New evidence reshapes the water story of distant worlds
A team led by ETH Zurich, with collaborators from the Max Planck Institute for Astronomy and the University of California, Los Angeles, has redefined what we should expect from water on planets beyond our solar system. Building on the discussion sparked by the April 2025 discovery of the exoplanet K2-18b, researchers now argue that sub-Neptunes—worlds larger than Earth but smaller than Neptune—likely do not host massive surface oceans. Their findings, published in The Astrophysical Journal Letters, cast doubt on the idea that many distant worlds are swimming in water and, by extension, on the ubiquity of Earth-like oceans in the galaxy.
Why this study matters: shifting the water budget of exoplanets
The researchers focused on how planetary formation and interior-atmosphere chemistry interact over time. Previous models tended to treat a planet’s interior and its atmosphere as separate components, potentially overestimating surface water inventories. Dorn, Werlen and colleagues incorporated a chemistry-driven coupling between a young planet’s magma ocean and its hydrogen-rich atmosphere. They simulated 248 planets across 26 chemical components, aiming to mirror the real, messy processes that shape planetary water budgets.
From magma oceans to dry surfaces
In the early stages, sub-Neptunes likely possessed deep magma oceans cloaked by hydrogen gas. The team’s simulations show that, as chemical reactions proceeded, hydrogen and oxygen preferentially bonded with metallic elements and silicates. Much of the water was effectively drawn into the interior and core, leaving only a thin veneer of surface water behind—at most a few percent of the planet’s mass. In short, the once-hypothesized Hycean worlds, with oceans spanning most of the planet, are much rarer than previously thought.
The surprising role of interior-atmosphere chemistry
“We found that the chemical equilibrium between the magma ocean and the atmosphere governs how much water ends up on the surface,” Werlen explains. This coupling overturns earlier expectations that distant planets simply accreted vast oceans during formation. Even when a planet begins life with a hydrogen envelope, much of the water can vanish into the interior as chemistry unfolds over millions of years.
What this means for life and future observations
The implication is stark: habitable conditions requiring long-lived surface water are likely to emerge on smaller planets, rather than on inflated sub-Neptunes. Detecting liquid water on such worlds will demand more sensitive instruments, potentially beyond the reach of the James Webb Space Telescope. The study also reframes how we interpret exoplanetary atmospheres: a water-rich atmosphere does not automatically equate to a water-rich planet at large.
Earth’s place in the cosmic context
Nothing in this work rules out the possibility of life elsewhere, but it positions Earth as possibly more typical than exceptional. Dorn notes that many distant planets may have water contents not vastly different from our own world, challenging the idea that Earth sits in a uniquely water-rich niche. An intriguing twist emerges: planets with the most water-rich atmospheres may have formed inside the snow line, not beyond it, through chemical reactions that generate water rather than importing it from ice alone.
Looking ahead
As exoplanet science advances, these findings will influence how researchers search for life. The new framework emphasizes the importance of interior-atmosphere dynamics in shaping a planet’s water budget and atmospheric composition. With next-generation observatories and more sophisticated models, scientists hope to refine which worlds might host surface liquid water and, by extension, which could be promising targets in the quest for extraterrestrial life.
