Hycean Worlds: Myth or Reality in the Oceans of the Cosmos?

Hycean worlds are hypothetical planets, typically sub-Neptunes, that are rich in water and hydrogen. It is believed that the conditions on such planets could be extremely favorable for the emergence of life. However, the question remains: do these worlds actually exist? Recent studies have shown that popular models of their formation often overlooked a crucial process—chemical interactions between the planet’s interior and its “volatile” envelope, which could cause these planets to lose much of their water.

Rising Interest in Water-Rich Sub-Neptunes

Interest in water-rich sub-Neptunes surged after observations of the exoplanet K2-18b using the James Webb Space Telescope. On this sub-Neptune, located 124 light-years from Earth, scientists detected large amounts of dimethyl sulfide—a compound that, in significant quantities on Earth, is produced by marine phytoplankton and certain bacteria. Although dimethyl sulfide can also form through geochemical processes, its concentration on K2-18b sparked debates about a possible biological origin.

Debates Surrounding K2-18b

The scientific community has yet to reach a consensus regarding the nature of the substances found on K2-18b. In 2024, researchers suggested that the detected signals might not come from dimethyl sulfide, but from methane. In August 2025, another group proposed that the signals could be from ethylene. Meanwhile, new data from the James Webb Telescope in April 2025 once again indicated the presence of byproducts of bacterial activity on this planet, further fueling interest in the topic.

How Hycean Worlds Form

In discussions about K2-18b, it is often assumed that this sub-Neptune is covered by a vast ocean. Such planets are thought to form far from their star, beyond the so-called “snow line”—the boundary in a protoplanetary disk where temperatures are low enough for volatile substances to freeze into ice. If such a planet migrates closer to its star, the accumulated ice melts, forming an ocean and a dense atmosphere. These worlds are called hycean planets (from the English words hydrogen and ocean). Estimates suggest that water could make up to 50% of the mass of such planets.

A New Perspective on Chemical Processes

The authors of a new study note that traditional scenarios for the formation of sub-Neptune ocean worlds did not account for an important factor: the chemical interaction between the planet’s interior and its volatile envelope after the planet forms. The researchers calculated the chemical “equilibrium” of 26 components for 248 simulated young sub-Neptunes and super-Earths.

The study suggests that, in the early stages of formation, these planets are covered by a deep, hot magma ocean. The team used a new model to calculate the chemical processes that occur when atmospheric gases interact with metals and silicates in the magma.

Why Hycean Worlds May Be Rare

It turns out that even sub-Neptunes with up to 30% of their mass as water lose most of it during the magma ocean stage. Nearly all water molecules are destroyed in chemical reactions: hydrogen and oxygen bond with metal compounds and “sink” into the depths of the core. As a result, the water content drops to just 1.5% of the planet’s mass.

“According to our calculations, there are no distant worlds covered by a massive water layer making up about 50% of the planet’s mass, as previously thought. Therefore, the existence of hycean planets with 10–90% water is highly unlikely,” explained study co-author Caroline Dorn, a professor of exoplanet science at ETH Zurich.

Alternative Scenarios

There is still hope for planets that formed inside the snow line. These planets accumulate fewer volatile compounds but may retain a water-rich atmosphere. However, due to high temperatures, water on these worlds does not form surface oceans.

Who Conducted the Study

The new research was carried out by scientists from ETH Zurich in collaboration with planetary scientists from the Max Planck Institute for Astronomy (Germany) and the University of California, Los Angeles (USA). The results were published in The Astrophysical Journal Letters.