K2-18b is a sub-Neptune about 124 light-years away from Earth first detected in 2015. Follow-up research found water vapour in its atmosphere, indicating that it could be a water planet, or what is called a ‘hycean planet‘. Hycean planets have thick hydrogen atmospheres and deep, global or near-global oceans. Or so scientists thought.
In April 2025, additional research generated renewed interest in the 8.6 Earth-mass exoplanet. It could have a deep ocean, and chemicals detected in its atmosphere could be biosignatures, researchers said. “The possibility of hycean worlds, with planet-wide oceans and H2-rich atmospheres, significantly expands and accelerates the search for habitable environments elsewhere,” they wrote.
Sadly, new research shows that not only is K2-18b unlikely to be a water planet, it’s also not a place where life is likely to persist. The research is titled “Sub-Neptunes Are Drier than They Seem: Rethinking the Origins of Water-rich Worlds,” and it’s published in The Astrophysical Journal Letters. The lead author is Aaron Werlen from the Institute for Particle Physics and Astrophysics at ETH Zurich in Switzerland.
“Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets,” the authors write in their research. Scientists think that these planets form beyond the snow-lines in their solar systems, and that as a result, they accrete large amounts of water. They can accrete as much as 30% of their mass in frozen water, then migrate inward. The ice melts and the result is a hycean world. A 2021 paper by Madhusudhan et al. claimed that hycean worlds could be up to 90% H20.
“Our calculations show that this scenario is not possible,” said co-author Caroline Dorn, also from the Institute for Particle Physics and Astrophysics at ETH Zurich.
“According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet’s mass, as was previously thought.” Caroline Dorn, ETH Zurich.
The researchers say that current models of hycean worlds neglect “chemical equilibration between primordial atmospheres and molten interiors.” In this research, they created a synthetic population of sub-Neptunes with magma oceans and computed their global equilibrium states. “We have now factored in the interactions between the planet’s interior and its atmosphere,” explains lead author Werlen in a press release.
“Although many initially accrete 5–30 wt% water, interior–atmosphere interactions destroy most of it, reducing final H2O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets,” they write.
The researchers assumed that like many other planets, sub-Neptunes go through a magma ocean phase. But in their case, a thick hydrogen atmosphere insulates the planet, and the magma ocean phase lasts much longer. The insulating shells maintain the magma phase for millions of years. So to understand hycean worlds, it’s necessary to understand how the magma ocean and hydrogen atmosphere interacted.
“In our study, we investigated how the chemical interactions between magma oceans and atmospheres affect the water content of young sub-Neptune exoplanets,” said Werlen.
The study included 248 model hycean planets, and calculated the energy equilibrium states for 26 separate components. The results show that hydrogen and oxygen atoms attach themselves to metallic compounds in the magma ocean and disappear into the planetary core.
“We focus on the major trends and can clearly see in the simulations that the planets have much less water than they originally accumulated,” explained Werlen. “The water that actually remains on the surface as H2O is limited to a few per cent at most.” For comparison, Earth is about 0.02% water, which covers about 70% of its surface.
This figure from the research compares the accreted and equilibrated water mass fractions of sub-Neptunes in their modelled sub-Neptunes. The black dashed line indicates the 1:1 correlation; in the absence of chemistry, all planets would lie along this line. The gray shaded region denotes the 10–90 wt% water mass fraction range proposed for Hycean planets by N. Madhusudhan et al. (2021). Clearly, none of the modelled planets even come close to 10% water when chemical reactions between the magma ocean and atmosphere are considered. Image Credit: Werlen et al. 2025. ApJL
The same research group already showed in earlier research that most of a planet’s water is likely buried in its interior. Earth’s interior contains a significant amount of water, mostly in its mantle transition zone. Now they’ve come up with evidence of how much water planets hold in total.
“In the current study, we analyzed how much water there is in total on these sub-Neptunes,” explained Dorn. “According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet’s mass, as was previously thought. Hycean worlds with 10-90 percent water are therefore very unlikely.”
This figure shows the H20 mass fraction for two groups of planets. On the left are planets that formed within the water ice line. On the right are planets that formed outside of that line. Planets formed inside the ice line are systematically depleted in carbon due to the lack of volatile ice accretion and exhibit higher envelope H2O mass fractions. In contrast, planets formed beyond the ice line retain lower H2O content despite higher bulk volatile abundances. Each pie chart shows the mean mass fraction of hydrogen in H2 (gas), H (metal), H2 (silicate), H2O (gas), and H2O (silicate). Planets that formed beyond the ice line store most hydrogen as H2 (gas) and H (metal), while those that formed inside the ice line retain a larger share of hydrogen in H (metal), H2 (silicate), and H2O (gas + silicate). Image Credit: Werlen et al. 2025. ApJL
If this is true, then it means a drastic reduction in the number of potentially habitable worlds. But there’s another implication, too: Earth’s water content isn’t some kind of wild outlier.
“The Earth may not be as extraordinary as we think. In our study, at least, it appears to be a typical planet,” said Dorn.
The research and the modelled planets also revealed an unexpected paradox. Instead of planets forming beyond the snow line having more water, the reverse is true. Planets that form within the snow line end up with more water because of interactions between the atmosphere and the magma ocean. In these cases, the water didn’t come from ice crystals accreted beyond the snow line. Instead, it was created when hydrogen in the atmosphere reacted with oxygen contained in silicates in the magma ocean, creating H2O.
“These findings challenge the classic link between ice-rich formation and water-rich atmospheres. Instead, they highlight the dominant role of the equilibrium between magma ocean and atmosphere in shaping planetary composition,” said Werlen.
“Our results, which focus on the initial (birth) population of sub-Neptunes with magma oceans, suggest that their water mass fractions are not primarily set by the accretion of icy pebbles during formation but by chemical equilibration between the primordial atmosphere and the molten interior,” the authors explain.
Not a single one of the modelled planets, regardless of their original H20 content, managed to retain more than 1.5%wt H20 after chemical equilibration. That’s a far cry from the 90% maximum from previous research.
“Our results challenge the conventional link between ice accretion and water-rich atmospheres, showing instead that H2O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line,” the researchers conclude.