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Welcome to Smooth World.

Most of this planet (at least two thirds, but preferably more) has only tiny differences in elevation, 30-40cm at most. No hills, no mountains, no valleys. Even plains found on Earth usually have a fair amount of difference in elevation, but not Smooth World. From a far enough distance the surface of this world seems to be mostly flat and smooth, hence its name.

The "smooth" surface of this planet is mostly covered in soil and a fairly even coverage of a grass like plant.

The remaining third (preferably less) of the world is comprised of whatever it takes to make this planet capable of supporting human life. I'm imagining this would be an ocean, but I am open to any ideas that makes this world capable of supporting life.

The world is approximately the same size as Earth, possibly the same kind of make up. I'm imagining that this world has no moon and is particularly old to facilitate the overall smoothness, but I am totally flexible on these points if they don't make any scientific sense.

How can I make Smooth World capable of supporting human life? I'm thinking of things like, a breathable atmosphere, protection from the radiation of space etc etc. We can assume that the humans living there have brought food, water and shelter with them.

The greater the "smooth" surface of this world, the better. If I have two good answers, but one of them has a greater percentage of smooth surface, that answer will get marked as correct. Ideally I'd want a world that was 100% smooth, but I don't think I'd be able to make it habitable if it was like this, but please correct me if I'm wrong here.

So, in addition to making this world scientifically feasible, I have two questions:

  • What will I need in this remaining "non-smooth" part to make this world habitable?

  • What will the weather be like on this world? More stormy or less stormy?

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    $\begingroup$ So the problem I see, is that you either make this a water world, or (if you must have land,) you won't have enough water to support life. Or the Ocean is an exception to the flat world thing but than you'd need to hand-wave that inconsistency... $\endgroup$ – Tezra Oct 24 '16 at 17:19
  • $\begingroup$ An important thing to think of is that the rotation of the planet will make it bulge at the equator. So you can have it be a smooth rotating ellipse, or a smooth (basically not rotating at all) stationary sphere. $\endgroup$ – Lacklub Oct 24 '16 at 18:12
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    $\begingroup$ @lacklub not necessarily. This world obviously has no tectonic movement. Its core might not be liquid. Might be stable enough material to not bulge. $\endgroup$ – SRM - Reinstate Monica Oct 24 '16 at 20:25
  • $\begingroup$ @SRM Mars has a larger equatorial bulge than earth despite longer days and no tectonic activity. $\endgroup$ – Lacklub Oct 25 '16 at 12:38
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    $\begingroup$ The Netherlands may be geographically boring, but it's not that boring, it can support life. (The Netherlands is flatter than a scaled up pancake) $\endgroup$ – Separatrix Nov 2 '16 at 14:19
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I think you might need to refine your definition of "smooth" some: for instance, you want there to be oceans as well, but by definition their bottoms will be at a significantly (hundreds to thousands of metres) lower elevation than the rest of the planet, so I guess you meant the landmasses? Also, would it be ok for you if the large "smooth" landmass would be at a higher elevation as a sort of dry steppe/desert highland? In that kind of setup, there would be large elevation differences involved in the transition between the deep sea to the shore to the highlands.

Further difficulties in having an ocean: if the planet has a moon of significant size, you'd get tides. These would wash up material (sand/soil), and since the land area is very flat, you'd get absolutely MASSIVE tidal flats. This kinds of makes it difficult to build anything lasting, which would make habitation tricky, but maybe not impossible. Maybe everything is built on platforms that float during high tide, and the local vegetation has adapted to repeated submersion. I just noticed you wrote that you were thinking you wouldn't have a moon, which would admittedly resolve the tide issue. I recalled a tidbit that the moon would have protected from asteroids, but based on a quick lookup just now, that seems dubious. Other benefits of having a moon have been suggested, however, including e.g. tidal forces helping to stabilize both oceans and the atmosphere, and the same tidal forces may also play a part in keeping the outer, liquid metal core flowing (due to friction against the tidal forces slowing down its cooling process). Having a moon is probably optional, but I'd say it would probably help in forming and maintaining a habitable planet, especially if the planet is supposed to have been capable of supporting life for a long time.

