Exoplantets Could Be Hiding Something Beneath Their Surface That Boosts Possibility of Alien Life

New research suggests that planets once assumed to be covered in massive global oceans may be more Earth-like than earlier models predicted, according to findings showing that water may be more likely to exist underground on many exoplanets rather than flooding their surfaces.

Many known exoplanets located near their stars have surfaces covered in magma oceans. In these environments, water dissolves into the molten silicate layer surrounding an iron core while carbon dioxide rises into the atmosphere.

Now, a research team led by Caroline Dorn, Professor for Exoplanets at ETH Zurich, in collaboration with Haiyang Luo and Jie Deng at Princeton University, has published a paper providing new insights into the unique role water plays in this process.

Molten Exoplanets and Earth’s Hidden Water

Four years ago, a team began to examine the interactions between water, iron, and silicates in an earlier study that simulated conditions on the young Earth to understand how water would behave under such conditions. Surprisingly, the simulations suggested that water equivalent to 80 of Earth’s oceans could be trapped beneath the surface. Subsequent experiments and seismological measurements supported these findings. Dorn’s team then explored what this might mean for the water thought to cover the entire surface of some exoplanets.

The current method for estimating the amount of water on an exoplanet is relatively straightforward. Astronomers measure the mass and size of exoplanets and use this data to create mass-radius diagrams that help determine the planets’ composition. However, this approach overlooks critical factors such as water’s solubility and distribution, potentially leading to a significant underestimation of water volume. Dorn notes that these simplistic models have led to false conclusions, stating, “Planets are much more water-abundant than previously assumed.”

According to their paper, the team conducted ab initio molecular dynamics simulations that revealed water’s metal–silicate partition coefficients at pressures up to 1,000 GPa. Using this data, they modeled the interiors of planets to examine the effects of water content on density, melting temperature, and water partitioning. These detailed computer models showed behavior consistent with earlier research on Earth’s water.

Experimental Findings

“The iron core takes time to develop. Initially, a significant portion of the iron is contained in the hot magma as droplets.” Dorn explained, noting how water attaches to these iron droplets as they fall towards the planet’s core. “The iron droplets act like an elevator, carrying the water downwards,” Dorn added.

According to Dorn, a key finding of the study is that “the larger the planet and the greater its mass, the more water tends to be drawn into the core along with the iron droplets. Under certain 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 as hydrogen and oxygen.”

Implications for Exoplanet Studies

The research suggests two possible fates for water on exoplanets: it could either become trapped in the core or dissolve into the molten layer, eventually rising from the mantle as an atmospheric gas. Dorn explains this significance for atmospheric surveys, noting that if astronomers detect water in a planet’s atmosphere, there is likely much more water within its interior. This is crucial because, as Dorn points out, the atmosphere is the only part of an exoplanet that can be directly measured. “Our group aims to connect atmospheric observations to the inner workings of celestial bodies,” she says.

While abundant water on a planet may sound promising, scientists caution that the global oceans once believed to cover Super-Earths—massive, Earth-like planets—may be inhospitable to life. On such planets, high-pressure ice could prevent the exchange of substances necessary for life to arise. However, this study challenges those concerns by showing that most of a planet’s water—up to 95%—is likely beneath the surface. As a result, planets previously thought to be covered in a single super-ocean may have Earth-like surface conditions.