What's the best mix of oceans to land for a habitable planet?
Earth is about 29% land and 71% oceans. How significant is that mix for habitability? What does it tell us about exoplanet habitability?
There are very few places on Earth where life doesn't have a foothold. Multiple factors contribute to our planet's overall habitability: abundant liquid water, plate tectonics, bulk composition, proximity to the sun, the magnetosphere, etc.
What role does the ratio of oceans to land play?
Our understanding of habitability is pretty crude at this point, though it is based on evidence. We rely on the habitable zone around stars to locate potentially habitable exoplanets. It's a factor that's easy to ascertain from a great distance and is based on the potential for liquid water on planets.
We're still drawing a bigger, more detailed picture of habitability, and we know that things like plate tectonics, bulk composition, a magnetosphere, atmospheric composition and pressure, and other factors play a role in habitability. But what about a planet's ratio of oceans to land?
A new study examines that ratio in detail. The paper's been submitted to the journal Astrobiology and is available on the preprint site arXiv. It hasn't been peer reviewed yet.
The authors are Dennis Höning and Tilman Spohn. Höning is from the Potsdam Institute for Climate Impact Research in Germany, where he focuses on the interface between planetary physics and Earth System sciences. Spohn is the Executive Director of the International Space Science Institute in Bern, Switzerland. Spohn was also the principal investigator for the InSight lander's "mole" instrument, the Heat Flow and Physical Properties Package (HP3.)
Continental shelves, shown in turquoise, are rich in nutrients due to the weathering of the continents. Credit: coastalwiki.org
The Earth’s mantle is convective, and the continents act as a blanket, helping Earth retain heat. As time passes, radioactive elements, which produce heat as they decay, are depleted in the mantle as they reach the crust via upwelling. Credit: Wikimedia Commons
This figure from the paper illustrates the feedback cycles linking continental coverage (green), mantle water concentration (blue), and mantle temperature (red). Credit: Höning and Spohn, 2022
In their paper, the authors considered the habitability of planets based on the ratio of land to oceans. They compared a mostly land planet, to Earth, to a mostly ocean planet. Credit: Europlanet 2024 RI/T. Roger