Another major complication regarding smoothness that I though of is weather, which you expect the world to have. If there's any significant amount of rainfall, either the surface would have to be very porous, with a water table fairly deep, or you'd end up with drainage issues, and any rainwater would quickly gouge channels into the landscape, forming rivers. Rain could also result in vast areas of soggy swampland.

Lastly, there's an issue of how did it end up this flat? You can't have active tectonics, or you'd get subduction, volcanoes, and tectonic plates colliding create mountain ranges as well. So the mantle, at least, would have to be cooled down to be a solid. You'd also need a lot of erosion to get rid of past meteor impacts, any mountains that used to exist, etc. Both of these would require a very old planet.

My suggestions on how to make this somewhat plausible: - Make it Earth-sized, for similar gravity. A high enough gravity is also required to retain an atmosphere; on the Moon or on Mars, gases slowly escape to space - A rocky, earth-like planet, but with a solid mantle that's been that way for eons, so there are no remaining signs of tectonic activity - However, to protect from solar winds and cosmic radiation, it would be preferable to have a magnetic field, i.e. the planet needs to have a liquid metal core - Save yourself the trouble of trying to work out tides etc. by making the entire world a flat desert. Think of the flat areas of Mars, or Tatooine. The flattest areas on earth are salt flats (not counting atolls, since they rise quite a bit from the seabed). - Maybe there are a few small hilly areas remaining, the last remnants of the largest mountain ranges of bygone eras. Or a couple of small but deep basins with shallow seas in them, if you really still want to have some oceans.

I think that's about it. I can't think of anything else required for a breathable atmosphere, even if it would probably be very thin and dry in this kind of environment. Protection from cosmic radiation is covered. You could still have Tatooine-style moisture collectors (or check out actual-modern day technology, e.g. http://themindunleashed.com/2016/10/this-wind-powered-water-condenser-can-pull-11-gallons-of-clean-water-out-of-air-each-day-for-drinking.html), and climate-controlled (cooled, heated, moister air, etc.) greenhouses for farming, even if you have to import the technology, fertilizer and such. Long-term this is cheaper and more comfortable than just importing all your actual food. Energy could be supplied by wind and solar power, both plentiful (more on that below), plus fission, fusion, or whatever future-tech you may want to use. The local grass could possibly be used as a raw material for fibers (including textiles, cardboard, chipboard...), bioplastics, maybe biofuel if that's needed, etc.

So, moving onto the area I have perhaps the most credentials for: weather (I've studied some Earth sciences in general, but meteorology most of all). Without a lot of moisture to handle heat transport from the equator to the poles, you'd get quite large differences in temperature depending on what latitude you're on. This would also create quite fast winds, at least sporadically if not constantly. On Venus, this has developed to an extreme situation where it has been suggested that Venus used to rotate in the same direction as the rest of the planets, but the runaway greenhouse effect of its dense atmosphere has created such strong winds in the direction opposite to that "usual" direction of rotation that over billions of years, the wind blowing in an opposite direction to the planet's rotation slowed, then stopped, and ultimately reversed it's direction of rotation. Take this with a grain of salt though, tidal locking effects from the sun probably played a part, if indeed it did rotate in the "common" direction originally. Plus for our "desert planet" thought experiment, we have a thin atmosphere, and it would also be further away from the sun, more similar to Mars or a slightly more distant and thus cooler Earth in that sense.

So the winds would be powerful. Have a look at the storm in the movie The Martian for inspiration. Solar power would also be relatively plentiful and above all reliable, if the atmosphere is fairly dry and thus there are little or no clouds (maybe some thin cirrus or such would form rarely). Other weather effects could include sparse fogs, dust/sandstorms, and Mars at least regularly sees snowfalls composed of dry ice (frozen carbon dioxide) near its poles, because it's so cold there. It sublimates directly from its gaseous state to a solid one, falls to the ground, and when the seasons turn/temperature rises, sublimates directly back to a gas again. Thus, if you have more of a "cold Tatooine" than Mars, water-based snow might be plausible in the colder regions near the poles. I mentioned earlier that outright rain would be problematic, but maybe some very light and sparse drizzle might happen on occasion, with the droplets just heavy enough they don't stay suspended in the air as a mist/fog. If this happens, it would happen during a fog, condensing out of the fog itself, not rain clouds.

I hope this was helpful, and provided some food for thought at least.

