K2-18b is a sub-Neptune about 124 light-years away from Earth first detected in 2015. Observe-up analysis discovered water vapour in its environment, indicating that it may very well be a water planet, or what is named a ‘hycean planet‘. Hycean planets have thick hydrogen atmospheres and deep, world or near-global oceans. Or so scientists thought.
In April 2025, extra analysis generated renewed curiosity within the 8.6 Earth-mass exoplanet. It may have a deep ocean, and chemical substances detected in its environment may very well be biosignatures, researchers mentioned. “The opportunity of hycean worlds, with planet-wide oceans and H2-rich atmospheres, considerably expands and accelerates the seek for liveable environments elsewhere,” they wrote.
Sadly, new analysis reveals that not solely is K2-18b unlikely to be a water planet, it is also not a spot the place life is prone to persist. The analysis is titled “Sub-Neptunes Are Drier than They Appear: Rethinking the Origins of Water-rich Worlds,” and it is revealed in The Astrophysical Journal Letters. The lead creator is Aaron Werlen from the Institute for Particle Physics and Astrophysics at ETH Zurich in Switzerland.
“Current claims of biosignature gases in sub-Neptune atmospheres have renewed curiosity in water-rich sub-Neptunes with floor oceans, sometimes called Hycean planets,” the authors write of their analysis. Scientists assume that these planets type past the snow-lines of their photo voltaic methods, and that consequently, they accrete giant quantities of water. They’ll accrete as a lot as 30% of their mass in frozen water, then migrate inward. The ice melts and the result’s a hycean world. A 2021 paper by Madhusudhan et al. claimed that hycean worlds may very well be as much as 90% H20.
“Our calculations present that this situation isn’t attainable,” mentioned co-author Caroline Dorn, additionally from the Institute for Particle Physics and Astrophysics at ETH Zurich.
“In accordance with the calculations, there are not any distant worlds with large layers of water the place water makes up round 50 % of the planet’s mass, as was beforehand thought.” Caroline Dorn, ETH Zurich.
The researchers say that present fashions of hycean worlds neglect “chemical equilibration between primordial atmospheres and molten interiors.” On this analysis, they created an artificial inhabitants of sub-Neptunes with magma oceans and computed their world equilibrium states. “We’ve got now factored within the interactions between the planet’s inside and its environment,” explains lead creator Werlen in a press launch.
“Though many initially accrete 5–30 wt% water, inside–environment interactions destroy most of it, lowering last H2O mass fractions to under 1.5 wt%. Consequently, none meet the brink for Hycean planets,” they write.
The researchers assumed that like many different planets, sub-Neptunes undergo a magma ocean section. However of their case, a thick hydrogen environment insulates the planet, and the magma ocean section lasts for much longer. The insulating shells preserve the magma section for tens of millions of years. So to know hycean worlds, it is necessary to know how the magma ocean and hydrogen environment interacted.
“In our research, we investigated how the chemical interactions between magma oceans and atmospheres have an effect on the water content material of younger sub-Neptune exoplanets,” mentioned Werlen.
The research included 248 mannequin hycean planets, and calculated the vitality equilibrium states for 26 separate elements. The outcomes present that hydrogen and oxygen atoms connect themselves to metallic compounds within the magma ocean and disappear into the planetary core.
“We concentrate on the key traits and might clearly see within the simulations that the planets have a lot much less water than they initially gathered,” defined Werlen. “The water that really stays on the floor as H2O is proscribed to a couple per cent at most.” For comparability, Earth is about 0.02% water, which covers about 70% of its floor.
This determine from the analysis compares the accreted and equilibrated water mass fractions of sub-Neptunes of their modelled sub-Neptunes. The black dashed line signifies the 1:1 correlation; within the absence of chemistry, all planets would lie alongside this line. The grey shaded area denotes the ten–90 wt% water mass fraction vary proposed for Hycean planets by N. Madhusudhan et al. (2021). Clearly, not one of the modelled planets even come near 10% water when chemical reactions between the magma ocean and environment are thought of. Picture Credit score: Werlen et al. 2025. ApJL
The identical analysis group already confirmed in earlier analysis that almost all of a planet’s water is probably going buried in its inside. Earth’s inside comprises a big quantity of water, largely in its mantle transition zone. Now they’ve give you proof of how a lot water planets maintain in complete.
“Within the present research, we analyzed how a lot water there may be in complete on these sub-Neptunes,” defined Dorn. “In accordance with the calculations, there are not any distant worlds with large layers of water the place water makes up round 50 % of the planet’s mass, as was beforehand thought. Hycean worlds with 10-90 % water are due to this fact most unlikely.”
This determine reveals the H20 mass fraction for 2 teams of planets. On the left are planets that fashioned throughout the water ice line. On the appropriate are planets that fashioned exterior of that line. Planets fashioned contained in the ice line are systematically depleted in carbon as a result of lack of unstable ice accretion and exhibit greater envelope H2O mass fractions. In distinction, planets fashioned past the ice line retain decrease H2O content material regardless of greater bulk unstable abundances. Every pie chart reveals the imply mass fraction of hydrogen in H2 (fuel), H (steel), H2 (silicate), H2O (fuel), and H2O (silicate). Planets that fashioned past the ice line retailer most hydrogen as H2 (fuel) and H (steel), whereas those who fashioned contained in the ice line retain a bigger share of hydrogen in H (steel), H2 (silicate), and H2O (fuel + silicate). Picture Credit score: Werlen et al. 2025. ApJL
If that is true, then it means a drastic discount within the variety of doubtlessly liveable worlds. However there’s one other implication, too: Earth’s water content material is not some form of wild outlier.
“The Earth is probably not as extraordinary as we expect. In our research, at the very least, it seems to be a typical planet,” mentioned Dorn.
The analysis and the modelled planets additionally revealed an sudden paradox. As an alternative of planets forming past the snow line having extra water, the reverse is true. Planets that type throughout the snow line find yourself with extra water due to interactions between the environment and the magma ocean. In these circumstances, the water did not come from ice crystals accreted past the snow line. As an alternative, it was created when hydrogen within the environment reacted with oxygen contained in silicates within the magma ocean, creating H2O.
“These findings problem the traditional hyperlink between ice-rich formation and water-rich atmospheres. As an alternative, they spotlight the dominant function of the equilibrium between magma ocean and environment in shaping planetary composition,” mentioned Werlen.
“Our outcomes, which concentrate on the preliminary (beginning) inhabitants of sub-Neptunes with magma oceans, recommend that their water mass fractions will not be primarily set by the accretion of icy pebbles throughout formation however by chemical equilibration between the primordial environment and the molten inside,” the authors clarify.
Not a single one of many modelled planets, no matter their authentic H20 content material, managed to retain greater than 1.5percentwt H20 after chemical equilibration. That is a far cry from the 90% most from earlier analysis.
“Our outcomes problem the standard hyperlink between ice accretion and water-rich atmospheres, exhibiting as a substitute that H2O-dominated envelopes emerge by chemical equilibration in hydrogen-poor planets fashioned contained in the snow line,” the researchers conclude.
