A hycean world — one with a global water ocean and a hydrogen-rich atmosphere — orbits a red dwarf star in this artist’s concept. Credit: A. Smith, N. Madhusudhan (University of Cambridge)
Scientists have reported new observations from NASA’s James Webb Space Telescope (JWST) that strengthen the case that the exoplanet K2-18 b has molecules in its atmosphere that, on Earth, are produced only by life.
The work, announced Wednesday, builds on previous observations from JWST published in 2023 by the same team that yielded weak hints of the molecule dimethyl sulfide (DMS) on K2-18 b, a planet that the team thinks could be covered with a global ocean. In the new data, additional spectra taken with a different onboard instrument have seen more evidence of the chemical signal of DMS, as well as signs of a related molecule, dimethyl disulfide (DMDS).
The team says they are not claiming to have discovered life, and that it is vital to collect more data to confirm the detection. However, they are not shying away from pointing out what they feel is the gravity of the moment — that detecting molecules that could be potential signs of life in other star systems are now within reach of our most powerful telescopes. In comments to media during an embargoed press conference on Tuesday reported by outlets like The New York Times, team leader Nikku Madhusudhan of the University of Cambridge, U.K. said that it was a “revolutionary moment.”
Madhusudhan also argued that the best explanation for the data is that K2-18 b hosts life. But the initial reaction from the astronomical community to that proposal has been met with a healthy dose of skepticism. Many praised the power of JWST to be able to attempt detections of such faint signals. But there was also widespread caution of interpretations that invoke life, as recent studies have suggested that DMS can also be produced without life.
Biosignature or not?
Lying 124 light-years away in the constellation Leo, K2-18 b orbits in the habitable zone of its red-dwarf host star — at a distance where the planet’s surface temperature can support liquid water. But at around 8 or 9 times as massive as Earth — equivalent to half the mass of the ice giant Neptune — it’s not clear what that surface is like.
The JWST observations that Madhusudhan’s team reported in 2023 showed clear signs of methane (CH4) and carbon dioxide (CO2) in its atmosphere. That, they argued, fits a scenario where a hydrogen-rich atmosphere surrounds a planet with a global water (H2O) ocean that could support life. They dubbed this scenario a “hycean” world, drawn from the words hydrogen and ocean.
At the time, the team also noted weak signs of DMS in the data, taken by JWST’s near-infrared spectrograph. DMS has been studied by astrobiologists for its potential to indicate life — what scientists call a biosignature — for over a decade. But the significance of the detection barely rose to a 2-sigma level, meaning there was a 5 percent probability of the signal appearing by chance — well short of the 5-sigma considered to be the statistical gold standard in science.
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The new data come from JWST’s mid-infrared spectrograph and bring the detection up to a 3-sigma level, according to the team’s analysis, meaning there is a 0.3 percent probability of the data fitting the expected model spectrum for those molecules as well as it does purely by chance. With 16 to 24 additional hours of observing time, they think they can hit the 5-sigma mark — a 0.00006 percent probability of it fitting the model that well by chance. However, such statistical analyses can sometimes be misleading in that they do not take into account the possibility that the model is wrong, or that the data captured some other chemical that is mimicking the appearance of the model.
Even if the detection is confirmed, the question remains: How reliable is DMS as a biosignature? On Earth, DMS is produced by life like phytoplankton. It’s part of the smell of a sea breeze. And as far as scientists know, life is the only way DMS is produced on Earth.
But that doesn’t mean it can’t be produced by nonliving means elsewhere in the universe.
A year ago, researchers reported a detection of DMS on the comet 67P/Churyumov-Gerasimenko — hardly a location brimming with life. (The team found the signal in archival data from the European Space Agency’s Rosetta mission.) And in September of last year, a team of researchers reported that in lab experiments, they were able to produce DMS by shining UV light on a simulated, hazy exoplanet atmosphere. This suggests that the reactions between a star’s photons and molecules in a planet’s atmosphere could provide a nonbiological way to produce DMS, again challenging the idea that the gas is a clear sign of life.
In press
The new JWST results were publicized April 16 in a wave of news stories published at 7 p.m. EDT, accompanied by a press release from the University of Cambridge.
According to those reports, the paper was published today in The Astrophysical Journal Letters. As of publication of this story, six hours later, the paper was not yet online at the link provided to journalists. However, a preprint of the manuscript was posted to the arXiv.
In the Cambridge press release, Madhusudhan noted, “It’s important that we’re deeply skeptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them. … That’s how science has to work.”
NASA, which issued a press release when Madhusudhan and his colleagues’ previous JWST observations on K2-18 b were published, did not issue a press release on the new work.
But the agency did appear to distance itself from speculation of life or “true biosignatures” in a statement to The Washington Post, suggesting that JWST data on its own was not sufficiently convincing. Science reporter Joel Achenbach posted on Bluesky that NASA told him a “detection of a single potential biosignature would not constitute discovery of life. We would likely need multiple converging lines of evidence to confirm true biosignatures and rule out false positives, possibly including independent data from multiple missions.”
In other reactions, planetary scientist Sarah Hörst of Johns Hopkins University took to Bluesky to reject the claim that DMS amounts to a biosignature, pointing to the lab experiments that show it can be produced without life.
David Kipping, an astronomer at Columbia University, posted on X that it was “Worth remembering that the detected molecule (DMS) does not necessarily mean life.” But, he added, it is ”exciting JWST can touch this sensitivity!”
This is a developing story and will be updated.
