Illustration of Exoplanet K2-18b. It is believed to have a thick layer of gas and do not have global oceans.
Detection of extraterrestrial life has just become a more distant reality. “There are no distant worlds with massive water layers. Hicean worlds with 10-90% water are therefore very unlikely.”
There is much less water on the surfaces of distant planets beyond our solar system of what was thought. These exoplanets have no thick layers of wateras speculated often. This is the conclusion of an international study led by ETH Zurich and The Astrophysical Journal Letters.
An exoplanet in orbit of an Earth’s 124-year-old star worldwide in April 2025. Cambridge University researchers said the planet could be a marine world with a deep and global life-filled ocean. However, the study now shows that the so-called sub-negles, such as K2-18 B, are highly unlikely to be water dominated by water and that the conditions there are far from being conducive to life. The very very webb found “clear signs” of life in another planet.
“Water on the planets is much more limited than you thought,” says Caroline Dorn, exoplanet teacher at ETH Zurich.
The study was conducted under the direction of ETH Zurich, in collaboration with researchers at the Max Planck Institute of Astronomy of Heidelberg and the University of California in Los Angeles. K2-18 B is larger than Earth, but smaller than Neptune, which puts it in a class of planets that do not exist in our solar system. However, observations show that they are common in the outer space. Some of these sub-negles probably formed very far from their central star, beyond the so-called snow line, where water becomes ice and then migrates inside.
So far, some of these planets have been thought to accumulate particularly large amounts of water during their formation and now housed deep and global oceans under a hydrogen-rich atmosphere. Experts refer to these planets as Planets I made: A combination of “hydrogen” and “ocean”.
Given the chemistry
“Our calculations show that This scenario is not possible“Says Dorn. This is because a fundamental vulnerability of previous studies was that they ignore any chemical coupling between the atmosphere and the interior of the planet.“ Now we have into account the interactions between the planet and its atmosphere, ”explains Aaron Werlen, a researcher of the Dorno team and the main author of the study.
The investigators assume that at an early stage of their formation, the sub-negles went through a phase in which they were covered by an ocean of warm and deep magma. A hydrogen gas shell ensured the maintenance of this phase for millions of years.
“In our study, we investigated the way chemical interactions between magma oceans and atmospheres affect the water content of young sub-neglestunus,” says Werlen.
To this end, the researchers used an existing model that describes planetary evolution for a specific period. They combined it with a new model that calculates the chemical processes that occur between gas in the atmosphere and metals and silicates in the magma.
Water disappearing inside
Researchers calculated the state of chemical equilibrium of 26 different components for a total of 248 model planets. Computer simulations showed that the Chemical processes destroy most water molecules. Hydrogen and oxygen bind to metal substances, which disappear largely in the nucleus of the planet.
Although the accuracy of these calculations has some limitations, researchers are convinced of the results. “We focus on the main trends and we can clearly see in the simulations that planets have much less water than they originally accumulated,” explains Werlen. “The water that actually remains on the surface is limited to a few percent at most.”
In a previous publication, Dorn’s group had already been able to show how most of the water of a planet is hidden inside. “In the current study, we analyzed the total amount of water existing in these sub-negles,” explains the researcher, “according to the calculations, There are no distant worlds with massive water layers, where water represents about 50 percent of the mass of the planet, as previously thought. Hicean worlds with 10-90% water are therefore very improviseds”.
This makes the demand for life extraterrestrial more difficult than expected. The conditions conducive to life, with enough liquid water to the surface, will probably only exist on smaller planets, which are likely to be observable only with even better observatories than the James Webb space telescope.
We are not special
Dorn considers the role of our land particularly exciting in the light of new calculations that show that the most distant planets have a water content similar to that of our planet. “The earth may not be as extraordinary as we think. In our study, at least, it seems to be a typical planet ”says.
The investigators were also surprised by a difference apparently paradoxical: The planets with the most rich water atmospheres are not the ones that accumulated the most ice beyond the snow line, but planets that formed into the snow line. In these planets, water did not come from ice crystals, but it was chemically produced when the hydrogen of the planetary atmosphere reacted with the oxygen of magma ocean silicates to form H2O molecules.
“These discoveries challenge the classic connection between ice -rich formation and water -rich atmospheres. Instead, they highlight the dominant role of the balance between the magma ocean and the atmosphere in the formation of planetary composition,” concludes Werlen. This will have great implications for planetary training theories and the interpretation of exopaplanetary atmospheres in the age of the James Webb Telescope.
