We know that the Earth has an iron core surrounded by a mantle of silicate rock, and that it has water (oceans) on its surface. This simple planetary model is still used by scientists today to study exoplanets – planets orbiting another star outside our solar system.

“It is only in recent years that we have begun to realize that planets are more complex than we thought,” says Caroline Dorn, professor of exoplanets at ETH Zurich.

Most of the exoplanets known today are located close to their star. This means that they are mainly hot worlds consisting of oceans of molten magma that have not yet cooled and formed a solid mantle of silicate rock like Earth. Water dissolves very well in these magma oceans – unlike, for example, carbon dioxide – which quickly outgasses and rises into the atmosphere.

The iron core is located under the molten shell of silicates. How is the water distributed between the silicates and the iron?

This is exactly what Dorn investigated in collaboration with Haiyang Luo and Jie Deng from Princeton University using model calculations based on fundamental physical laws. The researchers present their results in the journal Natural astronomy.

Magma soup with water and iron

To explain the results, Dorn has to go into a little more detail: “The iron core takes time to form. A large part of the iron is initially contained 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 to the core. “The iron droplets behave like an elevator that is carried downwards by the water,” explains Dorn.

Until now, this behavior was only known at moderate pressures, such as those found on Earth. What happens on larger planets with higher pressures in the interior was unknown.

“This is one of the most important results of our study,” says Dorn. “The larger the planet and the greater its mass, the more the water tends to anchor itself with the iron droplets in the core. Under certain circumstances, iron can absorb up to 70 times more water than silicates. However, due to the enormous pressure in the core, the water no longer takes the form of H₂O molecules, but is present in hydrogen and oxygen.”

Large amounts of water are also found inside the earth

The study was prompted by investigations into the Earth’s water content, which produced a surprising result four years ago: the oceans on the Earth’s surface contain only a small fraction of the total water on our planet. The contents of more than 80 oceans, however, could be hidden in the Earth’s interior. This is shown by simulations that calculate how water behaves under conditions that prevailed when the Earth was young. Experiments and seismological measurements are therefore compatible.

The new findings about the distribution of water on planets have dramatic consequences for the interpretation of astronomical observation data. With their telescopes in space and on Earth, astronomers can measure the weight and size of an exoplanet under certain conditions. From these calculations, they create mass-radius diagrams that allow conclusions to be drawn about the planet’s composition. If the solubility and distribution of water are ignored – as has been the case so far – the volume of water can be dramatically underestimated by up to ten times.

“There is much more water on the planet than previously thought,” says Dorn.

Understanding the history of evolution

The distribution of water is also important if we want to understand how planets form and evolve. The water that has sunk into the core remains trapped there forever. However, the water dissolved in the magma ocean of the mantle can lose gas as the mantle cools and rise to the surface.

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

This is the task of the James Webb Space Telescope, which has been sending data from space to Earth for two years. It is able to detect molecules in the atmospheres of exoplanets.

“Only the composition of the upper atmosphere of exoplanets can be measured directly,” explains the scientist. “Our group now wants to establish the connection between the atmosphere and the interior of celestial bodies.”

Particularly interesting are the new data from the exoplanet called TOI-270d.

“There, evidence was gathered that there are indeed such interactions between the magma ocean in its interior and the atmosphere,” says Dorn, who was involved in the corresponding publication on TOI-270d. Her list of interesting objects that she would like to study more closely also includes the planet K2-18b, which made headlines because of the probability that life exists on it.

Are aquatic worlds ultimately life-sustaining?

Water is one of the prerequisites for the development of life. There has long been speculation about the possible habitability of water-rich super-Earths – planets with a mass many times greater than that of the Earth and whose surface is covered by a deep, global ocean. But calculations suggested that too much water could be hostile to life. The argument was that in these watery worlds, a layer of exotic high-pressure ice would prevent the exchange of vital substances at the interface between the ocean and the Earth’s mantle.

The new study now comes to a different conclusion: planets with deep water layers are likely to be a rarity, since most of the water on super-Earths is not on the surface, as previously thought, but is trapped in the core. This leads scientists to believe that even planets with relatively high water content could have the potential to develop Earth-like living conditions. As Dorn and her colleagues conclude, their study sheds new light on the possible existence of water-rich worlds that could host life.