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I'm relatively new to all this. I'm thinking of studying a little Earth sciences in the hope that it tells me but, up front, I want to know if I'm completely off. I want to know if a world with no high altitudes can be habitable. What leads me to think it might not be is that mountains are indications of

  • tectonic activity and so a liquid core generating a magnetic field repelling radiation,
  • low meteorite activity.

The amount of activity sufficient to result in a world with only large hills and low peaks that never rise above the "tree line", that is the altitude at which plants grow. Can anyone tell me if such a world is feasible? Can I have a habitable world which also has nothing in the way of mountains without constant battering from meteorites? Am I overthinking this and I could just have tectonic plates that don't really move in a way so as to produce mountain ranges? Sorry if this is far too many questions for a single post.

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    $\begingroup$ Raise water level, increase winds and storms, increase gravity and vualà no mountains. $\endgroup$ – user56803 Nov 9 '18 at 19:33
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    $\begingroup$ @Eries is it habitable though? $\endgroup$ – Alexis Nov 9 '18 at 19:41
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    $\begingroup$ @Alexis If you enjoy swimming in cold waters in order to avoid spinal problems from the excess gravity then it won't be that bad, I guess one could get used to anything anyway. $\endgroup$ – user56803 Nov 9 '18 at 19:50
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    $\begingroup$ Great question, but I don't think it can be reasonably answered here. The things you point out make it less likely that the planet would be habitable than Earth is, based on what we know about Earth. Unfortunately, we have exactly one data point for habitable planets, so no one really knows if a mountain free planet is somewhat unlikely to be habitable or highly unlikely to be habitable. So this question really can't be answered, and I'm going to vote that it should be closed as "opinion based." I did give it a +1 because I think it is a good question, but it isn't a good fit for our site. $\endgroup$ – kingledion Nov 9 '18 at 19:54
  • $\begingroup$ Funnily enough, my world is intended to be quite wet, where the boundary between land and water bodies is not distinct. The world is covered in primordial plant life, mainly mosses and ferns, the largest being tree ferns and giant club mosses, similar to early earth. However I'm not sure high sea levels are an answer here if I can help it, I was hoping for fairly undulating land, where the land levels never rise considerably. If I increase weathering and gravity, would this be sufficient? What I don't want to end up with is a world with little land, and that which is there, is "peaky". $\endgroup$ – Jeremy Hunter Nov 9 '18 at 19:57
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This is actually a more thought provoking question than it seems at face value. You are not overthinking things and if anything by overthinking it you make it worth exploring.

Though the first challenge is the ambiguity in the usage of habitable. Starting with Europa, it potentially meets all the qualifications. I believe Europa's surface meets the shape requirements. As for habitability. Scientists believe that beneath the ice there is likely lots of liquid water with the possibility for life. If that is the case that then could be habitable with undersea colonies. Kinda like Rapture.

So Europa or a similar setup potentially passes the test Also extends to water worlds

If you meant more conventional habitability with land then that is much more difficult. NO It shouldn't be possible. To better understand tectonics imagine a balloon deflating. when it reaches a certain size you begin to see ripples and sag marks as the material contracts with no where to go but bulge. This is similar to what happens as a planets core cools. The rock contracts resulting in mountains forming. Even on Mars this process is still happening.

Point is a planet would have to be completely cold for tectonics to stop. Therefore it would be inhabitable.

The other way to achieve this and be geologically active would be to have hyper erosion. The chemistry and energy needed to make this happen would be in-hospitable for non microbial life.

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    $\begingroup$ Thanks for seeing why I think it is a challenging issue. It makes me think of that watery planet in Interstellar. For vast swathes of flat land to be covered in just a relatively thin layer of water with no land to break up waves allowing them to build up hundreds of feet, wouldn't it mean no tectonic activity, and with no activity, what would prevent radiation stripping away an atmosphere and freezing the water on that planet? Would I face a similar problem? $\endgroup$ – Jeremy Hunter Nov 9 '18 at 21:12
  • $\begingroup$ I guess by "inhabitable", I mean the atmosphere would be composed of sufficiently hospitable elements and of a temperature and moisture range that would host plant life (though nothing as serious as large, woody trees in "the wild") out in the open, and humans would not find it very difficult to construct settlements on the planet. The issue of Europa in a watery sense means the whole surface would need to be watery. Maybe floating islands would be a better place for humans to live on this planet? $\endgroup$ – Jeremy Hunter Nov 9 '18 at 21:15
  • $\begingroup$ nothing, a cold planet is dead and the atmosphere is getting stripped. The water planet is intersteler is a different story and was inhabitable as an example of hyper erosion. $\endgroup$ – anon Nov 9 '18 at 21:16
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The smoothest planet in the solar system is probably, AFAIK, Europa. It does have plate tectonics, of a sort, but it's ice plates floating on a water ocean. In terms of habitability, not great. Also, smoothness is here defined as maximum deviation from ellipsoid, not how steep those deviations are. I recently read (I forget where) that it might have fields of knife-like ice projections (which have a name that I also forgot) metres high and quite impassable.

Since you're probably looking for something more earth-like, I would guess that you can't avoid mountains. To develop complex life, you need a healthy mix of elements on the surface, plus a reasonable protection from radiation. You get the first from tectonics and volcanism, and the second from a molten ferromagnetic core that provides a magnetosphere. You also need surface volatiles (water, atmosphere) which in Earth's case got there from cometary bombardment, if memory serves. Someone correct me if I'm wrong.

