Nearby super-Earth K2-18 b may be a water-rich ocean planet: 'This has certainly increased the chances of habitability'
The saga surrounding Neptune-size "super-Earth" exoplanet K2-18 b just got a whole lot more interesting. For a quick recap, this is the world a team of scientists recently suggested could host life — to the dismay of other scientists in the community, who felt the announcement failed to include necessary caution.
While signs of life on the world have failed to conclusively present themselves to the James Webb Space Telescope (JWST), the powerful space telescope has discovered that this planet is so rich in liquid water that it could be an ocean, or "Hycean" world.
"This has certainly increased the chances of habitability on K2-18 b" Nikku Madhusudhan, the University of Cambridge scientist behind the original K2-18b discovery as well as the new study, told Space.com. "This is a very important development and further increases the chance of a Hycean environment in K2-18 b. It confirms K2-18 b to be our best chance to study a potential habitable environment beyond the solar system at the present time."
The story regarding the habitability of K2-18 b began back in April 2025, when Madhusudhan and fellow researchers from the University of Cambridge announced they had found what they called the "strongest evidence yet" of life beyond the solar system around this distant super-Earth (it's around nine times as massive as our planet).
The evidence came from the tentative detection of molecules that, when found in the atmosphere of Earth, are typically the result of biological processes of living things. The pressure was then on to confirm these potential biosignatures: dimethyl sulfide and dimethyl disulfide.
The team set about this by observing four separate instances of K2-18 b crossing, or "transiting," the face of its parent red dwarf star, located about 124 light-years away, during its roughly 33-Earth-day orbit. Because chemicals absorb and emit light at characteristic wavelengths, when light from a parent star passes through a planet's atmosphere, the molecules in that atmosphere leave their telltale fingerprints in the spectrum of starlight.
"With four additional transit observations using JWST, we have measured the spectrum of K2-18 b's atmosphere with unprecedented precision," Renyu Hu, the new study's team leader and a NASA Jet Propulsion Lab scientist, told Space.com. "The spectrum allowed us to conclusively detect both methane and carbon dioxide in the planet's atmosphere and to constrain their abundances. This information points to a planet with a water-rich interior."
Hu explained that the team searched for signals of dimethyl sulfide and other organic sulfur molecules in the spectrum using several independent models, but did not find conclusive evidence for their presence.
"This was not necessarily disappointing," Hu continued. "We're excited about establishing the planet's water-rich nature."
Is K2-18 b a ocean world?
Saying it's now confirmed that K2-18 b is water-rich, Hu explained that the next step is to discover if the planet possesses a global liquid water ocean.
Ironically, one of the most positive signs of such an ocean is the fact that the atmosphere of this super-Earth appears to lack water vapor.
"The spectrum we obtained does not show signs of water vapor. If the atmosphere truly lacks water, this suggests that water has been depleted — most likely through condensation," Hu said. "On Earth, this process is known as the 'cold trap,' and geoscientists consider it essential for retaining water over billions of years by preventing it from escaping to space.
"Observing a similar process on an exoplanet would be very exciting. Rigorously confirming the absence of water can by itself be a scientifically important goal for future observations," Hu said.
However, Hu cautioned that the spectrum detected by the JWST could also be explained by an alternative model in which the atmosphere actually contains abundant water vapor.
Establishing whether K2-18 b and other similar temperate, sub-Neptune-sized planets possess liquid water oceans, Hu says, will also require detecting the presence of a broader set of atmospheric gases beyond methane and carbon dioxide. It would also require an absence of molecules like ammonia, carbon monoxide and sulfur dioxide, which, as of yet, have indeed not been detected in the atmosphere of K2-18 b
"This conclusion is based on theoretical work by my group and several others," Hu added. "With the new observations providing valuable context, we've summarized these insights into a roadmap to help guide future observations and studies."
Meanwhile, the search for the biosignatures, dimethyl sulfide and dimethyl disulfide, is far from done; while not hitting the significance level required for a confirmation, this research did provide a stronger signal from these molecules than were provided by previous examinations.
"The evidence for dimethyl sulfide in the present work is significantly higher than what we had with our previous observations in the same near-infrared wavelength range," Madhusudhan said. "However, this evidence is still not high enough to claim a conclusive detection.
"We also need to be able to distinguish dimethyl sulfide from other possible contributors, such as methyl mercaptan, which is also a biosignature on Earth."
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It looks certain that K2-18 b will continue to hold the interest of astronomers for some time.
"It is great that we are able to infer tentative signs of potential biosignatures with current JWST observations, but significantly more time is needed for conclusive detections. A key question is whether the atmosphere contains one or more biosignatures," Madhusudhan said. "At the same time, extensive theoretical and experimental efforts are needed to robustly identify biological and non-biological pathways for candidate biosignature molecules."
One thing the team is sure of, though, is the progress made thus far in the study of K2-18 b wouldn't have been possible without the JWST. And, the $10 billion space telescope is set to play a key role in the future investigation of this super-Earth.
"Our observations and analyses add to the growing list of exciting discoveries that highlight the truly transformative science enabled by JWST," Hu concluded. "While we found its Near-Infrared Spectrograph [NIRSpec] particularly well suited to address the goals of our study, other JWST instruments or observational modes could provide complementary and highly valuable information to further enhance our understanding of this planet."
The team's research is available as a preprint on the paper repository arXiv.
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