Do Deep Exoplanet Oceans Hide Clues to Life? Exploring Hycean Habitable Zones

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

The quest for life beyond Earth has centered for decades on rocky, Earth-like planets, but a new class of enigmatic worlds called Hycean planets may upend this way of thinking about habitability. These ocean-laden exoplanets, cloaked in dense hydrogen atmospheres, could have great hidden seas thousands of miles below their surfaces, and tidal forces and geothermal heat might keep alien ecosystems going there, scientists reported Wednesday in the journal The Astrophysical Journal. But recent studies now indicate their habitable zones could be more exotic and maybe a lot tinier than before, with tidal heating likely playing an essential role in their ability to support life. Foremost among them, the mysterious world K2-18b has tantalized scientists, offering potential hints of biosignatures. Might these deep-ocean exoplanets be where we find extraterrestrial life?

The Hidden Oceans of Hycean Worlds

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

The majority of the water in our galaxy might not be where we think it is. While the water on Earth is spread across its surface, a new study of exoplanets, in particular super-Earths and mini-Neptunes, says it may instead become locked up deep within their mantles and cores, sealed off by incredibly high pressures. This “hidden water” phenomenon hugs traditional notions of habitability zones, in which surface liquid water was viewed as the gold standard for life.

Hycean worlds, which are hypothetical planets with global oceans located underneath hydrogen-rich atmospheres, could have ten times more water than estimated, although much of it might be chemically bound in their interiors instead of being flowing freely on their surface. That leaves open a fascinating possibility: If life exists on these planets, might it be basking in high-pressure abysses, beyond the touch of starlight?

Tidal Heating: The Unexpected Force Reshaping Habitability

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

Solar radiation decides whether or not life can exist on Earth. Tidal heating, on the other hand, may be the principal source of energy on Hycean worlds. Tidal forces might make exoplanets that orbit close to red dwarf stars very hot on the inside, exactly way Europa, one of Jupiter's moons, preserves an ocean below its surface by bending its gravity.

Joseph Livesey's new research at the University of Wisconsin–Madison indicates that tidal heating may reduce the size of the Hycean Habitable Zone (HHZ) by extending its inner limit outward. This happens because tidal friction generates heat, which could boil away water on the surface or make it difficult for life to exist. The effect is strongest around low-mass M-dwarf stars, where the gravitational pull of outer companion planets generates moderate orbital eccentricity, which keeps the heating cycle going.

K2-18b: A Hycean World Teeming with Clues and Controversy

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

K2-18b, a sub-Neptune 124 light-years away, has sparked the most debate about Hycean. The James Webb Space Telescope (JWST) found dimethyl sulfide (DMS) in its atmosphere. On Earth, this molecule is mostly made by marine phytoplankton. If this is true, it would be the best sign yet of life outside of our solar system.

But there is still doubt. Some scientists think that K2-18b could be a world of molten magma with a poisonous hydrogen atmosphere, where DMS might form through unknown abiotic processes. The signal is statistically significant, but it hasn't yet reached the "five-sigma" level needed for conclusive proof. Nikku Madhusudhan, the lead researcher on the JWST study, says, "Extraordinary claims need extraordinary evidence."

The Dark Hycean Zone: Life on the Night Side?

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

One of the most radical ideas in exoplanet science is the "dark Hycean habitable zone," which is where planets that are locked in place by tides could only support life on their cold night sides. These worlds might use chemosynthesis near hydrothermal vents instead of sunlight to make food, like deep-sea ecosystems on Earth.

Tidal heating could keep water in these dark places, making areas where life could thrive where there isn't much stellar radiation. As Livesey's team points out, "Strong tides on Hycean planets could provide a major power source for life and speed up biological evolution."

The Runaway Greenhouse Threat

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

Hycean worlds are very close to being able to support life and being a disaster. Their thick hydrogen atmospheres might protect them from stellar flares, but they could also start a runaway greenhouse effect if they trap too much heat. JWST hasn't yet found any clouds on K2-18b, but if there aren't any, these planets could turn into pressure-cooker environments, making it impossible for oceans to form on the surface.

The most important thing? How tidal heating helps keep the temperature stable. The oceans turn to gas if there is too much, and they freeze if there is too little. Finding the right balance may require a careful balance of the distance between stars, the composition of the atmosphere, and the effects of gravity.

The Future of Hycean Exploration

The Hidden Oceans of Hycean Worlds, Tidal Heating: The Unexpected Force Reshaping Habitability, K2-18b: A Hycean World Teeming with Clues and Controversy, The Dark Hycean Zone: Life on the Night Side?, The Runaway Greenhouse Threat, The Future of Hycean Exploration

The next steps are clear: More JWST observations! A confirmation of DMS on K2-18b would require an observing time of at least 16−24 hours to achieve a five-sigma level of confidence. Meanwhile, astronomers are working on models that can help tease out real Hycean planets from “false positives” such as a magma-ocean world.

Future missions, including ESA’s ARIEL and NASA’s Habitable Worlds Observatory, may widen the search for Hycean candidates, searching their atmospheres for molecules like methane, carbon dioxide, and crucially DMS.