The James Webb Space Telescope was not designed to find life. It was designed to see light, specifically infrared light, the kind that has been travelling for billions of years, stretched thin by the expansion of the universe. But light is information. And when starlight passes through the atmosphere of a distant planet, it picks up the chemical fingerprints of whatever it touched on the way through.
Since 2022, JWST has been reading those fingerprints. On worlds orbiting stars you will never see with your naked eye, the telescope has detected carbon dioxide, methane, sulfur compounds, and, on one planet 124 light-years away, what might be the faint chemical trace of biology. The James Webb Space Telescope and the question of life on other planets have become inseparable. Not because JWST has answered the question, but because it is the first instrument precise enough to ask it properly.
This is what we know. This is what we don’t. And this is what it would actually take to say we’ve found life.
How You Read an Atmosphere 124 Light-Years Away
The technique is called transmission spectroscopy, and its elegance belies its difficulty.
When a planet passes in front of its host star (an event called a transit), a thin ring of starlight filters through the planet’s atmosphere before reaching us. Different molecules absorb different wavelengths of that light. Carbon dioxide leaves a characteristic dip at one wavelength. Methane at another. Water vapour at another still. By comparing the star’s spectrum during a transit with its spectrum when the planet is out of the way, astronomers can isolate the atmospheric absorption signature: a chemical inventory of a world we will never visit.
JWST’s 6.5-metre primary mirror and its suite of infrared instruments (NIRSpec, NIRISS, and MIRI) make it uniquely suited for this work. Previous telescopes, including Hubble and Spitzer, could detect atmospheres on large, hot gas giants. JWST can reach smaller, cooler worlds: the ones where liquid water might persist, the ones that matter for the question of habitability.
It proved the concept early. In late 2022, JWST produced the first detailed transmission spectrum of WASP-39 b, a hot Saturn-mass planet 700 light-years away. The data revealed water, carbon dioxide, sodium, potassium, and, for the first time on any exoplanet, sulfur dioxide produced by photochemistry: chemical reactions driven by the host star’s light. WASP-39 b is far too hot for life. But the observation demonstrated that JWST could read atmospheric chemistry with a precision no previous telescope had achieved.
The challenge grows enormously when you point the telescope at smaller, cooler targets. An Earth-sized planet’s atmosphere might alter the starlight by parts per million. It takes dozens of transits, stacked and averaged, to extract a reliable spectrum. And even then, interpreting what the spectrum means requires ruling out every non-biological explanation first.
K2-18 b: The World That Won’t Let Go
If one planet has come to embody both the promise and the peril of biosignature science, it is K2-18 b.
In September 2023, a team led by astronomer Nikku Madhusudhan at the University of Cambridge published the first JWST transmission spectrum of this world. K2-18 b is roughly 2.6 times the radius of Earth and 8.6 times its mass. It orbits a red dwarf star 124 light-years away, completing one year every 33 days. Crucially, it sits in its star’s habitable zone: the orbital distance where liquid water could theoretically exist on the surface.
The JWST data, captured with the NIRSpec and NIRISS instruments, revealed carbon dioxide and methane in the planet’s atmosphere, with a notable absence of ammonia. That combination matters. On a planet with a thick hydrogen envelope and no ocean, ammonia should be abundant. Its absence, paired with CO2 and CH4, is consistent with a hypothesis Madhusudhan had proposed years earlier: a Hycean world, a planet with a hydrogen-rich atmosphere sitting atop a global water ocean.
There was also a tentative hint of something more provocative: dimethyl sulfide, or DMS.
On Earth, DMS is produced almost exclusively by marine phytoplankton. It is the molecule responsible for the briny smell of the open ocean. No known geological or atmospheric process produces it in significant quantities on our planet. If confirmed on K2-18 b, it would represent the first detection of a molecule that, here on Earth, requires biology to explain.
In April 2025, Madhusudhan’s team published follow-up observations using JWST’s MIRI instrument, covering longer infrared wavelengths between 6 and 12 micrometres. The new data strengthened the case. The spectrum showed features inconsistent with a featureless atmosphere at 3.4-sigma significance, and the team reported evidence of a related compound, dimethyl disulfide (DMDS), another molecule tied to microbial activity on Earth.
Three-sigma means a roughly 0.3% chance the signal appeared by random noise. That sounds compelling. But in physics and astronomy, the threshold for a discovery claim is five-sigma: a one-in-3.5-million chance of a false alarm. K2-18 b’s result is suggestive, not conclusive.
And then came the pushback.
