Most of a planet’s water is usually not found on its surface, but is hidden deep inside. This has an impact on the potential habitability of distant worlds, as model calculations by researchers at ETH Zurich and Princeton University show.

We know that the Earth has a core made of iron, a mantle made of silicate rock above it, and water masses (oceans) connected to the surface. This simple planetary model is also used in science to study so-called exoplanets, which orbit another star outside our solar system. “It is only in recent years that we have begun to take into account that planets are more complex,” says Caroline Dorn, Professor of Exoplanets at ETH Zurich.

Most of the exoplanets that we know of today are located close to their star. They are therefore mostly hot worlds that do not yet have a cooled mantle of silicate rock like the Earth, but rather oceans of molten magma. Water dissolves very well in these magma oceans – in contrast to carbon dioxide, for example, which quickly outgasses and rises into the atmosphere.

The iron core is located under the molten silicate mantle. What about the distribution of water between the silicates and the iron? This is exactly what Dorn investigated together with Haiyang Luo and Jie Deng from the American Princeton University using model calculations based on fundamental physical laws. The researchers present their results in the journal Nature Astronomy.

Magma soup with water and iron

To explain the results, study author Dorn has to go into a little more detail: “The iron core only forms over time. In the hot magma soup, there is initially a lot of iron in the form of droplets.” The water dissolved in the magma soup likes to combine with these iron droplets and sinks with them to the core. “The iron droplets act like an elevator that brings the water downwards,” explains Dorn.

Until now, this behavior was only known from 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 most important results of our study,” says Dorn: “The larger the planet and the more mass it has, the more the water tends to sink into the core with the iron droplets.” Under certain conditions, iron can absorb up to 70 times more water than silicates. Under the enormous pressure in the core, however, the water no longer exists in the form of H₂O molecules, but as hydrogen and oxygen.

Large amounts of water also in the earth’s interior

The reason for this study was research into the water content of the Earth, which four years ago came to a surprising conclusion: the oceans on the Earth’s surface contain only a small part of the total amount of water on our planet. The contents of more than 80 oceans could be hidden in the Earth’s interior. This was shown by simulations that calculated how water behaves under conditions such as those that existed on the young Earth. Experiments and seismological measurements are compatible with this.

The new findings about the distribution of water in planets have drastic effects on the interpretation of astronomical observation data. Using their telescopes in space and on Earth, astronomers can measure how heavy and how large an exoplanet is under certain circumstances. From this they create so-called mass-radius diagrams, from which conclusions can be drawn about the composition of the planet. If the solubility and distribution of the water are ignored – as has been the case up to now – the amount of water is drastically underestimated, up to ten times as much. “Planets are much richer in water than previously thought,” says Dorn.

Understanding development history

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. The water dissolved in the magma ocean of the mantle, on the other hand, can outgas and reach the surface when the mantle cools. “So if you find water in the atmosphere of a planet, there is probably a lot more of it inside,” explains Dorn.

This is exactly what the James Webb Space Telescope is looking for. It has been sending data from space to Earth for two years. It can detect molecules in the atmosphere of exoplanets. “Only the composition of the upper atmosphere of exoplanets can be measured directly,” explains the researcher. “In our group, we want to establish the connection between the atmosphere and the deep interior of the celestial bodies.”

New data from the exoplanet called TOI-270d is particularly interesting. “There, evidence was collected that such interactions between the magma ocean in the interior and the atmosphere actually exist,” says Dorn, who was involved in the corresponding publication on TOI-270d. Her list of exciting objects that she would like to study more closely also includes the planet K2-18b, which made headlines because it could possibly harbor life.

Are water worlds life-friendly after all?

Water is considered to be one of the prerequisites for the development of life. For a long time, there was speculation about the possible habitability of water-rich super-Earths, i.e. planets the size of several Earth masses, whose surface is covered by a deep, global ocean. Then calculations suggested that too much water could be hostile to life. On these water worlds, a layer of exotic high-pressure ice at the transition between the ocean and the planet’s mantle would prevent the exchange of vital substances, so the argument goes.

The new study now comes to a different conclusion: worlds with deep water layers are probably not common, because the majority of water on super-Earths is not on the surface as previously assumed, but is enclosed in the core. Therefore, planets with a relatively high water content could also have the potential to develop Earth-like, life-friendly conditions, the researchers suspect. Their study thus sheds new light on the possible existence of water-rich worlds that could host life, conclude Dorn and her colleagues.

Caroline Dorn is Professor for Exoplanets at the Department of Physics at ETH. Her research is part of the external National Centre of Competence in Research (NCCR) PlanetScall_made and the Center for Origin and Prevalence of Life (COPL) at ETH.

Luo H, Dorn C, Deng J. The interior as the dominant water reservoir in super-Earths and sub-Neptunes. Nature Astronomy, 20 August 2024, doi:externe Seite10.1038/s41550-024-02347-zcall_made

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