In April 2025, a headline swept the world: astronomers had found the "strongest hints yet of biological activity outside the solar system" [source: University of Cambridge, 2025]. Using the James Webb Space Telescope (JWST), a Cambridge-led team reported a gas in the atmosphere of a distant planet called K2-18b that, on Earth, is made almost entirely by living things. Within months, though, a string of independent reanalyses had pushed back hard, and by late 2025 the claim looked far shakier than the first headlines suggested.
This is not a story about whether we have found aliens. It is a story about how careful science actually works — and about the wide gap between a signal measured by a telescope and the interpretation that there is life at the other end of it. Let us separate the two cleanly: what JWST really recorded, what one team argued it means, why others disagree, and how the field decides when a hint becomes a discovery.
In this article
- What JWST actually measures — the signal
- The K2-18b claim — from spectrum to "biosignature"
- The pushback — reanalyses and the significance debate
- Even if the gas is real — is it a biosignature?
- The bigger picture — how well JWST reads other atmospheres
- How science decides — the standard of evidence
- Conclusion — what to watch
What JWST actually measures — the signal
Start with the measurement, because everything else is built on it. JWST does not photograph exoplanets or sniff their air. For a planet like K2-18b it uses transmission spectroscopy: when the planet passes in front of its star, a sliver of starlight filters through the planet's atmosphere on its way to us. Different molecules absorb different colors (wavelengths) of that light, so the spectrum arrives with faint dips at specific wavelengths — a barcode of what the atmosphere contains [source: NASA JPL, 2022].
The catch is scale. K2-18b lies about 124 light-years away in the constellation Leo, and the atmospheric signal is a change in starlight of only a few dozen parts per million. Teasing that out means stacking many hours of observation and modelling away the telescope's own instrument noise. So the raw "signal" is never a clean picture; it is a statistical inference, and how you reduce the data and which models you fit can move the result. Hold on to that point — it is the fault line the whole debate runs along.
K2-18b itself is an interesting target. It has about 8.6 times Earth's mass and 2.6 times its radius, placing it between Earth and Neptune in size — a "sub-Neptune." It orbits a cool red dwarf star every 33 days, inside the habitable zone (the region where a planet could hold liquid water) [source: University of Cambridge, 2025]. In 2023, JWST detected methane and carbon dioxide in its atmosphere, which some researchers read as consistent with a "Hycean" world — a planet with a hydrogen-rich atmosphere over a liquid-water ocean. That reading, too, remains a hypothesis, not a confirmed fact.
The K2-18b claim — from spectrum to "biosignature"
In April 2025, Nikku Madhusudhan and colleagues at the University of Cambridge reported that new JWST observations — this time with the mid-infrared instrument MIRI — showed a spectral feature consistent with dimethyl sulfide (DMS) and possibly dimethyl disulfide (DMDS) [source: Astrophysical Journal Letters, 2025]. On Earth, atmospheric DMS is produced overwhelmingly by life, chiefly marine phytoplankton. That is why it has long been discussed as a candidate biosignature — a gas whose presence might point to biology.
The numbers matter, and so does their framing. The team reported the feature at a confidence of about "three sigma" (3σ), meaning the odds of it being a random fluctuation are roughly 0.3% [source: Astrophysical Journal Letters, 2025]. Their atmospheric models implied a DMS abundance above 10 parts per million — thousands of times higher than the level on Earth. Taken at face value, that is a striking amount of a life-associated gas.
But the researchers themselves were careful to draw the line the headlines often erased. Madhusudhan stressed this was a "possible hint" of biological activity, "not the detection of life," and urged deep skepticism: "we should be very cautious," and "there could be unknown chemical processes at work" [source: University of Cambridge, 2025]. The team framed their result as a starting point that would need independent checking, and noted that in physics the conventional threshold for a "discovery" is not 3σ but five sigma (5σ) — odds of about one in two million. They estimated 16 to 24 more hours of JWST time might settle whether the signal reached that bar. In other words, the authors published a candidate signal and explicitly invited scrutiny. That invitation was taken up quickly.
The pushback — reanalyses and the significance debate
Within months, several independent groups reanalysed the same JWST data and reached more cautious conclusions. This is the part that rarely makes the front page, but it is where the science happens.
The signal may not survive the statistics
A study in Astronomy & Astrophysics combined the near-infrared and mid-infrared datasets and found "insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b," concluding the features were not statistically significant [source: Astronomy & Astrophysics, 2025]. A separate team led by Kevin Stevenson went further, arguing the MIRI feature could be explained by red noise — correlated instrument systematics rather than a real molecular signal — and reported that under their preferred data binning, 87.5% of their reanalyses failed to detect DMS or DMDS at all. Their paper's blunt title captured the mood: "K2-18b Does Not Meet The Standards of Evidence For Life" [source: arXiv, 2025].