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  • $\begingroup$ Sorry but the part about Venus slowing down due to its winds is impossible. Due to the very law of conservation of momentum. A spinning body cannot stop itself without any external force. Remember that the atmosphere on Venus was initially from Venus itself. It did not appear suddenly and started slowing the planet down. $\endgroup$ – Bloc97 Oct 24 '16 at 18:40
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    $\begingroup$ @Bloc97 According to the Wiki page on Venus here, the heating was caused by the sun over billions of years as well as potentially tidal-locking to the sun, so conservation of energy/momentum is fine - the sun is the external force $\endgroup$ – Mithrandir24601 Oct 24 '16 at 19:00
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    $\begingroup$ @Mithrandir24601 Are there statistical data about the influence of winds on a planet's rotation? I'm curious to know what are the significant effects over a long time period. $\endgroup$ – Bloc97 Oct 24 '16 at 19:11
  • $\begingroup$ @Mithrandir24601 see this Q about Venus’s rotation on Wikipedia. $\endgroup$ – JDługosz Oct 25 '16 at 0:42
  • $\begingroup$ @Bloc97: As Mithrandir24601 noted, the sun's gravitational pull, that may have caused tidal locking, is an external force. So is the heating provided by the sun's radiation, the same differential (the equators get more than the poles) heating which drives the wind circulation on Venus, Earth, and elsewhere. $\endgroup$ – Rundil Oct 25 '16 at 12:02
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Flatworld for sure is possible, but there need to be some special conditions to make it so.

One way would be a high erosion world. High erosion can be achieved with strong and continuous winds, that take away every elevation like a grinder. However, this is an environment very hostile to people, that you can't say it is a hospitable planet, and plant life might not be possible in the way we are used to - and building any structure there will create a massive dune just behind. Such planetoids will tend to be made from sand or similar fine grain material. Think Dune or Tatooine, sans any mountains.

Next way to get a flat planet would be a water world, placed just at the point where the whole thing doesn't result in a superheated steam atmosphere and no ice crust covering the planet covering ocean. However, this is not a solid soil surface as asked. Think... Waterworld vor visualisation.

A variant of the water planet would be to have the whole "ocean" be very very flat and turn the whole planet into a giant swamp. With small alterations, a planetwide mix of swamp, bog and fens could make a very interesting setting. On the other hand, it is still not a very hospitable planet, but one could very easily farm rice on it if one uses a little pump and can manage to get a rice variant that can cope with those conditions. In parts, it might look like Dagobah in Star Wars, if you need a visual reference.

Drying down the planet a bit more makes it very tough: as soon as large parts fall dry, erosion has to take over to keep the planet surface flat and keep it smooth. That is unless the planet has been made artificially and the population takes the place of erosion. In that case, a very regulated water system could provide the rain and drainage needed to keep the only slightly elevated land in an arable condition. Having a meter of depths into the furthest channels and then a tilt to the main channels and lakes where water can evaporate might suffice to get it running, but I am not an agriculture engineer. Anyway, such a planet dug into 'fields' might work. Your idea of 25-30% water surface water coverage in these channels and lakes to provide the rain might allow several variants of agriculture, which depend on artificial weather patterns created by where bodies of water are kept in what size - the more water a plant needs, the closer it has to be towards a water vapor source. In such a system, trees will be the main means of breaking the wind and keeping the surface from eroding the channels and turning the whole planet into a dried up dustball.

However, there is a third hospitable planet system, that might do the trick besides Swampworld and Farmworld: Jungleworld. Jungleworld will not rely on water from outside, it has a very wet mangrove floor and day weather cycles, where the coming evening comes with heavy rainfall and the ground swamping up, while the morning and early day are ok, the rest of the day it is dominated by a wet heat close to 100% of humidity. Some larger, ever changing rivers will cut the neverending woods in different areas, and as the trees grow upwards and fall, they build up soil that is washed into the rivers and then building up at other points to new shallows, new trees growing... you get the point: if left alone, maps of the planet will outdate in a manner of months, seasons are unheard of. It might look like Kashyk from Star Wars if you need to visualize it somewhat, but a better depiction might be documentations about the African or Amazonian rainforests.

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An ocean world that looks like a flat land world

If the smooth world has ocean over its entire surface, with floating interconnected plant rafts covering large regions, then it would have very little variation in elevation, and even with a moon tides would be minimal (significant tidal ranges on Earth are due to interaction with land masses). This would give the appearance of very flat land.

