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Earth is our only example of a habitable planet. The surface of Earth is 29% land and 71% ocean. I am wondering what other land ratios can create a habitable planet.

Half a billion years ago, Earth had little land and was covered in ocean. Yet, even back then Earth had life. So it is definitely possible for a habitable planet to have a surface that is 100% ocean and 0% land. I'm not sure if it is possible to have a life-bearing planet that is 100% land and 0% ocean though.

What is the maximum amount of land (ratio-wise) that a natural planet can have that humans can still live on? Assume the mass, volume, and distance from a main sequence star is the same as Earth.

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    $\begingroup$ You're asking the right quesiton in, perhaps, the wrong way. The question isn't how much land you can have, it's how much water you require to ensure an orderly evolution of life. I agree that there will be a practical distribution of that water, but the water volume minimum is more critical than the water distribution minimum. IMO. $\endgroup$
    – JBH
    Mar 13, 2023 at 1:19
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    $\begingroup$ "Half a billion years ago, Earth had little land and was covered in ocean": Citation needed. Where did you read such a thing? AFAIK Earth always had more or less the same amount of oceanic and continental crust. $\endgroup$
    – AlexP
    Mar 13, 2023 at 6:04
  • $\begingroup$ @Alexp Perhaps a mind trick due to the Pangaea? When you get one large land blob, the actual land-to-water proportions are more visible, like when you dunk a big cookie vs hundreds of tiny corn flakes in a milk bowl. $\endgroup$ Mar 13, 2023 at 13:23
  • $\begingroup$ halfa billion years ago earth had roughly the same amount of land. $\endgroup$
    – John
    Mar 13, 2023 at 22:06
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    $\begingroup$ Half a billion years was only slightly less land. Subduction zones continue to generate granite, and granite "floating" on the heavier basalt is what makes continents. Three billion years ago, the earth would have mostly been shallow oceans with a few archipelagos. $\endgroup$ Mar 14, 2023 at 18:24

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Best estimates say a world can be habitable (for a sufficiently broad definition of habitable) within the range of 75% land to 15% land.

That is just to have stable gas ranges and enough humidity to have consistent rain to keep the ocean supplied with minerals.

Note though that at higher land areas the amount of the land that is habitable is fairly small. And said planet will not remain habitable indefinity. Tectonics seems to matter more than total water content.

It is also worth mentioning that if the ocean is only in one hemisphere the other hemisphere will be too dry to be habitable.

Source

Source 2

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  • $\begingroup$ Excellent point about minerals washing down. $\endgroup$ Mar 14, 2023 at 18:26
  • $\begingroup$ I'm wondering, why is there a lower bound for land mass? Why not only 5% land, or even less? $\endgroup$
    – fgysin
    Mar 16, 2023 at 10:18
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    $\begingroup$ @fgysin because then there is not enough minerals coming into the water to support life and the surface is too smooth to force higher atmospheric mixing. the potential biological productivity is too low to produce an oxygen atmosphere, thus life never gets out of the single celled phase if it does evolve. there is also some fun geologic feedback loops that cause a ocean planet's plate tectonics to grind to a halt, and gas balance will cause a planet with too much ocean to cool rapidly turning into an ice planet. An ocean planet has too few ways to generate greenhouse gasses. $\endgroup$
    – John
    Mar 16, 2023 at 20:54
  • $\begingroup$ @John To what extent would submarine volcanism and subsidence from sea mounts etc act as a substitute? i.e, imagine an 'oceanic' planet with similar plate tectonics etc to Earth but with only say 10 % or less of its surface area being land (mostly in the form of continental mountain range chains and or mid ocean hots spots aka Hawaii etc). Would that provide enough minerals in solution? $\endgroup$
    – Mon
    Mar 16, 2023 at 21:12
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    $\begingroup$ @mon you could actually read the first paper, it goes into complete detail about waters effect on the mantle and plate tectonics, but no they would not provide enough minerals, if there was enough of them to do so they would make continents. that is how the current continents were made, a build up of lighter volcanic products that resist subduction. $\endgroup$
    – John
    Mar 16, 2023 at 21:21
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If what you mean by "ocean" is a large body of surface-level water, then a planet can theoretically have 100% land while being able to host life. If you are talking about no water, a planet cannot harbor life as we know it.

Now let's talk about the theoretical "no water" planet since it is easier to explain. (Note in this answer I will be talking about "life as we know it," not "life as we don't know it") Organisms on Earth need water because of it's molecular structure, according to this article Single celled organisms simply would not survive without water. A lot of cells have something called a phospholipid bilayer, which are lipid molecules (fat) arranged in a unique pattern where the polar "heads" face outwards while the nonpolar tails face inwards. This is because of water's unique polarity, which allows the heads to face outwards towards the water. If a cell or microorganism had little to no water, it would shrivel because of the absence of water inside the cell. When this happens, the bilayer begins to fall apart, and the cell dies.

