In a game-changing article published in Nature Astronomy, the researchers found out that exoplanets, in particular super-Earths and sub-Neptunes, were likely a hundred times more water-rich than previously surmised. This could change considerably our views on these distant worlds and the possibility of life that they hold.
The fluid is one of the most critical factors in planetary systems, from geological processes to habitability. In the past, it was believed that the bulk of the water occurring on other planets existed on their surface, forming gigantic oceans. However, the new study suggests that the biggest part of water on these planets is located in their deep interiors, secluded in their mantles and cores.
“Most of the exoplanets known today are located close to their star,” said Professor Caroline Dorn of ETH Zurich. “This means they primarily comprise hot worlds of oceans of molten magma that have not yet cooled to form a solid mantle of silicate bedrock like the Earth.” Under these extreme conditions, water dissolves well in the magma oceans, whereas carbon dioxide outgrows into the atmosphere rather fast.
The researchers, using advanced molecular dynamics simulations, probed how water would behave under the ferociously high pressures and temperatures in such molten settings. They observed that the water headed more towards the iron core rather than staying in the mantle rich in silicate. “The kind of iron cores that can be formed in rocky planets depends on how early their building blocks acquire water,” Dorn said in a statement. These iron droplets, if mixed with water instead, travel down toward the core and permanently entomb the water in the inner reaches of the planet.
This latter effect bucks the expected pattern in which water finds itself in the mantle or eventually at the surface, as it has here on Earth. “The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated into the core,” Dorn said. Under specific conditions, iron can incorporate up to 70 times more water than silicates. However, at the tremendous pressures within the core, the water does not exist as H2O molecules; instead, it has dissociated into individual hydrogen and oxygen atoms.
These findings represent more than just the distribution of water: it manifests itself in changing the way scientists interpret the mass and radius data exoplanets provide. Typically, mass and radius data on an exoplanet are applied to infer what composition a planet may have. This is often taken to mean that a planet bringing in a lot of mass will also translate into the formation of a thick atmosphere or a mass of surface water. But if the water is present deep inside the mantle of the planet, this might be a real underestimate of the water content.
The research paves the way for considering the possible habitability of water-rich super-Earths. Until now, earlier calculations by scientists led to the idea that life couldn’t be possible with too much water because high-pressure ice tended to form layers, blocking the movement or exchange of important substances between the oceans and the mantle of the planet. The new findings suggest planets with deep water layers are probably rare, with most of the water being trapped within the core. This would, therefore, imply that even planets with a relatively high amount of water could develop Earth-like, habitable conditions.
The data, which includes the observations from the James Webb Space Telescope through two years of sending data from space to Earth, will prove to be of importance in the confirmation of these products. It will help scientists learn how to watch molecules inside the exoplanet atmosphere, in a bid to track the connection from the atmosphere to the inner details of the celestial bodies.
Finally, his work, along with the current study, will challenge these many assumptions regarding exoplanets and their water content, which could make us look at water-content worlds from a new perspective, imagining them to host life. As Dorn and colleagues wrote, “Planets are much more water-abundant than previously assumed,” and surely this new estimate will fundamentally change the way our search for habitable worlds around other stars is conducted.