This would not require the ocean bed to be particularly flat, just restricted to never breaking the surface of the water. This means the planet need not be old. It can still have a liquid metal core with a magnetic field, providing shielding from radiation.

If the plant coverage of the ocean is sufficiently deep, and also spread out far enough to minimise movement due to waves, then it may not be immediately obvious to people walking on the surface that they are not on a land mass.

There is now no need to have an exception to the smooth landscape for one third of the planet's surface. Even if the majority of the ocean surface is obscured by the plant growth, preventing evaporation, there will still be plenty of water entering the atmosphere through transpiration of the plants. There should be sufficient oxygen and water in the atmosphere to make it comfortably breathable to humans. Water vapour will also contribute to the protection from radiation, and the oxygen rich atmosphere will allow for the formation of an ozone layer.

This would be a world covered in grassy plains, although not necessarily having any soil beneath. However, if soil is important to your setting then you could make the raft deeper. There could then be plants that grow on the surface, rooting into the decaying matter on the raft, rather than directly into the ocean below. The mat of organic material need not necessarily be solely plant based. The surface plants could be growing on an underlying fungal layer that gets no light but gets it nutrients from the plants above (perhaps in a symbiotic mutually beneficial relationship).

It isn't clear whether you require the presence or absence of animal life in your world. Depending on what suits your requirements, this could be a world where animal life did not develop, or developed only in the ocean beneath the plains, or made the transition to land/flying animals. The fact that the planet need not be old should allow for flexibility in the stage it has reached in animal evolution.

Over time there may be some loss of essential minerals as dead organisms sink to the ocean floor as marine snow. You may want to design a cycle involving organisms that bring up material from the ocean floor, in order to explain the long term stability of these huge rafts. If you want this level of detail, the explanation would need to show that as much material is brought up from the sea bed as is deposited as marine snow.

Weather

You ask about whether this world would be stormy. The wind speed will depend on the variation in surface temperature. The higher the difference in temperature between different regions, the higher the wind speed. An ocean world is likely to have less variation in temperature as heat is not trapped in fixed land masses - ocean currents are free to redistribute heat evenly.

If you want to fine tune the wind speed you could choose the length of day to give more or less temperature variation. A fast planetary rotation (a short day) will give less time for the day side to heat up and the night side to cool down, keeping the surface temperature more even. Making the day longer will cause more temperature variation and higher winds.

Increasing the temperature variation is also likely to increase rainfall, which can be triggered where regions of atmosphere at different temperatures meet.

The colour of the organic raft and the plant life growing on it may also affect the weather. If there are large regions with very dark coloured plants, and other regions with very light coloured plants, then the dark regions absorbing more light will become warmer, leading to heating of the atmosphere above, and cool winds blowing into the dark regions as the heated air rises.

If this leads to more rainfall on the lighter regions, derived from water vapour that rose over the darker regions, then this may reinforce the distinct regions. For example, the lighter plants may require rainfall, and the darker plants may be intolerant of it. Perhaps the lighter plants require fresh water and the darker plants have deep roots that prefer salt water from the ocean below.

So this type of world gives you plenty of flexibility in choosing weather that suits your purpose.

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    $\begingroup$ I considered writing about an ocean world as well, but didn't consider floating "continents" of plant matter as an explanation for landmass. An interesting option. I agree that tides would be less of a problem; even if the tidal variation is, say, several meters, there's really nothing to compare it to; the entire local "land"scape rises and falls together, and there are no fixed reference points of solid land to compare to. One thing these rafts might result in would be pseudo-tectonic phenomena, at least subduction of the edges of one raft under another. Rifts might also be an issue. $\endgroup$ – Rundil Oct 25 '16 at 12:25
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    $\begingroup$ It might even be possible to have mussels and corals on the underside, providing at least some minerals, and more solidity to the entire raft. Beautiful idea, by the way. So long as you don't get shipwrecked with a tiger ;-) $\endgroup$ – Burki Oct 25 '16 at 12:50
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    $\begingroup$ I love this idea, its definitely thinking outside the box! $\endgroup$ – Jimmery Nov 6 '16 at 13:45
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To make this world habitable, you will need to consider the following:

What made the world flat? If it was geological (i.e. erosion), the planet would have to be tectonically dead. Life as we know it would cease to exist, as there would be no recycling of the lithosphere. Furthermore, a geologically dead planet does not have a magnetosphere, solar flares can cause mutations or even mass extinction events. Though if we consider alien lifeforms, they might survive by having a different composition. If it was external (i.e. another civilization deciding to flat the world out), the consequences can be mostly ignored, as alien tech cannot be analyzed scientifically.