Finally, I would think that thermodynamics on a flat planet would be too bland. We have no idea how life got started (ok, we have too many ideas), but what is sure is that you need an mixture of gradients: one chemical species diffusing from A to B, a temperature gradient going in a different direction, a liquid current carrying everything the third way etc. That is why all models of biogenesis, starting with Darwin's warm little pond are on some kind of interface. If the planet is too flat, I suspect the conditions might be too uniform. I haven't checked this suspicion in any way, and I doubt anyone has, so it could be completely wrong. But I have a hunch.

Of course, you might not want life to develop there, just to be able to terraform and settle it. In that case, apart from probable lack of magnetosphere, I don't see any objections. Your ecosystems might be a bit boring, though.

Follow-up

An idea that might work: take an old planet around an old sun, and make it the only planet in the system. You could have a red dwarf that got too close to some energetic young suns and had the rest of its planets stripped away. Leave a circum-stellar dust ring that provides just enough micro-bombardments for any mountains to be eroded to low hills.

Now you have the right shape, but you lack water and atmosphere. No problem: the micro-bombardment has gradually shifted the planet's orbit outward (would that happen? Um...) and the outer layers of the dust ring are water-rich (which would happen). So a layer of water accumulates on the planet. Take it from there.

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  • $\begingroup$ No meteor impact can make a meaningful shift in a planet's orbit. Even a planet-shattering collision with something of comparable size will only produce an asteroid belt in roughly the former planet's orbit. $\endgroup$ – Mark Nov 9 '18 at 21:04
  • $\begingroup$ @Mark what no, in the early solar system, planets got knocked out of orbit all the time. $\endgroup$ – anon Nov 9 '18 at 21:18
  • $\begingroup$ "The smoothest planet in the solar system is probably, AFAIK, Europa." Smoother than the Earth (which is very smooth compared to it's diameter)? $\endgroup$ – RonJohn Nov 9 '18 at 21:36
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    $\begingroup$ Europa's a planet? Cool! $\endgroup$ – only_pro Nov 9 '18 at 22:33
  • $\begingroup$ @only_pro I've always missed pluto, now we got back up to 9! $\endgroup$ – Reed Nov 9 '18 at 22:37
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What leads me to think it might not be is that mountains are indications of a) tectonic activity and so a liquid core generating a magnetic field repelling radiation,

That's correct.

and b) low meteorite activity.

I don't think that's correct. (Low circular hills, yes, but mountains? No.)

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  • $\begingroup$ Can you expand on your second point please? If anything was going to eradicate mountains, it would be sufficiently heavy bombardment as to effectively melt the surface, resulting in a new face, this is what I mean. $\endgroup$ – Jeremy Hunter Nov 9 '18 at 21:09
  • $\begingroup$ @JeremyHunter what happens when you melt something tall (like an ice cube or lump of butter)? It melts into liquid. Liquids are notoriously poor at being mountains. They are -- in fact -- much better at flatness. Thus, that's what you need if you want a mountain-free planet. $\endgroup$ – RonJohn Nov 9 '18 at 21:13
  • $\begingroup$ That's what I thought. When you said you don't think it's correct, I think there might have been a misphrasing on my part. The low meteorite activity was referring to habitability (which it wouldn't help), rather than eradicating mountains (which it would be good at) $\endgroup$ – Jeremy Hunter Nov 9 '18 at 21:17
  • $\begingroup$ @JeremyHunter how low is "low meteorite activity"? Earth certainly has low meteorite activity. Such activity does nothing to hinder our planet's habitability and certainly doesn't eradicate mountains. $\endgroup$ – RonJohn Nov 9 '18 at 21:28
  • $\begingroup$ yes I know, I'm saying meteorite activity would have to be below a certain level to make it habitable, but without significant meteorite activity, mountains wouldn't be levelled. It's a catch 22. There aren't many other options. $\endgroup$ – Jeremy Hunter Nov 10 '18 at 9:53
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Earth was like this sometime in the Archean Eon

Life started in the early Archean (Earth was habitable in the Archean).

The first continents formed in the Archean. Mountains are caused by two continents colliding or rubbing together.

Some continent had to be the first continent. So there was a time when Earth was habitable but couldn't make a mountain because it didn't have two continents to rub together.

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  • $\begingroup$ See I thought about this too, but you have to realize that what the Archean is thought to look like is incredibly theoretical at the moment. Much of the geologic record from this time has been lost which is part of the ambiguity. Also if you consider the fact that this time must have been plagued with rampant volcanos then theres no way it can meet the requirements $\endgroup$ – anon Nov 9 '18 at 21:39
  • $\begingroup$ Since the planet I'm constructing is not in fact Earth, does the postulation of what post Hadean Earth looked like matter that much? Could a world without life that recently left its own Hadean stage not produce the world I'm looking for, a relatively flat world with a lot of water? $\endgroup$ – Jeremy Hunter Nov 10 '18 at 10:03
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Less likely,but not improbable. And habitable for what? Life is not all about humans. Meaby you have more or less flat continent and all that not-so-common activity is down below in your oceans. Harsher weather is not something you can not overcome and adapt to. If you fear meteors better to have less them in your solar system and have good pal gas giant collecting them for you.

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  • $\begingroup$ I definitely mean habitable for humans. Considering we don't know what extraterrestrial life requires, we can only go with what the various forms of life on earth need. And a world inhabitable only by tardigrades or algae is not a very interesting one except maybe in the case of evolution over millions of years. $\endgroup$ – Jeremy Hunter Nov 10 '18 at 9:59
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An Iron planet they have no plate tectonics or strong magnetic field as they cool rapidly after formation. This leaves you with a pretty barren planet though.

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