Planets hold more water in their interiors than previously believed

Planets hold more water in their interiors than previously believed
by Robert Schreiber
Berlin, Germany (SPX) Aug 21, 2024

For years, scientists have modeled planets based on Earth's structure-an iron core, a silicate mantle, and surface water. This model has often been applied to exoplanets, or planets outside our solar system. However, "it is only in recent years that we have begun to realize that planets are more complex than we had thought," explains Caroline Dorn, Professor for Exoplanets at ETH Zurich.

Exoplanets, especially those close to their stars, are typically hot with surfaces dominated by molten magma rather than solid rock. In these conditions, water can dissolve into the magma, unlike gases such as carbon dioxide, which tend to escape into the atmosphere.

Researchers, including Dorn and colleagues Haiyang Luo and Jie Deng from Princeton University, have conducted model calculations to explore how water is distributed between a planet's silicate mantle and its iron core. Their findings, recently published in 'Nature Astronomy', suggest that larger and more massive planets are likely to trap significant amounts of water within their cores.

Water's Journey to the Core
Dorn describes the process in detail: "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." As these iron droplets sink to the core, they carry water with them. "The iron droplets behave like a lift that is conveyed downwards by the water," she explains.

Previous research showed this phenomenon under pressures similar to those found on Earth, but Dorn's study extends this understanding to larger planets with much higher pressures. "This is one of the key results of our study," Dorn notes. In larger planets, the water-to-iron absorption ratio can be up to 70 times greater than that of silicates. Due to the intense pressure at the core, this water is no longer in the form of H2O molecules but exists as separate hydrogen and oxygen.

Earth's Hidden Water
This investigation was inspired by research on Earth's water content, which revealed that the oceans on the surface represent only a small portion of the planet's total water. In fact, simulations suggest that more than 80 times the volume of Earth's oceans could be hidden deep within its interior, a conclusion supported by seismological data and experiments.

These findings have significant implications for how astronomers interpret data from exoplanets. When measuring an exoplanet's size and mass, scientists create mass-radius diagrams to infer the planet's composition. If the potential solubility and distribution of water are overlooked, the water volume could be underestimated by as much as tenfold. "Planets are much more water-abundant than previously assumed," Dorn says.

Implications for Planetary Evolution and Habitability
Understanding how water is distributed within a planet is essential for grasping its formation and development. Water trapped in the core is likely to remain there permanently, while water in the magma ocean may rise to the surface during cooling. "So if we find water in a planet's atmosphere, there is probably a great deal more in its interior," Dorn explains.

The James Webb Space Telescope, which has been collecting data for two years, aims to identify molecules in the atmospheres of exoplanets. "Only the composition of the upper atmosphere of exoplanets can be measured directly," Dorn points out, adding that her group seeks to link atmospheric data to the internal structure of these planets.

Recent observations of the exoplanet TOI-270d suggest interactions between its interior magma ocean and its atmosphere. Dorn, who contributed to the study, is also eager to investigate K2-18b, a planet that has gained attention due to its potential to harbor life.

Rethinking Water Worlds
Water is a key ingredient for life, and there has been much debate about whether water-rich Super-Earths-planets significantly larger than Earth-could be habitable. Earlier theories posited that too much water might be detrimental to life by creating high-pressure ice layers that block essential chemical exchanges between the ocean and the mantle.

However, this new study suggests otherwise: most of the water on Super-Earths may be locked within their cores, not on their surfaces. This finding implies that even water-abundant planets could potentially support Earth-like conditions. As Dorn and her colleagues conclude, their research offers new insights into the habitability of water-rich worlds.

Research Report:The interior as the dominant water reservoir in super-Earths and sub-Neptunes