September 19, 2025

Exoplanets are (probably) not water worlds

Many “sub-Neptunes” were thought to be covered by deep global oceans. A new ETH Zurich–led study suggests the opposite: when a young planet’s hydrogen atmosphere interacts with its magma ocean, most water gets locked away deep inside—making true “water worlds” far rarer than hoped. Many exoplanets discovered so far are larger than Earth but smaller than Neptune—so-called sub-Neptunes. For years, some researchers have speculated that a fraction of these worlds could be “Hycean planets”: covered by deep global oceans beneath a hydrogen-rich atmosphere. A new international study led by ETH Zurich suggests this scenario is unlikely.

The work was prompted in part by the debate around K2-18b, a sub-Neptune about 124 light-years away that made headlines in April 2025 as a possible “water world”. ETH Professor Caroline Dorn and colleagues now conclude that planets like K2-18b are very unlikely to be dominated by water. “Water on planets is much more limited than previously assumed,” Dorn says.

Why the oceans disappear

The key difference to earlier studies is chemistry. Many models treated a planet’s atmosphere and interior separately. The ETH-led team explicitly included the chemical coupling between a young planet’s hydrogen atmosphere and its hot magma ocean.

Using computer simulations, the researchers calculated chemical equilibrium for 26 components across 248 model planets. The results show that most H₂O molecules don’t survive: hydrogen and oxygen tend to bind to metallic compounds, and a large fraction of these materials sinks into the planet’s core.

What remains at the surface is limited—at most a few percent as water (H₂O), rather than the massive water layers (tens of percent of a planet’s mass) sometimes proposed for Hycean worlds.

Implications for habitability in the JWST era

If sub-Neptunes are generally drier than they seem, then finding truly ocean-covered exoplanets—and potentially life-friendly environments—may be harder than hoped. The study suggests that more promising targets for surface liquid water could be smaller planets, which will require even more capable observatories than today’s instruments to characterize in detail.

At the same time, the findings reframe our place in the cosmos: in these simulations, Earth’s water fraction appears not exceptional but typical.

Publication: Aaron Werlen et al., The Astrophysical Journal Letters (2025), doi: 10.3847/2041-8213/adff73.

Source: ETH News (German)