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[edited after feedback in comments] I've had an idea for a story that takes place on a planet that is primarily shallow oceans (average depth of no more than 100m or so, max depth of no more than 1000m). In general, the story I'm trying to tell would be set on a world just barely on the edge of habitability. I want it to be harsh or difficult in some way.

The planet would have no plate tectonics (like Mars). The only land masses would be volcanic islands formed similarly to mid-plate volcanic islands (Think Hawaii), with at least one island that is very large, like Olympus Mons (which is also a shield volcano like the Hawaiian islands).

The planet would be tidally locked around an M-class star. So not only would the inhabitants be spread across these small islands, but there would be a relatively narrow band of habitability and likely some pretty harsh weather, especially winds.

Otherwise, the planet would be similar to Earth in most other ways that count for habitability (mass and gravity, atmospheric composition, etc.)

My question is, could a tidally locked planet with no plate tectonics plausibly have some way of generating a magnetosphere or some other mechanism for maintaining an atmosphere? Would such a planet be habitable?

I have also read some research recently around tidally locked S-type planets in binary systems that suggests such a planet orbiting an M-class star may be more habitable if the M-class star system was orbiting a larger G-class star similar to our sun. The added irradiation would help raise temperatures on the dark side of the locked planet, and would create some kind of variation in illumination even if it would never truly be "night."

DMacc1917
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    How exactly do you envision no tectonic plates AND volcanic islands? – L.Dutch Mar 14 '23 at 06:20
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    Hello @DMacc1917, welcome to [worldbuilding.se]. This is a tough question to answer, and it has some problems that violate the rules you'll find in our [tour] and our [help]. But it might be salvageable. (a) You can have volcanoes without fault lines, they're called hotspot volcanoes. But hotspot volcanoes without tectonics would be difficult to explain. Let's ignore that. (b) A tidally-locked planet would result in a completely frozen sea on the back side. Is that all right? If not, you'll need to remove the tag. ... – JBH Mar 14 '23 at 07:11
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    ... (c) Finally, we have a number of rules. Stack Exchange doesn't do discussions (see [tour]), it expects specific problems that have an actual solution. Those problems are expected to be narrow enough to be answered without a book-length treatise (see [help/dont-ask]). Humans have survived in many inhospitable locations, so the real question is whether or not your planet has the minimum requirements for life (sufficient water, climate within reason (let's say +38℃ to -25℃), and enough fertile soil to grow plants and husband animals. Etc. So long as those minimums are there, you're good. – JBH Mar 14 '23 at 07:15
  • With an average depth of only 100 m (and hence likely vast areas with depths lower than 50 m), it would be no big deal to construct artificial islands or cities anchored to the bottom with poles (like oil platforms), given industrial-age technology. Ocean lifeforms should provide plenty of food (provided it is compatible with human biology) and materials for textiles and more. – Klaus Æ. Mogensen Mar 14 '23 at 13:47
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    As an alternative to volcanic islands (which may be difficult to explain in the absence of plate tectonics), you could consider impact craters, which would create ring-shaped islands (possibly with a central peak or multiple rings for complex craters). The geologically young Tooting crater on Mars (29 km diameter) has a central peak 1052 m high and >9 km wide) and a rim wall 386-893 m tall. Higher gravity leads to higher crater rim walls due to higher impact velocity - crater rims on Earth are 5x-8x as tall compared to diameter than on the Moon. – Klaus Æ. Mogensen Mar 14 '23 at 14:11
  • Putting aside the geology... How is it different from Polynesia? Rare islands, lots of ocean, people living there. – Boba Fit Mar 14 '23 at 14:44
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    @JBH apologies if the OP didn't conform to some of the community rules. First-time poster here. Re: volcanoes (this also responds to @Questor), I was thinking that the volcanic activity on this planet would be similar to Mars (which lacks/lacked plate tectonics) and mid-plate volcanoes such as the Hawaiian Islands. I guess that perhaps the better question to ask in my OP would have been if it is plausible for a tidally-locked planet with no plate tectonics to have a magnetosphere or some method of retaining an atmosphere sufficient for habitability. – DMacc1917 Mar 14 '23 at 21:20
  • @BobaFit I would be certainly be looking to Polynesia for some inspiration, but there would be some differences. First, the islands I would be focusing on would be much larger (Iceland-sized or larger). Second, if the planet was tidally-locked, my understanding is that the weather would likely be much harsher than it is in Polynesia, and depending on where you were located in the planet's habitable band, it could be much colder. – DMacc1917 Mar 14 '23 at 21:25
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    @DMacc1917 That's why no one has voted to close. We get it, it takes time to learn the rules. Note that SE only allows one "@" user per comment. The first one is notified, the rest are ignored. Finally, never trust that anyone will read your comments, please [edit] your post and add all clarifications to it. Thanks. – JBH Mar 14 '23 at 23:08
  • @L.Dutch volcanoes don't require plate tectonics. Hotspot volcanoes form active volcanoes even on planets with dead tectonics, like mars. said volcanic islands however will be huge and very far apart. – John Mar 31 '23 at 21:31

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I mean... you can brute force humans living anywhere with symbiosis with another lifeform. In this case, all they really need is shelter and wind stability from a really big sedentary lifeform.

