Minimum amount of land a planet can have and still be habitable?

Question

I read in a paper, posted by L.Dutch, that:

It turns out that water worlds may be some of the worst places to look for living things. One study presented at the meeting shows how a planet covered in oceans could be starved of phosphorus, a nutrient without which earthly life cannot thrive. Other work concludes that a planet swamped in even deeper water would be geologically dead, lacking any of the planetary processes that nurture life on Earth. On Earth, rainwater hitting rocks washes phosphorus and other nutrients into the oceans. But without any exposed land, there is no way for phosphorus to enrich water on an aqua planet over time [...] There would be no ocean organisms, such as plankton, to build up oxygen in the planet’s atmosphere, she says — making this type of world a terrible place to find life.

But this leaves me wondering, what is the minimum amount of land I can get away with and it be reasonable/plausible, and still be able to give the ocean enough phosphorus to support life. Like, would 10% on a planet 1.5x Earth radii be enough (so around 114764506.18 km2, or 77.37% the area of Earth's land)? Could I maybe just make it low amounts of land and just have it be very rich in posphorus?

I want to try to have the minimum amount of land that I can get away with and have a planet that supports life. As it is the next best thing to having no land (like I wanted to, but it now seems that I can't do that without magic, which is very dissapointing). So, I am just trying to see what the expected minimum would be, at least what the minimum amount is that I can go with and it be at least feasible.

Summary

How much land is needed to keep a sufficent amount of phospherous in the ocean as to sustain life, looking for the lower end/minimum amount that I can get away with and it at least be scientifically feasible.

As I really want to do a planet with a minimal amount of land, while still developing life.

Notes

• I know you will likely say that I should do whatever I want without feeling creatively constrained by science, and while you are totally correct, I like keeping myself to hard sci-fi.
• I didn't use hard-science, but I would still like citations to papers and such if at all possible. It would be much appreciated.
• By habitable, I mean that life can develop and survive in the oceans, I do not mean habitable to human and human settlement. I also mean for the water to be habitable, I do not care for land-based life in this scenario, so please do not give examples based upon what would be necessary for land-based life to develop. : )

Answer 1

There are many scientists who believe life started on Earth in and around hydrothermal vents.

As AlexP said, you don't need land, you need a crust. The phosphorous and other chemicals may be brought up from the crust through hydrothermals.

However, this will greatly limit the places where life will exist. You may have obly hotspots with very simple forms of life. Life on the surface might be improbable, and a civilization much less likely.

If you want to have some landmass, there is no lower threshold in space, but there is one in a combination in space and time. The oceans on Earth were seeded with minerals taken by rivers over billions of years. Less landmass would just take longer to achieve that, and more landmass would get there faster. But depending on the difference in masses we might be talking over as short a span as a few million years to maybe billions of years.

Answer 2

You don't need any dry land at all.

What you need is continental crust. Continental crust does not have to be above water. Here on Earth we have an entire continent, Zealandia, which is almost completely submerged, except for New Zealand and New Calendonia.

Answer 3

We can look into Earth's past for answers:

At the end of the Cryogenian Period & It's snowball-earths 720-635 million years ago, there was a spike in phosphorus levels in the ocean, which most likely was a deciding factor for the evolution of animals in the following Ediacaran period due to it stimulating the production of algae.

It has been proposed that the phosphorus levels might have been caused by abundant underwater volcanism, delivering phosphorus-rich stuff to the surface where chemical reactions would conspire to release the phosphorus into the water.

So life on 100% ocean planets seems plausible as long as the planet has enough volcanism and maybe has more phosphorus-rich magma for good measure

Answer 4

Short answer: There is no lower limit of landmass, it depends on the amount of life you want.

Life can evolve without landmass. We know this because when life evolved on earth, the earth was probably covered by water. I guess you want a bit more life than a handful protozoa. The current marine life consists of ~ 6 billon tons of biomass-carbon. With the amount of land you mention (~77% of the current earths landmass) and minor tweaking of geological conditions (fewer endorheic basins, moderately higher concentrations of phosphorous in minerals), maintaining the same amount of biomass should be possible. Keep in mind that the planet you describe has a bigger surface, so it would still be less life per area, but it would probably be concentrated around the continents/islands anyway. If you want smaller landmasses, you could go substantially lower than 10% of the surface. Marine life would decrease accordingly, but probably not die out as a whole (although the "interesting" organisms usually die first). With smaller landmasses, the life would concentrate more and more around the land (and a few hydrothermal vents), leaving the open sea mostly deserted.

Answer 5

This NASA article considers some similar questions in the context of the solar system's ice-shell moons. Although it doesn't directly reference extra-solar liquid-surface ocean planets, it does seem to imply that life is likely possible on such planets. On a quick scan, the big challenge seems to be the potential lack of geological activity. If you've got tidal forces, plate tectonics, or similar effects which can cause mixing at the ocean/seafloor interface, then you should be able to get sufficient nutrients into the ocean for life to survive.

Note that "tidal forces" are going to be larger when your world is gravitationally bound to a large partner - either a double planet, or a moon orbiting another planet, or an eccentric orbit of a large moon, or something of that sort. A larger world may require a correspondingly-larger partner / driving force to maintain plate tectonics.

Also, as mentioned in the NASA article above, strong magnetic fields in a companion planet can induce currents in the ocean world's metallic core (and salty oceans), which can contribute to the mixing.

There's also a "heat of formation" leftover from when planets condense out of asteroids, comets, dust and gas. That's supplemented by the decay of long-lived radioisotopes. There are some simulations suggesting even deep ocean worlds could sustain enough mixing to support life. If the oceans are deep & cold enough, they'll have ice at the base, but the linked article argues that ice would still allow the nutrient-mixing necessary to sustain life.

Also note that there's are two possible ways to look at the criteria for a planet's habitability. One is requiring the planet to be such that life could have evolved there. A contrasting, looser standard is that life be able to survive there after being seeded from elsewhere.

Although it's possible there's a minimum landmass / surface-percentage to get life, it's still an active area of research. Serious researchers are at least open to the possibility of life on 100% liquid-surface planets.

To address your potential concerns about there being interesting, complex, and varied life on the planet, I suggest making the planet's conditions varied in both time & space. Give it plate tectonics, a metallic core and a relatively large moon to help drive a variety of interactions on different timescales. Give it both shallow and deep oceans, ice caps, volcanic vents, subduction, mid-ocean ridges, and other structures that allow different life forms to specialize in different areas of the ocean. Provide multiple energy sources so that lifeforms can specialize in one or the other, or try to shift between them: 1) solar / photosynthesis, 2) geothermal, 3) chemical consumption/oxidation. Energy sources and local life forms will vary by location. Most obviously, photosynthetic life will tend to concentrate near the surface.

Read some of the linked articles and use them to inspire the sorts of structures & justifications which fit your narrative.

Answer 6

Have you read A gift from Earth, by Larry Niven? It is set on a planet named "Plateau"

... a colony in the Tau Ceti system, ... settled by humans some 300 years before the plot begins. The colony world itself is a Venusian-type planet with a dense, hot, poisonous atmosphere. It would be otherwise uninhabitable, except for a tall monolithic mesa that rises 40 miles (64 km) up into a breathable layer in the upper atmosphere. This gives the planet a habitable area about half the size of California. The Captain of the first colony vessel named the feature Mount Lookitthat (from his interjection at first sight of it), and the colony became known as Plateau.

Half a California is 206,000 sq km, or, in more familiar units, $9 \times$ Wales.