Sub-native exoplanets are unlikely to be oceanic worlds rich in water, say researchers

An exoplanet in orbit around a dwarf star 124 light years from the earth made the headlines of the world in April 2025. Researchers from the University of Cambridge reported that the K2-18B planet could be a marine world with a deep world ocean teeming with life.

However, a study now shows that the so-called sub-neptuans such as K2-18B are very unlikely to be worlds dominated by water and that the conditions are far from being conducive to life.

“Water on the planets is much more limited than we thought before,” notes Caroline Dorn, professor of exoplanets at Eth Zurich.

The work appears in Astrophysical newspaper letters.

The study was carried out under the direction of Eth Zurich, in collaboration with researchers from the Max Planck Institute for Astronomy in Heidelberg and the University of California in Los Angeles. K2-18B is greater than the earth but smaller than Neptune, placing it in a class of planets that do not exist in our solar system. However, observations show that they are common in space. Some of these sub-neptunes have probably formed far from their central star, beyond the so-called snow line where water freezes in the ice and migrates later inward.

Until now, it has been assumed that some of these planets could accumulate particularly large quantities of water during their formation and now deep global oceans under a atmosphere rich in hydrogen. Experts refer them as hycean planets: a combination of “hydrogen” and “ocean”.

In consideration in chemistry

“Our calculations show that this scenario is not possible,” explains Dorn. Indeed, a fundamental vulnerability of previous studies was that they ignored any chemical coupling between the atmosphere and the interior of the planet. “We have now taken into account the interactions between the interior of the planet and its atmosphere,” explains Aaron Werlen, researcher of the Dorn team and the study of the study.

The researchers assume that at an early stage of their formation, the sub-neptuss crossed a phase in which they were covered by a hot and hot magma ocean. A gaseous hydrogen shell assured that this phase was maintained for millions of years.

“In our study, we have studied how the chemical interactions between the oceans and the atmospheres of the magma affect the water content of young sub-nevant exoplanets,” explains Werlen.

To do this, the researchers used an existing model that describes planetary evolution over a specific period of time. They combined this with a new model that calculates the chemical processes that take place between gas in the atmosphere, and metals and silicates in magma.

Water disappearing inside

The researchers calculated the state of chemical equilibrium of 26 different components for a total of 248 model planets. IT simulations have shown that chemical processes destroy most₂O water molecules. Hydrogen (H) and oxygen (o) are attached to metallic compounds, and these disappear largely in the nucleus of the planet.

Even if the accuracy of these calculations has certain limits, the researchers are convinced by the results.

“We focus on the main trends and can clearly see in the simulations that planets have much less water than they accumulate,” said Werlen. “The water that really remains on the surface like H₂O is limited to a few percent at most. “”

In a previous publication, the Dorn group was already able to show how most of the water on a planet is hidden inside.

“In this study, we have analyzed the amount of water in total on these sub-neptus”, explains the researcher, “according to calculations, there are no distant worlds with massive layers of water where water represents about 50% of the mass of the planet, as it previously thought. Hycean worlds with 10 to 90% of water are therefore very unlikely.”

This makes the search for extraterrestrial life more difficult than hoped for. The conditions conducive to life, with sufficient liquid water on the surface, are likely to exist only on smaller planets, which will probably only be observable with even more powerful observatories than the James Webb space telescope.

Earth is not a special case

Dorn finds the role of the earth particularly exciting in the light of new calculations, which show that most distant planets have a water content similar to our planet.

“The earth may not be as extraordinary as we think. In our study, at least it seems to be a typical planet,” she said.

The researchers were also surprised by an apparently paradoxical difference: the planets with the atmospheres richest in water are not those that have accumulated the most ice beyond the snow line, but rather the planets that were formed in the snow line. On these planets, water did not come from ice crystals, but was produced chemically when hydrogen in the planetary atmosphere reacted with the oxygen of silicates in the magmatic ocean to form h₂O Molecules.

“These results question the classic link between the training rich in ice and the atmospheres rich in water. Instead, they highlight the dominant role of the balance between the magmatic ocean and the atmosphere in the formation of the planetary composition”, concludes Werlen.

This will have large -scale implications for theories of planetary training and the interpretation of exoplanetary atmospheres in the era of the James Webb telescope.