The gas is not uniquely DMS
Another angle is uniqueness. A systematic search led by Lorenzo Pica-Ciamarra fit 661 different candidate molecules to the spectrum and found that DMS was only weakly preferred, and that other sulfur- and carbon-bearing molecules could match the data about as well [source: arXiv, 2025]. If a feature can be explained by many molecules, attributing it specifically to a life-associated one is not justified by the data alone.
The confidence shrinks under fresh eyes
In July 2025, a NASA Jet Propulsion Laboratory analysis led by Renyu Hu — with three teams applying independent Bayesian models — confirmed water, methane and carbon dioxide but put any DMS signal at only about 2.7σ, below the discovery threshold [source: Astronomy, 2025]. Crucially, the same work noted that ordinary atmospheric chemistry, driven by starlight (photochemistry), could generate detectable DMS without any life at all. Notably, Madhusudhan was among the co-authors — a sign that this is a field converging through argument, not a feud. The through-line of all these reanalyses is consistent: the feature sits near the noise floor, and small, defensible choices in how the data are handled make it appear or vanish.
Even if the gas is real — is it a biosignature?
Suppose, for the sake of argument, that DMS truly is present. Would that prove life? Not on its own — and this is the second place where signal and interpretation must be kept apart. The logic "DMS means life" comes from Earth, where we know of no large abiotic (non-biological) source. But that assumption has weakened considerably.
In 2024, researchers reported evidence for DMS in the pristine material of comet 67P/Churyumov-Gerasimenko, from data gathered by the Rosetta mission — a source with no biology involved [source: Astrophysical Journal, 2024]. In 2025, a team detected DMS for the first time in the interstellar medium, in a molecular cloud near the center of our galaxy, again with no life anywhere in the picture [source: Astrophysical Journal Letters, 2025]. Together with the photochemistry seen in the NASA reanalysis, these findings show DMS can arise without biology. That does not mean DMS is worthless as a clue; it means it cannot, by itself, serve as a fingerprint of life. Ruling out false positives is the whole game.
The bigger picture — how well JWST reads other atmospheres
To judge the K2-18b claim fairly, it helps to see what JWST can do elsewhere. On large, hot planets its atmospheric readings are robust. For the hot gas giant WASP-39b, JWST delivered the first unambiguous detection of carbon dioxide in an exoplanet atmosphere, and even spotted sulfur dioxide produced by photochemistry — the first clear evidence of that process on another world [source: Nature, 2022]. These are strong, widely accepted results.
The picture changes for small, cool worlds. When JWST measured the Earth-sized rocky planet TRAPPIST-1 b, it found a dayside so hot — a brightness temperature of about 490 kelvin — that the data showed no sign of a substantial atmosphere at all, effectively ruling out a thick one [source: Nature, 2023]. The lesson is not that JWST is unreliable, but that small, temperate planets sit at the very edge of its capability. K2-18b is exactly such a target, which is precisely why its biosignature signal lands so close to the noise — and why reasonable experts can read the same data differently.
How science decides — the standard of evidence
So how does the field move from "interesting hint" to "accepted discovery"? Two ideas anchor the answer. The first is statistical: the 5σ threshold that separates a genuine detection from a fluke. A 3σ result is worth reporting and following up, but it is not a discovery — and roughly a third of past 3σ "signals" in physics have faded with more data.
The second is a framework built specifically for this problem. In 2021, a group of scientists called in Nature for a standard way to report evidence for life beyond Earth, proposing a "Confidence of Life Detection" (CoLD) scale with multiple levels [source: Nature, 2021]. The key move is to treat ruling out non-biological explanations as an explicit, staged requirement — first excluding known abiotic sources, then addressing new ones raised after a claim. By that yardstick, a candidate gas detected near the noise floor, explainable by other molecules and by non-biological chemistry, is an early rung on the ladder, not the top. That is not a criticism of the K2-18b team; it is the process functioning exactly as intended.
Conclusion — what to watch
Pull the threads together and the honest summary is this. JWST measured a faint spectral feature in K2-18b's atmosphere. One team argued it is consistent with a life-associated gas and, carefully, called it a possible hint rather than a discovery. Multiple independent reanalyses then found the signal weak, non-unique, or explainable as instrument noise — and even if the gas is real, non-biological sources of DMS now exist. Every layer between the measurement and the word "life" is still contested.
What should you watch from here? First, more JWST observations of K2-18b, which the original team said could push the signal toward or away from 5σ. Second, whether independent groups converge as data accumulate, rather than diverge. Third, whether the community adopts shared standards like the CoLD scale so that future claims arrive with their false positives already addressed. The search for life beyond Earth is advancing for real — but the distance between a candidate signal and a confirmed discovery is exactly the distance this debate is measuring.