There is more water on exoplanets than previously thought.

Exoplanets could contain much more water than previously thought. This could radically change the interpretation of astronomical data.

ZURICH – For a long time, the model of Earth as a planet with an iron core, a mantle made of silicate rocks and oceans on the surface was the standard in research. But four years ago, a study turned that assumption upside down, showing that there may be more water inside the Earth than in all its oceans combined. The discovery led to a new study on the water content of exoplanets. “It’s only in the past few years that people have started to take into account that planets are much more complex,” says Caroline Dorn, professor of exoplanets at ETH Zurich.

Most of the more than 5,500 known exoplanets—planets that don’t orbit our Sun—are close to their star. As such, they are mostly hot worlds without a cooling silicate rocky mantle. Instead, they have magma oceans and an iron core underneath. Water dissolves well in these magma oceans. But how is water distributed between silicates and iron? A research team led by Dorn took up this question. Your findings In the specialized magazine Astronomy Nature Published.

How is water distributed in exoplanets?

To interpret the results of the study, Dorn says something“The iron core only forms over time. At first, a large proportion of the iron is still present as droplets in the hot magma soup,” the researcher says. Water dissolved in the magma soup combines with the iron droplets and sinks with them to the core. “The iron droplets act as an elevator that brings the water down,” the researcher says.

Until now, this behavior has only been known at moderate pressures like those on Earth. However, what happens inside larger, high-pressure planets has been unclear. “This is one of the most important results of our study,” Dorn says. “The larger and more massive the planet, the more water tends to sink into the core with iron droplets under certain conditions.” Iron can absorb up to 70 times more water than silicates. However, under the enormous pressures in the core, water no longer exists as a liquid H₂O molecules but in the form of hydrogen and oxygen.

The amount of water on exoplanets is underestimated by a factor of ten.

These findings could have a major impact on the interpretation of astronomical observational data. Under certain conditions, astronomers can measure the size and mass of an exoplanet and draw conclusions about its composition. But if you ignore the solubility and distribution of water, as has been the case so far, you underestimate the amount of water by a factor of ten, the study shows. “The planets are much richer in water than previously thought,” Dorn says.

The distribution of water in exoplanets is also important for another reason, the researcher explains: “If you find water in the atmosphere of a planet, there is probably a lot of it inside.” This is because as it melts into magma, ocean water can release gases as the mantle cools and reaches the surface. However, the water in the core remains permanently trapped there. Water has also recently been discovered beneath the surface of Mars, which is already well explored.

It is clear that life-hostile aquatic worlds are fairly rare.

Previous calculations had assumed that large amounts of water on planets would be hostile to life. On these watery worlds, a layer of exotic, high-pressure ice at the transition between the ocean and the planetary mantle would prevent the exchange of vital materials, the hypothesis went. However, the study by Dorn and her team came to a different conclusion: Planets with deep layers of water are likely to be rare. Most of the water is not on the surface, as previously assumed, but is confined to the core.

The research team suspects that even planets with relatively high water content could evolve to Earth-like conditions suitable for life. Dorn and her team conclude that the new study sheds new light on the possibility of water-rich worlds that could harbor life. (unpaid invoice)