Recent studies are shaking up our understanding of sub-Neptune planets, often labeled as potential “water worlds.” It turns out, they might be more barren than we previously thought.
For a long time, scientists believed these planets—larger than Earth but smaller than Neptune—formed far from their stars, gathering ice as they traveled inward. This led to the idea that they could host hidden oceans beneath thick hydrogen atmospheres, giving rise to the term “Hycean planets,” a mix of “hydrogen” and “ocean.” However, a new study challenges this view.
Caroline Dorn, a physicist at ETH Zürich, co-led the research and explained, “Our calculations show that this scenario is not possible.” The study reveals that most sub-Neptunes likely have far less water than scientists had anticipated.
The findings come shortly after K2-18b, an exoplanet 124 light-years away, generated excitement for possibly harboring life due to signs of a biomarker gas, dimethyl sulfide, detected by the James Webb Space Telescope. Many hoped this meant K2-18b was a water-rich planet. However, some experts warned that sub-Neptunes may not be ocean worlds but rather dominated by hostile atmospheres that obscure any potential habitability.
Dorn and her team studied how these planets evolve. Unlike earlier work, they factored in chemical reactions between magma and atmosphere. Their models showed that, of the 248 planets they examined, none could sustain the massive oceans previously theorized. “Hycean worlds with 10-90 percent water are therefore very unlikely,” Dorn noted.
Interestingly, they found that hydrogen and oxygen—the essential components of water—are mostly trapped within the planet’s rocky interior. Even those with initial ice content ended up with less than 1.5% of their mass as surface water, which is a sharp contrast to earlier expectations.
Aaron Werlen, another researcher on the team, said, “Planetary simulations clearly show that they have much less water than we thought.” Most water-rich atmospheres appeared on planets closer to their stars, formed through chemical reactions rather than carried ice.
This has significant implications for the search for extraterrestrial life. If Hycean planets are a myth, the best prospects for liquid water may lie on smaller, rocky planets similar to Earth.
Nonetheless, K2-18b continues to fascinate researchers. As a sub-Neptune, it offers insights into planetary system formation and may challenge our notions of Earth’s uniqueness. Dorn observed, “The Earth may not be as extraordinary as we think. It appears to be a typical planet.” The full study is available in The Astrophysical Journal Letters.
