Exoplanets may contain more water than previously thought

We know that the Earth has an iron core surrounded by a mantle of silicate rocks and water (oceans) on its surface. Science has used this simple planetary model to date 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. They are therefore mainly made up of oceans of molten magma that have not yet cooled to form a solid mantle of silicate rocks like Earth. Water dissolves very well in these oceans of magma, unlike carbon dioxide, for example, which quickly degasses and rises into the atmosphere.

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

This is precisely what Dorn studied in collaboration with Haiyang Luo and Jie Deng of Princeton University using model calculations based on the fundamental laws of physics. The researchers present their results in the journal Astronomy of nature.

Magma soup with water and iron

To explain the results, Dorn has to go into detail: “The iron core takes time to form. A large part of the iron is initially contained in the hot magma soup in the form of droplets.” The water sequestered in this soup combines with these iron droplets and flows with them to the core. “The iron droplets behave like an elevator that is carried downward by the water,” Dorn explains.

Until now, this behavior was only known for moderate pressures, such as those that also prevail on Earth. It was not known what happened in the case of larger planets, where the internal pressure was higher.

“This is one of the key results of our study,” Dorn says. “The larger the planet and the greater its mass, the more water tends to follow the iron droplets and become embedded in the core. Under certain circumstances, iron can absorb up to 70 times more water than silicates. However, due to the enormous pressure at the core, the water no longer takes the form of H2O.”₂“O molecules but is present in hydrogen and oxygen.”

Large amounts of water are also found inside the Earth.

This study was initiated following a study of the Earth’s water content, 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 of the Earth’s oceans could be hidden inside it. This is shown by simulations calculating the behavior of water under conditions similar to those that prevailed when the Earth was young. The experiments and seismological measurements are therefore compatible.

New findings on the distribution of water on planets have dramatic consequences for the interpretation of astronomical observation data. Using their telescopes in space and on Earth, astronomers can, under certain conditions, measure the weight and size of an 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 water are neglected, as has been the case so far, the volume of water can be significantly underestimated by up to ten times.

“Planets are much richer in water than previously thought,” Dorn says.

Understanding the history of evolution

The distribution of water is also important for understanding the formation and development of planets. Water that has sunk into the core remains trapped there forever. In contrast, water dissolved in the mantle’s magma ocean can lose gas and rise to the surface as the mantle cools.

“If we find water in a planet’s atmosphere, there’s probably a lot more inside,” Dorn says.

This is what the James Webb Space Telescope is trying to discover, which has been sending data from space to Earth for two years. It is capable of tracking molecules present in the atmosphere of exoplanets.

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

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

“There is evidence that there are such interactions between the magma ocean inside and the atmosphere,” says Dorn, who contributed to the corresponding publication on TOI-270d. Among the interesting objects she wants to examine more closely is also the planet K2-18b, which has been in the news because of the likelihood that it may harbor life.

Are aquatic worlds ultimately suitable for life?

Water is one of the prerequisites for the development of life. There has long been speculation about the potential habitability of water-rich super-Earths, i.e. planets with masses several times that of Earth and a surface covered by a deep ocean. Calculations then suggested that excessive water could be hostile to life. The argument was that in such water worlds, an exotic high-pressure ice layer would prevent the exchange of vital substances at the interface between the ocean and the planet’s mantle.

The new study comes to a different conclusion: planets with deep water layers are probably rare, because most of the water in super-Earths is not on the surface, as previously thought, but is trapped in the core. The scientists conclude that even planets with relatively high water content could have the potential to develop habitable conditions similar to those on Earth. As Dorn and his colleagues conclude, their study thus sheds new light on the potential existence of water-rich worlds that could support life.