“Planetary bodies are richer in water than previously thought”

Water is found on many exoplanets, but not necessarily on the surface. A study shows that the amount of water has been underestimated until now.

ZURICH – Earth has an iron core in its interior, a mantle of silicate rock above, and vast oceans on its surface. Researchers have been working on this simple planetary model for a long time – but just four years ago a study made a surprising discovery: There could be far more water in the interior of the Earth than in all the oceans above the Earth’s surface combined. The discovery also prompted a new study into the water content of exoplanets.

Iron core, then silicate mantle, then surface – planets are more complex

“Only in recent years have people begun to take into account that planets are more complex.” He explained Caroline Dorn, Professor of Exoplanets at ETH Zurich. Most of the more than 5,500 known exoplanets (planets that do not orbit our Sun) are close to their star. For this reason, they are mostly hot worlds, and do not yet have a cooling mantle made of silicate rock.

Instead, they have oceans of molten magma, with an iron core underneath. Water dissolves well in these magma oceans. But how exactly is water distributed between the silicates and the iron? A research team led by Dorn investigated this question. Your findings In the specialized magazine Astronomy Nature Published.

Most of the water on planets is collected in their cores.

To make the study's findings understandable, study author Dorn explains something like this: “The iron core only forms over time. Initially, a large proportion of the iron is still present as droplets in the hot magma soup, and the 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,” says Dorn.

Until now, this behavior has only been known at moderate pressures, as on Earth. What happens inside larger planets with high pressures has not been known. “This is one of the most important results of our study,” Dorn says. “The larger the planet and the more massive it is, the more 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. However, under the enormous pressure in the core, water no longer comes in liquid form as H₂O molecules but in the form of hydrogen and oxygen.

Water Distribution on Planets: Study Will Change Research

But what does this result mean for the research? According to the ETH, it is likely to have radical implications for the interpretation of astronomical observation data. Under certain conditions, astronomers can measure the size and mass of an exoplanet. Conclusions about the planet’s composition can also be drawn. But here’s the gist: if you ignore the melting and distribution of water – as was done before – you underestimate the amount of water by up to a factor of ten. “Planetary bodies 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.” That’s because water is in magma—the ocean melts, the gas can escape as the layer cools and reaches the surface. Water in the core, on the other hand, stays locked in there forever. A research team has also just discovered water beneath the surface of the already well-studied planet Mars.

Planets with lots of water could also be life-friendly.

Previous calculations had assumed that large amounts of water on planets might 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 might not be so common after all. After all, much 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 that are 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)