Most of the planet would be inhospitable without a lot of help from symbiosis . Though there will be a narrow band where the two sides meet where humans could live. The planet wouldn't be mostly oceans due to water going to the cold side, but the band could be due to glacial meltwater.

Planets can have a magnetosphere without plate technonics

By the way, the oceans and coasts are teaming with life. They are among the easiest places for humans to live, so that's nice for the humans there.

So, all in all humans could live on that planet, but they would be completely confined to a relatively little band without a lot of help from symbiosis .

Aseku Vena
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You probably need to make the planet an old super-earth strongly deficient in radioactive elements. M-class stars are known for their flares. This activity shreds planetary atmospheres and over millions of years causes planets to loose their air. All red dwarfs, even old ones, have hazardous flare activity. For instance, the 10 billion year old Barnard's Star has been found to have hazardous flares. [1]

As a result, Earth sized planets cannot hold on to a Earth-like atmosphere for more than a few hundred million years. A typical method of prolonging the lifespan of the atmosphere is to increase the mass of the planet. This make it harder for gases to escape. If the planet's gravity is crushing enough, it could probably hold onto a nitrogen-oxygen atmosphere indefinitely. However, with Earth-like radioactive element concentrations a Super-earth will be geologically active for billions upon billions of years.

Therefore, one should reduce the amount of radioactive elements present in the planet drastically. With enough radioactive reduction, the planet will have a geologic history similar to Mars. For the story and the carbon cycle, you would want to get a geology similar to Mars 2 billion years ago. Currently, Martian volcanic activity is pitiful. Eruptions are rare and small, with a handful every million years all concentrated on one area of the planet. [2]

Sources

[1] Chandra Press Release about Barnard's Star. It links to a paper which goes into greater detail.

[2] Scientific American Article on Martian volcanic activity. It focuses on the discovery of a volcanic eruption 50,000 years ago in the Cerberus Fossae.

Mars Is Ours
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They wouldn't survive... Also your planet wouldn't have oceans... Or liquid water.

Plate tectonics are caused because, at the center of the earth is a giant molten ball of rock and metal that the crust floats on which has currents (motion). If your molten mantel does not have currents (or is not molten) then you won't have molten iron flowing around at the core so no magnetosphere. Without a magnetosphere your planet's atmosphere will be blasted off by solar radiation (Much like mars). This will make your planet inhostible to life..

Questor
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    re: blasting off atmosphere. I believe that's more of a "over the course of millions of years" kind of thing. I'd read before that if we could terraform Mars, giving it an atmosphere but no magnetosphere, it would be fine for quite a long time. Less of a blasting and more of a very slow wheedling away. Also depends on other factors. Venus has no active core, either, but still retains quite a heavy atmosphere (the heavier gravity helps it here, vs Mars). – JamieB Mar 14 '23 at 20:36
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    That is a good point, a stronger enough gravity well would keep the atmosphere for longer. But Venus has another thing going for it, an extremely dense atmosphere made of carbon dioxide (1.5x the density of an oxygen/nitrogen atmosphere, 2.5x the density of water vapor) and a highly charged/dense ionosphere which produces an induced magnetic field as Venus is bombarded by solar rays.. So saying that Venus doesn't have a magnetic field is not completely accurate. – Questor Mar 14 '23 at 20:58
  • Venus also has very little water vapor/oxygen with most of it probably blown away by solar radiation (based on looking at Venus "meteor tail" composition (Mostly Free Oxygen compared to Venus's atmospheric composition 96.5% CO2). – Questor Mar 14 '23 at 21:01
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    @JamieB Millions of years is a very small time frame astronomically speaking. Odds aren't exactly good for colonists arriving to this system in that time-frame. – Negdo Mar 15 '23 at 08:19
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No.

A planet tidally-locked with its star is a terrible place.

Temperature on the other side of the planet will be extremely low. Water will quickly evaporate on the sunny side (huge ocean surface always bombarded by energy), fall as snow on the dark side and never come back.

Ocean will dry out very fast, a matter of years. Sunny side will become nearly completely dry. Vegetation is impossible. Unless your humans can eat rocks, the only habitable area is around the terminator.

D'Monlord
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No but yes with some alterations and a given value of habitable.

Most of what you describe is survivable given you have enough other earth life present, earths atmosphere is mostly maintained by earth life. but It will only stay habitable for a few million years at best.

As others have mentioned tidal locking is out, that will make the planet unhospitable very quickly too quickly for seeded earth life to stabilize.

Not tectonics means your planet will only stay hospitable for a few million years, you can't maintain the gasses in your atmosphere with an ocean and no tectonics, so it is slowly loosing oxygen and co2 and not just to space.

Shallow oceans and little land means lots and lots of big hurricanes, your planet may even have perpetual storms. This also means where your land is is very important, put it at the wrong latitude and your islands are deserts. This also means your oceans have no way to get rid of salt, so your ocean is either very very salty if it is old(to salty for earth animals) or the ocean extremally young and freshwater.

Also without land your planets temperature is much more at the mercy of the suns output. so again habitably is only short lived.

Not this is only the broadest sense of habitable, your locals will never get out of the stone age, and could easily be wiped out by a single volcanic eruption. The biggest downside of a hotspot volcano island without plate tectonics is they a huge and can be unpredictably explosive unlike normal shield volcanoes.

John
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