Study attempts to uncover the best land-to-ocean ratio for habitable exoplanets: ScienceAlert

Earth is about 29% land and 71% oceans. How important is this combination for habitability? What does it tell us about the habitability of exoplanets?

There are very few places on Earth where life does not have a foothold. Multiple factors contribute to the overall habitability of our planet: abundant liquid water, plate tectonics, mass composition, proximity to the Sun, magnetosphere, and so on.

What role does the ratio of oceans to land play?

Our understanding of habitability is pretty crude at this point, even though it is based on evidence. We rely on the habitable zone around stars to identify potentially habitable exoplanets. It is a factor that is easy to ascertain from a distance and depends on the likelihood of liquid water being present on the planets.

We are still painting a bigger and more detailed picture of habitability, and we know that things like plate tectonics, mass composition, magnetosphere, atmospheric composition and pressure, and other factors play a role in habitability.

But what about the ratio of oceans to land on a planet?

A new study examines this ratio in detail. The study is titled “Diversity of Earth’s Partitions on Earth-like Planets and Implications for Their Habitability” and the paper was submitted to the journal Astrobiology Preprint is available at It has not been peer reviewed yet.

The authors are Denise Huing and Tillman Spoon. Hoening is from the Potsdam Institute for Climate Impact Research in Germany, where he focuses on the relationship between planetary physics and Earth system science.

Spoon is the executive director of the International Space Science Institute in Bern, Switzerland. Spohn was also the principal investigator for Insight’s “mole” instrument, the Heat Flow and Physical Properties Package (HP3).

Plate tectonics and related factors are at the root of the problem. Plate tectonics is the movement of continental plates on Earth’s surface as they travel along the top of the mantle.

Plate tectonics is still an active area of ​​research, and even with all we’ve learned, there’s still a lot scientists don’t know.

One of the crucial factors in the movement of tectonic plates is the “conveyor belt” principle. It says that when plates are subducted back into the mantle at convergent plate boundaries, new oceanic crust is formed at divergent boundaries, called seafloor spreading. The result is that the ratio of land to ocean remains constant.

With this ratio remaining constant, other factors also remain constant. And if these factors encourage a biosphere, that’s good for habitability. One of those things is nutrients.

Exposed earth is exposed to the elements, which transport nutrients around the world. Earth’s continental shelves are biologically rich regions. One reason is that all of the nutrient runoff from the continents ends up on the shelves. So the continents and their shelves contain most of the Earth’s biomass, while much less is found in the ocean depths.

Heat is another factor in plate tectonics and habitability. Continents act as a blanket over the mantle, helping Earth retain warmth. But this overall effect is mitigated by the depletion of radioactive elements in the mantle.

The radioactive decay of elements such as uranium in the mantle generates heat trapped by the mass effect of the continents.

At the same time, crustal renewal through tectonics brings more of these elements into the crust, where their heat is more efficiently disposed.

The Earth’s carbon cycle is also critical to sustaining life. This cycle is influenced by the movement of tectonic plates and also by the land-to-ocean ratio. The weathering of the continents removes carbon from the atmosphere roughly in equilibrium with the carbon emitted from the mantle by volcanoes.

Then there is the water content in the mantle. More water in the mantle lowers the mantle’s viscosity, which is defined as the resistance to flow. Mantle water content is part of a feedback loop with mantle temperature. The more water enters the mantle, the more easily it flows. This increases convection, causing more heat to be released from the mantle.

As the paper explains, all of these factors are linked, typically in feedback loops.

All of these factors and more on Earth combine to create a strong habitability. If the land-to-water ratio were biased toward more land, the climate would be drier, large parts of the continents could be cold and dry, and the biosphere might not be large enough to produce an oxygen-rich atmosphere.

Conversely, if there is too much water, there may be a lack of nutrients from continental weathering. This nutrient deficiency also prohibits a biosphere large enough to produce the oxygen-rich atmosphere necessary for complex life and a richer biosphere.

There is an extraordinary amount of detail in Earth’s tectonics, and it’s impossible to model all of it. Especially since scholars did not reach a consensus on many details. Much of it is hidden from researchers. They don’t have enough evidence to draw strong conclusions yet.

This study relied on scientific modeling to understand how planets have different land-to-ocean ratios.

Höning and Spohn modeled the three main processes that create the land-ocean ratio: growth of the continental crust, exchange of water between reservoirs at and above the surface (oceans, atmosphere) and in the mantle, and convective cooling.

of paper:

These processes are linked through convection and plate tectonics to:

  • Melting and volcanism associated with the subduction zone, and continental erosion governing the growth of the continents
  • Dumping of mantle water through volcanoes and re-gassing through subduction governs the water budget
  • Heat transfer through convection that controls thermal evolution.”

The authors reach one foundational conclusion. They write: “…the prevalence of continental coverage on Earth-like planets is determined by the strengths of positive and negative feedback in continental growth and the relationship between thermal coverage and radioisotope depletion upon continental crust growth.”

“The uncertainty in these parameter values ​​represents the main uncertainty in the model.”

These feedback loops would be present on any planet with tectonic activity and water. It is difficult to determine the relative strength of these rings. There are potentially a bewildering number of factors at play throughout the exoplanet group.

Researchers cannot model each factor individually, but this research comes down to feedback loops between all factors and whether they are positive or negative.

They conclude that strong negative feedback “…would lead to an evolution largely independent of the starting conditions and early history of the planet, implying a single present-day stable value of continental surface area”.

However, strong positive feedback loops create different outcomes. “For strong positive feedbacks, the evolutionary outcome may be very different depending on the onset conditions and early history,” they wrote.

The question is, do the same feedback loops form the exoplanets? Could exoplanets with plate tectonics also strike a balance between land and ocean coverage? Would a planet roughly the size of Earth with a similar temperature balance become similar to Earth, with its life-enabling stability?

First of all, research shows that land planets and ocean planets are possible, which shouldn’t be surprising. And of course we know that hybrid planets like Earth are possible.

In a previous paper, the same authors concluded that terrestrial planets are the most likely outcome. The next most likely outcome is ocean planets.

The authors point out that there are skepticism in all of this work, of course, and that there is a lack of data. However, their work highlights the mechanisms that create different land-to-ocean ratios on planets.

“Our discussion aims to provide a better qualitative understanding of feedback processes; we acknowledge the lack of data necessary for a detailed understanding of quantitative differences,” they wrote.

Other researchers have also addressed this problem. A 2015 study looked at planets around M dwarfs, which are the most common type of star in the Milky Way and where we’re likely to find most exoplanets.

And that study found “…a bimodal distribution similar to the area of ​​emerging Earth, with the surface of most planets either completely covered in water or with much less surface water than Earth,” the authors write.

However, that study looked at other factors and did not just focus on continental growth.

What does this study mean for the Earth? How can we answer the question in the headline: “What is the best combination of ocean lands for a habitable planet?”

As anthropocentric or terrestrial as they may seem, we can live on the answer.

This article was originally published by Universe Today. Read the original article.

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