For a long time, scientists have relied on a simple model to understand the composition of planets: Earth has an iron core, a mantle of silicate rock, and water on its surface. This model is also often used to study exoplanets – planets that orbit stars outside our solar system.

However, recent research has shown that planets are more complex than this model suggests, especially those located close to their stars. These exoplanets are often extremely hot and have oceans of molten magma, rather than a solid silicate mantle like Earth.

Under such molten conditions, water can dissolve much more easily than gases such as carbon dioxide, which tend to escape into the atmosphere.

“It is only in recent years that we have begun to realize that planets are more complex than we thought,” says Caroline Dorn, an astronomer at ETH Zurich.

Research into water distribution on distant planets

Dorn, along with Haiyang Luo and Jie Deng of Princeton University, studied how water is distributed within these molten exoplanets, focusing on how water interacts with their iron cores.

The study recently published in the journal Natural astronomyused model calculations based on fundamental physical laws to investigate these processes.

“It takes a while for the iron core to develop. A large part of the iron is initially contained in the form of droplets in the hot magma soup,” explains Dorn. As these iron droplets sink and form the planet’s core, they bring water with them. “The iron droplets behave like an elevator that is carried downwards by the water,” she adds.

Large planets have more water in their core

So far, this behavior has only been observed at the moderate pressures in the Earth’s interior. However, the new study shows that the phenomenon is even more pronounced on larger planets with higher internal pressures.

“The larger the planet and the greater its mass, the more the water tends to float with the iron droplets and integrate into the core,” notes Dorn. Under such conditions, the iron can absorb up to 70 times more water than silicates.

Due to the enormous pressure in the core, the water does not remain as an H2O molecule, but breaks down into hydrogen and oxygen.

The Earth may have water equivalent to 80 oceans more

The study was inspired in part by previous research into Earth’s water content, which led to a surprising discovery four years ago: the water in Earth’s oceans represents only a small fraction of the planet’s total water.

Simulations have shown that the equivalent of more than 80 Earth’s oceans could be stored inside the planet. This assumption is consistent with seismological measurements and experimental data and provides a new understanding of how water is stored inside planets.

Planets are richer in water than expected

These new findings have significant implications for the way astronomers interpret exoplanet data. When astronomers measure the mass and size of an exoplanet using telescopes, they often construct mass-radius plots to draw conclusions about the planet’s composition.

If the distribution and solubility of water are not taken into account, the amount of water present can be underestimated by ten times. “Planets are much richer in water than previously thought,” said Dorn.

The distribution of water within a planet is crucial to understanding its formation and evolution. Water that has penetrated into the core of a planet remains permanently trapped there.

However, water dissolved in the molten mantle can rise to the surface as the planet cools. “So if we find water in a planet’s atmosphere, there’s probably a lot more of it inside,” she added.

Molecules in the atmosphere of exoplanets

The James Webb Space Telescope, in operation for two years, is helping scientists explore these mysteries by detecting molecules in the atmospheres of exoplanets.

Dorn’s research group is particularly interested in linking atmospheric composition data with the internal structures of these distant planets.

One of the interesting exoplanets is TOI-270d, which shows evidence of interactions between the planet’s inner magma ocean and its atmosphere.

“Evidence has been gathered there for the actual existence of such interactions between the magma ocean inside it and the atmosphere,” said Dorn. Another interesting planet is K2-18b, which has attracted attention due to the possibility that life could exist there.

Habitability of water-rich “super-earths”

Water is considered essential to life, and there has been much speculation about the habitability of water-rich “super-Earths” – planets larger than Earth and possibly covered by deep, global oceans.

Some previous studies suggested that an excess of water could be harmful to life because a thick layer of ice could form under high pressure, preventing the exchange of vital nutrients between the ocean and the Earth’s mantle.

Dorn’s new research, however, challenges this view. The study suggests that planets with extensive layers of water on the surface may actually be rare.

Instead, much of the water on these super-Earths is likely trapped in the planet’s core rather than on the surface, as previously thought.

From this, scientists conclude that even planets with high water content may have Earth-like conditions that could support life.

Photo credit: NASA, ESA, CSA, Dani Player (STScI)

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