American astrophysicists from the University of Wisconsin have discovered that the habitable zones of hycean planets — hypothetical ocean-covered worlds surrounded by thick hydrogen atmospheres — may be much narrower than previously believed. The study, published as a preprint on the arXiv platform, highlights the role of tidal forces in shaping life-supporting conditions on such planets.

What Are Hycean Planets?

Hycean planets are thought to be water-rich worlds with deep global oceans and dense hydrogen atmospheres. Although no such planet has been definitively confirmed, scientists are investigating candidates like the exoplanet K2-18 b. Its atmosphere has previously shown signs of water vapor, carbon dioxide, methane, and possibly dimethyl sulfide (DMS) — a potential biomarker on Earth associated with marine phytoplankton.

The Role of Tidal Heating

Traditional models of the habitable zone consider only the balance of stellar radiation needed to sustain liquid water. However, as shown by the team led by Joseph Livesey, tidal heating plays a crucial role for hycean planets, especially those orbiting red dwarfs — stars with less than 0.3 solar masses. “Just like the subsurface oceans of Europa and Enceladus, hycean planets can receive heat not only from their star but also through tidal deformation,” the researchers explained.

Tidal heating arises from gravitational interactions between a planet and its star or a massive moon, such as a gas giant. This heat shifts the habitable zone farther from the star and makes it narrower, particularly for planets around 7 Earth masses and roughly 1.7 Earth radii. The more massive the host star, the less significant the tidal effect becomes.

The Influence of Outer Companions

The study also found that the presence of a massive outer companion, like a gas giant, can amplify tidal effects through periodic orbital variations. This provides the planet with additional internal heat that can keep water in a liquid state even at a greater distance from the star. As a result, such planets could remain potentially habitable even outside the classical habitable zone.

Implications for Biological Evolution

Unlike Earth, where tidal energy is largely dissipated, hycean planets with deep oceans might allow this energy to be efficiently used by living organisms — if they exist. Researchers suggest tidal heating could accelerate biological evolution by creating more dynamic environmental conditions, making these planets particularly interesting for astrobiology.

Future Research

The findings stress the need to revise existing models of habitable zones for oceanic worlds. Scientists plan to continue studying candidates like K2-18 b using advanced telescopes, including the James Webb Space Telescope, to verify the presence of biomarkers and better understand the impact of tidal forces. These efforts may help identify the conditions necessary for life beyond the Solar System.

Conclusion

The new research shows that tidal forces can significantly restrict the habitable zones of hycean planets, especially those orbiting red dwarfs. However, those same forces may also create unique conditions that support liquid water and potentially a biosphere. This discovery highlights the complexity of the search for habitable worlds and the importance of accounting for tidal effects when studying exoplanets — bringing us closer to understanding where life might exist in the universe.