An exoplanet orbiting a dwarf⁤ star⁣ 124 light-years from Earth made headlines around the world in April 2025. Researchers at the⁤ University of Cambridge reported that planet K2-18b could be a marine world with a deep, global ocean teeming with life.However, a study now shows that so-called sub-Neptunes such as K2-18b are highly unlikely to be worlds dominated ⁣by water and that conditions there are far from conducive to life. “Water on planets is much more limited than previously believed,” notes Caroline Dorn, professor of exoplanets at ETH ‍Zurich.

The Sub-Neptune Puzzle

The study was conducted under the leadership of ETH Zurich, in collaboration with researchers from⁤ the Max Planck Institute for Astronomy in ⁣Heidelberg and the University of California ⁣in Los angeles. K2-18b is larger ⁤than Earth but smaller than Neptune, placing it in a class of planets that do not exist in our solar system. However, observations from missions like Kepler and TESS show that they are remarkably common in ⁣outer space – ⁤representing a significant portion of exoplanets‍ discovered to date. some of these sub-Neptunes were likely formed far from their ⁤central star, beyond ⁢the so-called snow line where water freezes into ice, and later migrated inwards.

This migration pattern led to speculation that these planets could accumulate vast amounts of water during their formation,possibly harboring deep,global oceans beneath a hydrogen-rich atmosphere. This concept gave rise to the term “hycean planets” – a portmanteau of “hydrogen” and “ocean” – and fueled excitement⁢ about ⁣the possibility of finding habitable worlds beyond our solar system.

Ignoring the Chemistry: A Critical Flaw

Until now, it had been assumed that some of these planets were able⁣ to accumulate particularly large amounts of⁤ water during ⁣their formation and now harbour deep, global oceans⁣ beneath a hydrogen-rich atmosphere. Though, ⁢Dorn and her team argue that previous studies overlooked‍ a⁣ crucial factor: the chemical interactions between the planet’s atmosphere and its‍ interior.

“Our calculations⁣ show that this scenario is not possible,” says ⁢Dorn.”A essential vulnerability of previous studies was that they ignored any chemical coupling between the atmosphere and the interior of the planet.” ⁤ The research, ⁢published in‍ The Astrophysical Journal Letters, demonstrates that these interactions significantly alter the planet’s ‍water content.

“We have now factored in the interactions between the planet’s interior