Water on Exoplanets? Earth-Like Worlds May Hold Water in Their Cores

Scientists have discovered that up to 95 percent of the water on exoplanets might be trapped deep within their iron cores, which could make these planets more potentially habitable.

Caroline Dorn, a professor of exoplanets at ETH Zurich in Switzerland, stated, “Only recently have we begun to understand that planets are more complex than we previously thought. They are much more water-rich than we assumed.”

In the early stages, these exoplanets are composed of molten magma oceans that have yet to cool down. As they cool, a solid mantle of silicate bedrock forms. In these magma oceans, water dissolves more readily than carbon dioxide, which tends to outgas and rise into the atmosphere.

Dorn, along with researchers Haiyang Luo and Jie Deng from Princeton University, studied how water is distributed between silicates and iron on these planets.

“The iron core takes time to form,” Dorn explained. “Initially, a significant portion of the iron exists in the hot magma as droplets.”

She further described how water becomes part of the iron core in rocky planets. Water combines with iron droplets, which then sink towards the core. “The iron droplets act like an elevator, carrying water downwards,” Dorn said.

“This is a crucial finding from our study. The larger and more massive the planet, the more likely it is that water will merge with the iron droplets and integrate into the core. Under specific conditions, iron can absorb up to 70 times more water than silicates. However, due to the immense pressure at the core, the water no longer exists as H2O molecules but is instead present as hydrogen and oxygen,” she added.

The researchers used computer models to simulate how water interacts with molten magma on a young planet. Their findings suggest that a significant portion of water on even larger planets, like super-Earths, could end up in the planet’s interior.

“The bigger the planet and its mass, the more likely water is to be absorbed by the iron droplets and become part of the core,” Dorn reiterated. “Under certain conditions, iron can take in up to 70 times more water than silicates. But because of the intense pressure in the core, the water is present as hydrogen and oxygen rather than H2O. So, if we detect water in a planet’s atmosphere, there is likely much more water stored deep within its interior.”