How high is the "continent" with respect to the sea? To have seas, you need to have two different landmasses with different hights. If the continent is too low (i.e. under 5 meters), any variation of the ocean level will flood all the habitable lands near the oceans. (Think about waves, they can reach a meter high. Even higher during storms.)

Waterways would be impossible. No mountains, no high altitude glaciers. No glaciers, no rivers. No rivers... well no rivers and lakes. You would either have seas or oceans. Did I also mention that there would be no islands in the ocean, as islands are basically mountains. Furthermore, you have to get rid of tropical areas with too much rain, as an abundance of rain will carve rivers throughout the land, and you want to avoid that.

Since there would be no rivers, a huge part of a big continent might be inhospitable (arid).

So going back to your question:

The best and most habitable smooth world would be a ocean world with smaller continents, with spread-out seas within it. Most people would live near the seas within the continent, and stay far away from the ocean, as slight variations of the ocean level floods the land. Some plant life can deal with the rising oceans, so maybe some clever humans might find a way to live on top of those plants.

If you go the easy route a less habitable world (arid world) works too, no oceans, only many scattered seas. And humans only live near the small sea-oases.

P.S. weather patterns cannot be deduced from the geological standpoint only, there are many other factors. That question warrants a new post/topic entirely.

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Two major features are unavoidable on this planet, from which you can draw other conclusions:

1. Heinous wind

Weather patterns caused by heat differences across the planet would cause heinous wind basically everywhere, and chances are they are as regular and predictable as tradewinds. The upper atmosphere of gas giants (look at Jupiter timelapses) give you some idea of what standing pattern winds could look like on a planet with no surface features to impede weather.

2.The last third is likely a desert

All your water is in aquifers, or at least, if your soil-supported plant life lost a considerable amount of its biomass, it would be. You state that two thirds of your planet is covered in soil, and you could explain this as a way to have a regular water cycle: Water in the soil and the plants themselves is more readily evaporated, clouds condense and drop rain somewhere else. But anywhere that your highly regularized weather pattern doesn't drop rain would be a desert. If you use the previous Jupiter example and establish latitudinal weather bands, you could have some very interesting configurations: One huge green band in the middle, or alternating, or what-have-you.

The other thing I find interesting is what your biomes will look like. With no waterways, your plantlife could be homogeneous across an entire latitude, instead of locally differentiated depending on a plant's specific adaptations.

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    $\begingroup$ Jupiter isn't necessarily the best example here, because due to being a gas giant, it can and does have different rotational speeds at different latitudes. $\endgroup$ – Rundil Oct 25 '16 at 12:28
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Well, that's not the complete answer, just a "proposal":

What will I need in this remaining "non-smooth" part to make this world habitable?

Think of Stanislaw Lem's "Solaris": some kind of sentient being, instead of an actual ocean.

What will the weather be like on this world? More stormy or less stormy?

With smooth surface, Sun-like star and a regular "water ocean" the "seashores" of your continent will be somewhat habitable, however the deeper inside - the less life (desert).

So, I propose the ocean as a somewhat "magical" sentient being, that actually can make the whole continent habitable, depending on it's powers or nature.

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[Edit] Well, the first issue that comes to mind is wind. One of the major causes of local weather patterns is the mixing of pockets of atmosphere whose temperature and humidity levels differ greatly. Relative elevation differences create places where higher/colder and lower/warmer fronts meet. So that's one major factor that makes Smooth World very atmospherically stable. Alternatively, you could create weather without elevation through Smooth World's surface materials, i.e. if there were unevenly distributed surface mineral deposits or blanket megaflora with significantly different capacities to absorb and release radiant energy, this would create regions of relatively different temperatures, causing wind/weather.)