In contrast, a planet with no surface water might be able to host life, but it will be unlikely. First of all, many scientists theorize that life/the chemicals needed for life came from asteroids. If they did, cells or single celled organisms would have needed water to be able to arrange themselves together. If there was a surface-level ocean that got baked by a solar flare, well, tough luck.

The reason why I say that life could be likely is because of geothermal vents. The first known life forms on earth were chemosynthesizers living around geothermal vents. If your world has a large underwater ocean or even a small aquafer, all it takes is some geothermal activity and a lot of luck.

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No oceans would be fine, but life like ours cannot evolve without a lot of of water in some form. Even in your percentages, we have lakes, ponds, creeks, rivers, rainfall, snowfall, even moist mists, that some desert insects and plants can collect and thrive upon.

Without any water, there would be no plants or biology as we know it.

That said, the entire planet can be like the interior of the Americas, land with a weather system. Lakes, rivers, aquifers (underground lakes). And we have the full spectrum of climates, from desert to rainforest. No reason your planet cannot be like that. As long as water is available in some form.

Yes, much of the rain is evaporated from the oceans. It doesn't have to be that plentiful, and probably would not be, coming from lakes and rivers. It just has to be available.

Like modern desert life (both plants and animals) life would evolve mechanisms from the start to get by on less water and to internally store more water. I'd focus my story research on desert life.

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  • $\begingroup$ Without oceans there is no weather system, to need oceans ot have enough humidity ot get rain. $\endgroup$
    – John
    Mar 13, 2023 at 22:30
  • $\begingroup$ @John As I said, water is necessary. I'd go further, a water cycle is necessary. But lakes and rivers can suffice, water evaporates from them and eventually comes down as rain somewhere. There is life in deserts where it doesn't rain for years on end, but there is still a water cycle, and life finds water, even in fog. I didn't say the planet would be covered in rain forests. A sprinkling of lakes and rivers would suffice. $\endgroup$
    – Amadeus
    Mar 14, 2023 at 10:49
  • $\begingroup$ No they really can't, you need at least a quarter of the surface covered in water to have a water cycle, nearly as much water as there is land on earth. oceans not a lakes. And there is humidity and groundwater in a desert, which why life can exist, your planet will not have either. also foggy deserts are coastal deserts. without oceans you have no humidity you have no rain, there is not enough water to get anything close to fog there is not enough water in the atmosphere to build up to get rain or fog. Humidity will be so low the driest desert on earth will seem like a swamp by comparison. $\endgroup$
    – John
    Mar 14, 2023 at 21:46
  • $\begingroup$ @John. Ah well, I don't believe that is true. If a quarter of the surface is lakes and rivers, with underground water in addition (aquifers are just underground lakes), I see no specific demand for oceans. It is the spinning the planet that causes winds and moves and concentrates evaporated water, it doesn't have to be all from an ocean. Tides occur even on the Great Lakes. I'll need to see some reputable science of your claim, until then I think we have a difference of opinion, and I don't care to argue it further. $\endgroup$
    – Amadeus
    Mar 15, 2023 at 11:32
  • $\begingroup$ you should read my answer if you want the science. if a quarter of the earths surface were covered in lakes the lakes are big enough to become saline and thus oceans. a quarter of the earths surface is close to the area of all the land on earth today. You need that much water just for evaporation to keep water in the atmosphere. All the lakes and rivers on earth don't even add up to 1% of the total land area. $\endgroup$
    – John
    Mar 15, 2023 at 20:41
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It may be possible with special chemistry of the planet's crust. It is rich in hygroscopic minerals that can absorb many times their weight in water while still solid. Many such compounds exist in nature, such as containing calcium and magnesium. Life could evolve to extract necessary water out of these and humans are able to do it too simply by heating the rocks. However some care needs to be taken, they are irritating and caustic to humans.

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In addition to Juraj's answer, it would also be possible for there to be no open water on the planet but abundant sub-surface water, with puddles existing for a short-ish time and then are absorbed into the ground. Think of a beach where when it rains the sand gets wet, but never actually creates a puddle. The water could also still be easily illuminated depending on the structure of the "sand" (e.g. Star Trek TNG; The episode with the silicon based life form that formed at the border of the light and dark while being underground).

So your question potentially becomes: Can humans live on a planet without open water?

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100%. Not entire surface will be perfect for life, but the same is true for Earth. We have Sahara mostly surrounded by water. Life exists in Sahara.

Some people mention that oceans necessary for life development. Well, life exists on Earth for ~3.42 billion years. It is plenty of time for planet to lose significant part of water. So, even if "habitable" somehow means "good enough for humans to evolve", the answer is the same. 100%.

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