The Earth’s iron core is surrounded by a mantle of rock and water on the surface. Scientists study exoplanets using this simple planetary model. However, Caroline Dorn from ETH Zurich suspects that planets are more complex than previously thought.

Most exoplanets are located close to their star. As a result, they are boiling hot and form oceans of molten rock that have not yet cooled down. Water dissolves very well in these magma oceans – unlike carbon dioxide, for example, which quickly outgasses and rises into the atmosphere.

The iron core lies beneath this molten rock. Dorn, together with researchers from Princeton, used physical models to study how water spreads between the molten rock and the iron core. To do this, they used model calculations based on fundamental laws of physics.

Nearly four years ago, a study suggested that Earth’s oceans contain only a small fraction of our planet’s water. The contents of more than 80 of Earth’s oceans may be hidden inside them. This is the conclusion reached by simulations and experiments that show how water behaved when the Earth was young.

The latest findings on the distribution of water on planets will have a significant impact on the interpretation of data from astronomical observations. Under certain circumstances, astronomers can use their telescopes in space and on Earth to measure the mass and size of an exoplanet. From these calculations, mass-radius diagrams can be created that allow conclusions to be drawn about the planet’s composition. The volume of water can be drastically overestimated by up to ten times if, as has been the case so far, the solubility and distribution of the water are ignored.

Dorn said: “There is much more water on the planet than previously thought.”

Water distribution is an important factor in understanding planet formation and evolution. Once water has entered the core, it stays there indefinitely. However, as the mantle cools, the water dissolved in its magma ocean can degas and rise to the surface.

“So if we find water in the atmosphere of a planet, there is probably a lot more of it inside,” explains Dorn.

Webb is aiming for the same thing. The telescope has the potential to detect molecules in the atmosphere of an exoplanet. Direct measurements are only possible for the composition of the upper atmosphere of exoplanets. Scientists want to determine the connection between the atmosphere and the inner depths of celestial bodies.

The new data on the exoplanet TOI-270d are particularly fascinating. They show clear signs of interactions between the planet’s magma ocean and its atmosphere.

According to this new study, planets with deep water layers are likely rare. As previously thought, most of the water on super-Earths may be trapped in the core rather than on the surface.

On this basis, scientists assumed that Earth-like life conditions could develop on planets with a high water content. Dorn and her team are convinced that their findings open up a new perspective on the possibility of water-rich planets on which life might be possible.

Dorn said: “The iron core needs time to develop. A large part of the iron is initially present in the form of droplets in the hot magma soup. The water trapped in this soup combines with these iron droplets and sinks with them into the core. The iron droplets behave like an elevator that is carried downwards by the water.”

“This behavior was previously only known for moderate pressures, such as those found on Earth. What happens in larger planets with higher pressures in the interior was unknown. This is one of the key findings of our study. The larger the planet and the greater its mass, the more the water tends to enter the core with the iron droplets. Under certain circumstances, iron can absorb up to 70 times more water than silicates.”

“Due to the enormous pressure in the core, the water is no longer in the form of H2O molecules, but in the form of hydrogen and oxygen.”

Journal reference:

1. Luo, H., Dorn, C. & Deng, J. The interior as the dominant water reservoir in super-Earths and sub-Neptunes. Nat Astron (2024). DOI: 10.1038/s41550-024-02347-z