Readings from NASA’s James Webb Space Telescope confirm the presence of carbon-based molecules including methane and carbon dioxide in the atmosphere of a distant planet known as K2-18 b — which supports previous suggestions that it might be the kind of ocean-covered world capable of harboring marine life.
K2-18 b, which was first detected in 2015 using data from NASA’s now-retired Kepler space telescope, lies in the habitable zone of a star system that’s about 120 light-years away in the constellation Leo. It’s about 8.6 times as massive as Earth, and astronomers regard it as a sub-Neptune — a type of planet that doesn’t exist in our solar system.
Since its initial detection, astrobiologists have been interested in the world’s potential for habitability. They suspected it might be what’s called a Hycean planet — a type of world that’s massive enough to hold onto a hydrogen-rich atmosphere and a globe-spanning sea. The evidence has been piling up: In 2019, researchers reported the chemical signature of water vapor and clouds in K2-18 b’s atmosphere.
The fact that JWST detected an abundance of methane (CH4) and carbon dioxide (CO2), along with a shortage of ammonia (NH3), is consistent with the atmospheric modeling for Hycean planets.
“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of a paper about JWST’s findings, said in a news release. “Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations.”
The observations were made with JWST’s NIRISS and NIRSpec instruments in infrared wavelengths. The confidence level for the detection of methane is higher than it is for the carbon dioxide.
Madhusudhan and his colleagues also saw the signature of another compound with implications for the search for life: dimethyl sulfide. On Earth, DMS is produced almost exclusively through biological processes, primarily by microbes such as marine phytoplankton.
The confidence level for DMS’s presence on K2-18 b is still low. “While the present evidence is not as strong as that for CH4 or CO2, upcoming JWST observations of K2-18 b will be able to robustly constrain the presence and abundance of DMS,” the researchers say.
The newly reported findings sparked an online buzz. The Mars Society’s founder and president, Robert Zubrin, said in a posting to X / Twitter that the results were “VERY interesting.”
“If there is CO2 & H2 in the atmosphere there is a source of energy via the exothermic reaction: CO2 + 4H2 => CH4 + 2H2O,” he wrote. “If the atmosphere is mostly H2, any significant amount of CO2 would be way outside of equilibrium. Life!”
In contrast, Oxford astrophysicist Chris Lintott counseled caution. “I’d take the detection of a signature of life with a big pinch of salt (as the authors do too). Lots of work to understand the chemistry of these odd worlds,” he wrote on X / Twitter.
In astronomical terms, a distance of 120 light-years is in our celestial neighborhood. But it’s unlikely that Earthlings will be able to check out K2-18 b at close range anytime soon. Even if a starship were to travel continuously at the fastest recorded velocity for a spacecraft, it would take more than 220,000 years to get there.
Nevertheless, solid chemical evidence for biological activity on an alien world is likely to mark a giant leap in the search for life beyond Earth — even if we never make it to K2-18 b.
“Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe,” Madhusudhan said. “Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.”
Update for 6 p.m. PT Sept. 12: Rory Barnes, an astrobiologist at the University of Washington, said the newly reported findings highlight the potential for JWST to boost the search for chemicals associated with life on exoplanets.
The space telescope gathered data about the composition of K2-18 b’s atmosphere by analyzing the light that was filtered through the air of the planet as it crossed the disk of its parent star.
“What’s super-impressive is that they were able to make these detections just based on two transits,” Barnes said. “So it does point to the fact that if we even doubled or maybe tripled those numbers, we would actually start to be able to tighten up our understanding of how much methane and carbon dioxide is in the atmosphere. Maybe we would also then be able to see the dimethyl sulfide. Maybe we would detect water, ammonia and other kinds of species.”
Barnes emphasized that the results shouldn’t be considered a “smoking gun” in the search for biosignatures on K2-18 b. “There’s not any one single molecule in isolation that is a smoking gun,” he said. But the more data JWST is able to collect, the more confidence he and other astrobiologists will have that they’re on the right track.
The newly reported research definitely raises K2-18 b’s profile in the search for signs of life on distant planets, and Barnes said other planetary systems could provide even better targets for JWST observations in the future. He’s particularly interested in the TRAPPIST-1 system, which has three habitable-zone planets.
However, it will take more work to study the habitability of the TRAPPIST-1 planets, because those worlds are less weighty and are thought to have much thinner layers of atmosphere.
“The expectation for TRAPPIST-1 is that it’s going to take more like dozens of transits to make a similar type of observation. … But certainly the one that we’re all waiting for right now with JWST is TRAPPIST-1,” Barnes said.
The TRAPPIST-1 system is just 40 light-years away from Earth, but that’s still too far away for humans to travel there and back on a realistic time scale, given the current state of propulsion technology.
For what it’s worth, SETI astronomers checked TRAPPIST-1 for signs of radio transmissions several years ago. It sounds as if K2-18 b hasn’t yet been surveyed in the search for extraterrestrial intelligence — but this week’s reports have gotten the right people talking about looking for radio signals from the planet, just in case.
The research paper based on JWST’s data, “Carbon-Bearing Molecules in a Possibly Hycean Atmosphere,” has been accepted for publication in The Astrophysical Journal Letters and was the subject of a presentation today at the First Year of JWST Science Conference in Baltimore. In addition to Madhusudhan, the paper’s co-authors include Subhajit Sarkar, Savvas Canstantinou, Mans Holmberg, Anjali Piette and Julianne Moses.