Researchers have recently discovered that most of a planet’s water is not found on its surface, but is hidden deep inside. This has implications for the potential habitability of distant worlds, as model calculations by scientists at ETH Zurich and show. Princeton University.

• Using computer simulations, researchers have calculated a new model for the distribution of water on exoplanets – planets that orbit another star outside our solar system.
• Their results show that most of the water on young planets is not on their surface, but deep inside them. In addition, the total potential water volume on exoplanets has been dramatically underestimated.
• The new model is important for understanding planet formation and assessing the potential habitability of exoplanets.

The Earth has an iron core surrounded by a mantle of silicate rock and water (oceans) on its surface. Until now, science has used this simple planetary model to study exoplanets – planets that orbit 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 consist mainly of hot worlds made up of oceans of molten magma that have not yet cooled down to form a solid mantle of silicate rock like the Earth. Water dissolves very well in these magma oceans – unlike carbon dioxide, for example, which quickly outgasses and rises into the atmosphere.

The iron core is located under the molten silicate mantle. How is the water distributed between the silicates and the iron? This is exactly what Dorn investigated together 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 Nature 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.

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 results of our study,” says Dorn. “The larger the planet and the greater its mass, the more the water tends to enter the core with the iron droplets. 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 H.₂O molecules, but is present in hydrogen and oxygen.

Large amounts of water are also found inside the earth

The reason for this study was research into the water content of the Earth, which four years ago produced a surprising result: 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.

“Most of the water on exoplanets is deep inside, not on the surface.”

Caroline Dorn, Professor for Exoplanets

The new findings on 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, under certain conditions, determine the weight and size of a Exoplanet. From these calculations, they create mass-radius diagrams that allow conclusions to 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 can be dramatically underestimated by up to ten times. “Planets are much richer in water than previously thought,” says Dorn.

Understanding the history of evolution

The distribution of water is also important if we want to understand the formation and development of planets. The water that has sunk into the core remains trapped there forever. The water dissolved in the magma ocean of the Earth’s mantle, on the other hand, can degas and rise to the surface as the mantle cools. “So if we find water in the atmosphere of a planet, there is probably a lot more of it inside,” explains Dorn.

This is the goal of NASA’s 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 wants to establish the connection between the atmosphere and the interior of celestial bodies.”

The new data from the exoplanet known as TOI-270d are particularly interesting. “There we have collected evidence 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.

New considerations on the habitability of water worlds

Water is 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 – planets with several times the mass of Earth and a surface 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 planet’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.

Reference: “The Interior as the Dominant Water Reservoir in Super-Earths and Sub-Neptunes” by Haiyang Luo, Caroline Dorn and Jie Deng, August 20, 2024, Natural astronomy.
DOI: 10.1038/s41550-024-02347-z

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