Water is an important component of exoplanets, and its distribution, whether on the surface or deep within, fundamentally affects the properties of the planet. According to new research, for Earth-sized planets and planets with more than six times the mass of Earth, most of the water is located deep in the planet’s core.

“Most of the exoplanets known today are located close to their star,” says ETH professor Caroline Dorn.

“This means that they are primarily hot worlds whose oceans are made up of molten magma that has not yet cooled enough to form a solid mantle of silicate bedrock like Earth.”

“Water dissolves very well in these magma oceans – unlike carbon dioxide, for example, which quickly evaporates 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?”

“The iron core needs 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 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.

It was not known what happens to larger planets with higher pressure conditions in their interior.

“This is one of the most important findings of our study,” said Professor Dorn.

“The larger the planet and the greater its mass, the more the water tends to combine with the iron droplets in the core.”

“Under certain circumstances, iron can absorb up to 70 times more water than silicates.”

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

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 fraction of the total water supply of our planet.

The contents of more than 80 of Earth’s oceans could be hidden inside it.

This is shown by simulations that calculate the behavior of water under conditions that existed when the Earth was young. Experiments and seismological measurements are therefore compatible.

The new insights into the distribution of water on planets have dramatic consequences for the interpretation of astronomical observational data.

Using 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 composition of the planet.

“If the solubility and distribution of the water are ignored – as has been the case so far – the amount of water can be dramatically underestimated by up to ten times,” says Professor Dorn.

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

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 Earth’s mantle 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,” said Professor Dorn.

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.

Then calculations suggested that too much water could be hostile to life. The argument was that in these water 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 current study 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 assumed, but is enclosed in the core.

This leads astronomers to believe that even planets with a relatively high water content may have the potential to develop Earth-like life conditions.

“Their study therefore sheds new light on the possible existence of water-rich worlds that could support life,” the authors said.

The study was published in the journal Natural astronomy.

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H. Luo et al. The interior as the dominant water reservoir in super-Earths and sub-Neptunes. Nat Astronpublished online on August 20, 2024; doi: 10.1038/s41550-024-02347-z