in wind speeds is turbulence. Turbulence increases near rough surfaces like the ground and in systems with more energy. Note that the wind speed in the gas giant planets where the atmosphere is very thick is both high and increases with distance from the sun.

So, to have a world with a high ground-level wind speed, you would need a thick atmosphere, a short day, low temperatures, a low day/night temperature differential and a smooth ground surface.

Unfortunately, these conditions are also not terribly great for supporting life.

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