Planets are hiding much more water than we thought

For a long time, scientists have relied on a straightforward model to understand planetary composition: Earth has an iron core, a mantle made of silicate bedrock, and water on its surface. This model has also been commonly applied when studying exoplanets – planets that orbit stars outside of our solar system. 

However, recent research has begun to show that planets are more complex than this model suggests, particularly those located close to their stars. These exoplanets are often extremely hot, featuring oceans of molten magma rather than solid silicate mantles like Earth’s. 

In such molten conditions, water can dissolve much more easily compared to gases like 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 had thought,” said Caroline Dorn, an astronomer at ETH Zurich.

Exploring water distribution in distant planets

Dorn, along with Haiyang Luo and Jie Deng from Princeton University, has been investigating how water is distributed within these molten exoplanets, focusing on its interaction with their iron cores. 

The research, recently published in the journal Nature Astronomy, used model calculations based on fundamental physical laws to explore these processes.

“The iron core takes time to develop. A large share of the iron is initially contained in the hot magma soup in the form of droplets,” Dorn explained. As these iron droplets descend to form the planet’s core, they bring water with them. “The iron droplets behave like a lift that is conveyed downwards by the water,” she added. 

Large planets have more water in their core

Previously, this behavior had only been observed under the moderate pressures found inside Earth. However, this new study shows that in larger planets with higher internal pressures, the phenomenon is even more pronounced. 

“The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated in the core,” noted Dorn. In such conditions, the iron can absorb up to 70 times more water than silicates. 

Due to the immense pressure at the core, the water doesn’t remain as H2O molecules but instead breaks down into hydrogen and oxygen.

Earth may have 80 more oceans’ worth of water

The study was partly inspired by earlier research on Earth’s own water content, which produced a surprising discovery four years ago: the water in Earth’s oceans represents only a small fraction of the planet’s total water. 

Simulations suggested that the equivalent of more than 80 Earth oceans could be stored within the planet’s interior. This idea aligns with seismological measurements and experimental data, providing a new understanding of how water is stored within planets.

Planets are more water-abundant than expected

These new findings have significant implications for how astronomers interpret data from exoplanets. When astronomers measure an exoplanet’s mass and size using telescopes, they often create mass-radius diagrams to infer the planet’s composition. 

If the distribution and solubility of water aren’t accounted for, the amount of water present can be underestimated by as much as tenfold. “Planets are much more water-abundant than previously assumed,” Dorn said.

The distribution of water within a planet is crucial for understanding its formation and evolution. Water that has sunk into a planet’s core remains trapped there permanently. 

However, water that is 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 is probably a great deal more in its interior,” she added.

Molecules in the atmosphere of exoplanets

The James Webb Space Telescope, which has been operational 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 connecting the data on atmospheric composition with the internal structures of these distant planets.

Among the exoplanets of interest is TOI-270d, where evidence suggests interactions between the planet’s internal magma ocean and its atmosphere. 

“Evidence has been collected there of the actual existence of such interactions between the magma ocean in its interior and the atmosphere,” Dorn said. Another planet of interest is K2-18b, which has gained attention due to the potential for life to exist there.

Habitability of water-rich “Super-Earths” 

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

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

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

Instead, much of the water on these Super-Earths is likely trapped within the planet’s core, not on the surface as previously thought. 

This leads the scientists to conclude that even planets with a high water content might still have Earth-like conditions that could support life. 

Image Credit: NASA, ESA, CSA, Dani Player (STScI)

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