We know that as a planet increases in size, its surface gets covered by oceans (more water is captured by gravity and shape is more spherical). Therefore, a planet larger than Earth can not have a large or significant landmass.

For Earth, oceans were very important for the origin of life. However, because most of the ocean seabed does not get sunlight and there are not enough nutrients in the surface of the oceans, life is not as dense in water as it is on land. This drastically reduces the ability for a larger planet to host civilization.

My question is this: What are potential realistic ways life can evolve to get energy and nutrients simultaneously (like plants) while living in an ocean? Such life must be able to proliferate enough to make dense ecosystems like forests and grasslands that gave rise to civilization.

I have read that potentially, volcanic systems could host life. But I do not believe that would be enough energy for large amounts of life especially considering that large planets have less activity afaik.

P.S: I am assuming that dense ecosystems are necessary for civilization. Is that true?

  • $\begingroup$ Yes, I am saying that landmass has higher biomass per area (which at least according to a bio course a few years is true). Evolutions of civilized beings seems (I am not an anthropologist) to require ecosystems like forests and grasslands that enable higher specialization in response to increase in complexity in predation and competition rather than just surviving harsh elements and depending on very few other species. As for the later part of the your question, oceans cover 75% of the surface, yet they have less biomass than the land, which is only 25%. $\endgroup$ – Kavi Vaidya Jun 3 '18 at 11:54
  • $\begingroup$ I stand corrected on the issue of biomass - thanks for that! - I still doubt the assumption of dense ecosystems being necessary for civilization, but have no material to back that up, besides pointing to feats of organisation and problem solving in ocean-dwelling animals. $\endgroup$ – bukwyrm Jun 3 '18 at 12:06
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    $\begingroup$ You are describing early stage of life on our own Earth, so what's your actual problem? $\endgroup$ – Mołot Jun 3 '18 at 12:22
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    $\begingroup$ The statements you make in your first paragraph are not things we "know". The origin of Earth's ocean's is not really known. Please delete that first paragraph and read Wikipedia's page on the Origin of Water on Earth. $\endgroup$ – StephenG Jun 3 '18 at 12:47
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    $\begingroup$ @jamesqf We do have examples of planets larger than Earth but not gas giants (some exoplanets ), but of course we don't have any idea if they have oceans or even atmospheres. Apologies if that seems pedantic, but ... err ... it is. :-) $\endgroup$ – StephenG Jun 3 '18 at 19:11
  • You assert that sunlight and nutrients are necessary for life.
  • You assert that oceans are nutrient poor, presumably because nutrients do not stay in solution and settle at the bottom. In a water world, large swaths might be nutrient poor.
  • You assert that a large world will have less volcanism.

Sunlight is not a problem. You can have as much sunlight as you want your star to provide.

If nutrients settle down the bottom of the sea, there are 2 options. 1: Bring in more nutrients from elsewhere. 2: Stir them up from the bottom

1: From elsewhere. In this question I proposed a world with a constant meteor bombardment. What is a plausible way to fuel highly active volcanism and seismic activity on my planet?. Your world could have a rain of meteors which fertilizer the sea and keep the sky looking awesome at night.

2: Stir it up. You could have volcanism and eruptions at the sea floor. No doubt you are aware of the deep sea ecosystems fueled entirely by such eruptions and working without any energy input from the suns. http://wwf.panda.org/our_work/oceans/deep_sea/vents_seeps/ If you have lots of plate tectonic action you will probably have lots of volcanism. You can have a huge molten core with magnetodynamic motion fueling this.

Or you could have a massive planet sized moon orbiting close. The tidal forces exerted by this moon will flex and stretch the crust of your world - and also the waters. Tidal forces might be enough to keep the water moving and reduce settling of suspended nutrients.

But the best idea: giant ocean sea worms!

sea worm!


Of extra planetary origin, these colossal worms plow slowly through the sea bed, stirring up clouds of sediment in their wake much as earth worms plow and turn the dirt, moving nutrients in the process. The sea worms are chemoautotrophic and process great quantities of minerals to get the energy they need.

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    $\begingroup$ Just to be clear, my assertion are based on commonly held beliefs. Sunlight is not necessary for life, but all I was saying was that normal life could not exist. $\endgroup$ – Kavi Vaidya Jun 4 '18 at 12:09

First to speak to your assumption about civilisation; populations need rich life supporting habitats but civilisation seems to advance better in harsher environments where societies are challenged by factors including but not limited to scarcity of food and water. When you look at where and how human techno-civilisation has advanced it's been drought, deserts and the cold of temperate climates that has pushed us forward not the lush forgiving tropics.

Now for a plant-based ecology on a world without land something surface dwelling and free-floating like Sargasso Weed would be the basic requirement, not the special tolerances of Sargassum just it's ocean going habits and the fact that it's photosynthetic that's pretty important too. You'd need some form of upwellings that bring mineral rich waters up from depth, these can be either geological, caused by heat from rifting, or thermohaline, caused by evaporation and density shifts, in nature. That gets the basic nutrient and gas cycles going from which primary consumers can start to feed and evolve. After that it's a crap shoot because evolution is not a clearly understood process.

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    $\begingroup$ I think perhaps you have a personal bias, which has led you into error. To a lot of people, those "lush forgiving tropics" are disease-ridden and debilitatingly hot, while the "cold of temperate climates" is actually quite comfortable. $\endgroup$ – jamesqf Jun 3 '18 at 18:26
  • $\begingroup$ @jamesqf My bias, if any, is based on comparisons of complexity of material culture contrasting tropically settled Polynesian groups like the Tahitians and later temperate climate groups like the Maori. My reading indicates that Tahiti did, and does, have amazing oral histories and traditional rituals but it was very limited in physical records, visual art forms and unique tools. In New Zealand the climate is relatively unforgiving and food is harder to come by the material culture though younger and an offshoot of the same parent culture is rich with unique symbolism and form language.... $\endgroup$ – Ash Jun 3 '18 at 18:42
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    $\begingroup$ @jamesqf ...continued I can make similar arguments about the level of societal complexity, structure and stratification contrasting areas like Sub-Saharan Africa and Turkey or even the Arabian Peninsula based on the findings at contemporaneous archaeological sites. Where climate is mild and food is abundant humans have less societal adaptation pressure, some studies suggest there is more physical adaptation among those native to the tropics, particularly around disease resistance and heat tolerance, but civilisations seem to stagnate without a challenge. $\endgroup$ – Ash Jun 3 '18 at 18:48
  • $\begingroup$ @Ash But look at the following early civilization: 1. Mesopotamia 2. India 3. Egypt and 4. China. They have started in river banks in temperate regions (with exception of egypt). $\endgroup$ – Kavi Vaidya Jun 4 '18 at 12:07
  • $\begingroup$ @KaviVaidya Exactly, Egypt is in a desert the rest are temperate, none of them are in the Tropics. $\endgroup$ – Ash Jun 4 '18 at 12:10

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