For decades, scientists have scoured the cosmos for planets that resemble our own. The logic has been simple: if life evolved on Earth, then Earth-like planets should be the best candidates for hosting life. After discovering over 5,500 exoplanets, our perspective has now changed. There are a wide variety of worlds on which life can potentially exist, including ice moons with subsurface oceans, water worlds, Super-Earths, and even exoplanets in orbits around dead stars. Among the many exotic potential incubators of biology, are a new class of exoplanets, known as Hycean worlds, that particularly expand the specifications of what a habitable world could be.
Hycean worlds—named for their hydrogen-rich (“Hy”) atmospheres and vast global oceans (“cean”)—represent a compelling alternative to rocky, Earth-like planets. These sub-Neptunes, larger than Earth but smaller than ice giants, exist in a wide range of stellar environments and have conditions that may support life. Unlike terrestrial exoplanets, their thick atmospheres and vast oceans could create stable, life-friendly environments. In some ways, their thermodynamic conditions resemble Earth’s own oceans, but their expanded size and extended atmospheres make them easier to study than smaller, rocky worlds.
The concept of Hycean worlds gained momentum with the study of K2-18 b, a sub-Neptune in the habitable zone of its star. Observations suggested that K2-18 b could harbour liquid water beneath its thick atmosphere, sparking interest in the possibility that such planets might not only be common but also viable hosts for alien life. Even more intriguing, the habitable zone for Hycean worlds is broader than the one traditionally considered for Earth-like planets, meaning that many more exoplanets could be potential havens for life.
Chemistry in an Alien Ocean
Hycean planets differ from Earth in fundamental ways, particularly in their atmospheric composition. Their thick hydrogen-dominated atmospheres are quite unlike the nitrogen-oxygen mix we breathe. Instead, gases such as methane (CH4), ammonia (NH3), and water vapor (H2O) dominate, their precise abundances shaped by temperature, planetary composition, and interactions between the ocean and atmosphere.
A key question in assessing habitability is whether these oceans contain bioessential elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—at levels sufficient to sustain life. Unlike Earth, where geological activity and hydrothermal vents supply nutrients, Hycean worlds may face a challenge in delivering these elements from their rocky cores due to the presence of high-pressure ice layers. However, alternative sources exist: asteroid impacts, cosmic dust, and the slow diffusion of minerals through deep ice layers could provide essential ingredients for life. Some models suggest that prebiotic molecules such as hydrogen cyanide (HCN), a building block of life, could form within Hycean atmospheres or arrive via interstellar impacts, creating conditions where biology could emerge in ways unfamiliar to us.
The Hunt for Biosignatures
The launch of the James Webb Space Telescope (JWST) has revolutionized the search for extraterrestrial life by allowing scientists to study exoplanet atmospheres with unprecedented precision. For Hycean worlds, this means detecting potential biosignatures—molecular traces that could indicate biological activity.
One promising target is dimethyl sulfide (DMS), a compound known to be produced by life on Earth but rarely formed through non-biological processes. Methyl chloride (CH3Cl) is another intriguing candidate, commonly associated with biological activity on Earth. Recently, JWST observations of K2-18 b revealed the presence of methane (CH4) and carbon dioxide (CO2), raising hopes that further observations might uncover stronger biosignatures.
Yet, interpreting biosignatures is fraught with challenges. Methane, for example, can be produced abiotically through volcanic activity or reactions in the atmosphere. To distinguish biological from non-biological sources, scientists look for specific ratios of gases—high CH4/NH3 or CO2/CO ratios, for example—that would be difficult to explain through non-living processes alone. If future observations confirm the presence of such chemical patterns, it could mark the first compelling evidence for alien life.
Life in an Alien Ocean
If life exists on Hycean worlds, what might it look like? One promising factor is that their thermodynamic conditions—temperature, pressure, and chemical environment—may be remarkably similar to Earth’s deep oceans, where microbial life thrives. The key requirements for life as we know it—liquid water, an energy source, and essential chemistry—could all be met within a Hycean ocean.
Energy sources on Hycean worlds could include stellar radiation penetrating the atmosphere and chemical reactions between hydrogen and carbon dioxide, much like the methanogenesis that occurs in deep-sea hydrothermal vents on Earth. If such processes support life, it might be microbial, potentially floating freely in the upper ocean layers where sunlight and nutrients are more accessible. Alternatively, in warmer Hycean worlds, evolution could proceed at an accelerated pace, leading to the emergence of complex life forms earlier. Cooler, dimly lit Hycean planets might instead host simpler microbial ecosystems, with biosignatures that are more difficult to detect.
Elusive Aliens
Despite the excitement surrounding Hycean worlds, significant uncertainties remain. One of the biggest challenges is determining their exact internal structures—some may turn out to be solid-surfaced sub-Neptunes with little or no water, while others might be too hot for life. Theoretical models suggest that precise atmospheric measurements could help resolve this ambiguity, but observational constraints remain.
JWST’s ongoing observations will provide critical insights, but confirming the presence of life will require more than detecting individual molecules. Scientists need detailed spectral signatures of Hycean atmospheres, ideally measuring the relative abundances of multiple gases to confirm the presence of an ocean and rule out false positives. Future telescopes, both space and ground-based, will help refine these measurements, paving the way for a more definitive assessment of Hycean habitability.
The search for life beyond Earth has long been guided by the assumption that habitable worlds must resemble our own. But as we explore the cosmos, it is becoming increasingly clear that life may exist under conditions we never anticipated.
Cover Image: Artist’s illustration of K2-18 b, NASA, CSA, ESA, J. Olmsted (STScI), Science: N. Madhusudhan (Cambridge University)
Sources:
Carbon-bearing Molecules in a Possible Hycean Atmosphere
The Hycean Paradigm in the Search for Life
On the ocean conditions of Hycean worlds
Prospects for biological evolution on Hycean worlds
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