I agree with you, you'll need the prerequisite ambient gas mix. You don't want to get too goofy with the basic mechanics of why they can breathe on the planet. As for the protection from solar radiation, you *could get away with a weak to non-existent magnetosphere if you gave the planet a really thick hydrosphere, which would provide a means of watering the planet with constant mist and protect living organisms from radiation. (Water is a damn good radiation sink/shield, at least in terms of UV and radioactive decay types.)

If you're gonna ditch the oceans, you'll need a PROLIFIC mega-flora that will put water back out into the atmosphere, unless the planet is just very minimally populated. That would diminish the robustness of the global ecosystem, however. If you have a minimally populated planet, and its not super young, you'd have to back up to the solar system level and explain why almost no interstellar bodies/debris have disrupted the orbital equilibrium and wiped out life.

I assume you'll keep the whole chlorophyll/photosynthesis shtick, otherwise, you'll need a microbial alternative to act as an energy capture base AND carbon capture/O2 producer for all organic life.

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  • $\begingroup$ Could you elaborate on the elevation causing different "weather pockets"? I have a hard time following your answer. $\endgroup$ – Bloc97 Oct 24 '16 at 18:26
  • $\begingroup$ I was thinking just the day & night cycle would cause wind convection and get the ball rolling. $\endgroup$ – Ross Oct 24 '16 at 18:40
  • $\begingroup$ Sorry but -1, This answer at its current state is confusing, I have no idea who upvoted this. If @jSuich would elaborate further I would gladly +1. $\endgroup$ – Bloc97 Oct 24 '16 at 18:49
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Imagine a planet covered in moss, whose growth is at the same time spurned and limited by the chemical reactions between the moss, the vapours it produces, and isotope variants of the vapour compound derived through radioactive decay. This is inspired by chemical the balance needed to keep a yeast colony alive, and the derivation of protective patinas or oxides that protect deeper levels of a metal at the expense of surface levels.

The moss produces a compound with a short half life that reduces to water after a certain amount of time, which then nourishes the moss to continue this cycle. Moreover, the release of radiation caused by radioactive decay kills the moss, but only the spores (or whatever moss is made of) at the surface. These older mosses might have used up their supply of whatever it is that makes them produce the compound, which might be associated with reproductive systems as well as with the development of protective cellular structures or iodine-producing glands that inhibit decay caused by radiation. This might also explain why no genetic mutations occur, and why the entire landscape is covered by this one species of moss and nothing else (it was probably introduced artificially through terraforming or whatnot). The compound floats like a vapour due to the atmospheric conditions of the planet, which provide a suitable balance of pressure and temperature for achieving this effect. The water might sink to the soil, though it would not be heavier than the broken-down compound so that still remains to be explained.

The entire soil surface of the planet is thus covered by a moss whose growth is gradual and consistent due to a regular trimming process. A radioactive vapour envelopes the layer of moss, forming a sort of lower atmosphere, which is itself bound by an upper stratosphere.

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One of the main questions I would have thought is about water distribution. The water has to be shallow enough that it doesn't cover the entire planet, but on a totally flat planet, how can this happen? Shouldn't it be entirely covered or entirely dry?

The answer is in the spin of the planet. The centripetal force attracts the water to be deepest at the equator, meaning it could be deeper there than at the poles, in fact, all other things being equal, the poles are deserts and you'd have to have the habitable zone between the poles and the equator.

Second are the tides. This all depends on satellites (e.g. the Moon) and the local star (the Sun). With these you could cause complete chaos with the water if you wanted, dragging it all over the planet, so that at some stage or another there is nowhere that's left untouched. If life on the planet is reasonably amphibious (or learned to float a long time ago) then tides could be used to fertilise the land.

Then you have problem of the "water cycle". That is Ocean => Evapouration => Clouds => Rain => River => Ocean. Our rain usually happens when warm air evapourates some water, then comes in contact with cold air which condenses it again. The problem now is what happens with the fallen rain without hills and slopes to carry it away? I imagine a lot of stagnant pools in the slightly lower areas not currently occupied by oceans, just waiting to be evapourated again or to seep underground. Tidal & centripetal forces would not necessarily be able to drag it back with any great force, and probably not with a consistent enough direction for it to carve out rivers.

In fact I think you'd need the planet to have a very precise amount of water - enough that some of it is above surface level and it doesn't all just seep into the ground, but not enough that it entirely covers the surface.

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