A NASA-led reanalysis of the same data found no conclusive evidence of DMS. Other independent groups demonstrated that processing the data with slightly different reduction methods yielded different results; some analyses showed a DMS signal, while others showed a flat line. The detected concentrations were also puzzling: over 10 parts per million, roughly 10,000 times higher than DMS levels in Earth’s atmosphere, where they rarely exceed one part per billion. Could biology produce that much DMS? Could something else entirely, some unknown chemistry in a hydrogen-rich atmosphere with no solar system analogue, account for the signal?
For now, the answer is that we do not know. K2-18 b remains the most tantalizing case in exoplanet astrobiology. But tantalizing is not the same as confirmed.
TRAPPIST-1: Seven Worlds, Hard Answers
If K2-18 b is the single most studied exoplanet atmosphere, the TRAPPIST-1 system is the most studied exoplanet system. Seven roughly Earth-sized planets orbit a cool red dwarf star just 40 light-years away. Three of them sit in the habitable zone. It is, on paper, the best natural laboratory we have for testing whether small, rocky worlds can retain atmospheres, and perhaps host the conditions for life.
JWST has been watching. The results so far are sobering.
The two innermost planets, TRAPPIST-1 b and TRAPPIST-1 c, appear to be bare rock. Thermal phase curve measurements published in Nature Astronomy show that TRAPPIST-1 b has a dayside temperature of roughly 490 kelvin, no significant nightside emission, and no phase offset. These are the signatures of a dark, airless surface, a world stripped of whatever atmosphere it may once have had by its star’s relentless radiation. TRAPPIST-1 c may retain a tenuous, oxygen-dominated atmosphere, but even that remains uncertain.
The real prize is TRAPPIST-1 e, the habitable-zone world. In late 2025, JWST observations by Ana Glidden and collaborators brought both promise and caution. The data did not lean strongly for or against an atmosphere. Venus-like and Mars-like compositions were disfavoured. Hydrogen-rich atmospheres containing methane and CO2 were excluded. But nitrogen-rich atmospheres with traces of methane and CO2 remained permitted. The door is open, barely.
One complication: TRAPPIST-1 is an active star. Its surface is magnetically restless and spotted, and those stellar features can contaminate transit signals, mimicking atmospheric absorption where none exists. The team found that previously reported hints of an atmosphere on TRAPPIST-1 e were more likely stellar noise than planetary signal.
To address this, researchers devised an elegant workaround. TRAPPIST-1 b and TRAPPIST-1 e trace nearly the same path across the star’s disc during their transits. Since TRAPPIST-1 b appears to be bare rock, any spectral features shared between the two planets’ transit data are probably from the star, not from an atmosphere. Using TRAPPIST-1 b as a control allows astronomers to subtract the stellar contamination and isolate whatever atmosphere TRAPPIST-1 e might possess. These tightly spaced observations are now underway.
It is painstaking work, the kind that unfolds over years rather than press conferences. But it is the methodological rigour the search for life demands.
The Chemistry Problem
There is a fundamental difficulty at the heart of this science, and it is worth sitting with: the detection of a molecule that life produces is not the same as the detection of life.
A biosignature is a phenomenon that may have been produced by biology. The word “may” carries all the weight. Methane is a biosignature on Earth because biology is its dominant source here. But methane also emerges from volcanic activity, serpentinization of rock, and cometary impacts. Carbon dioxide is exhaled by every animal on our planet and released by every erupting volcano. Oxygen, long considered the gold standard of biosignatures, can accumulate abiotically through photolysis (ultraviolet radiation splitting water molecules) on planets orbiting red dwarf stars, with no biology required at all.
DMS is more promising because, on Earth, we know of no significant abiotic source. But Earth is one planet. K2-18 b is a sub-Neptune with a thick hydrogen atmosphere, a world with no analogue in our solar system. Chemistry behaves differently under different pressures, temperatures, and radiation environments. The possibility that some unknown abiotic process could produce DMS at 10,000 times terrestrial concentrations cannot be dismissed simply because we have not observed it here.
This is what scientists call the false positive problem. The chemistry of life overlaps with the chemistry of geology and photochemistry. Distinguishing one from the other requires not just detecting a molecule, but understanding the full planetary context: the star’s radiation, the atmosphere’s total composition, the planet’s interior chemistry, and the thermodynamic plausibility of every alternative explanation. As researchers in a 2025 PNAS review put it, the extraordinary claim of life should be the hypothesis of last resort, invoked only after every conceivable abiotic alternative has been exhausted.
What Comes Next
Madhusudhan’s team estimates that 16 to 24 additional hours of JWST observation time on K2-18 b could push the DMS detection from three-sigma to five-sigma, if the signal is real. Those observations are being planned. The result, whichever way it falls, will be significant: either the strongest evidence yet of biology beyond Earth, or a demonstration of how carefully the data must be handled to avoid false conclusions.
The TRAPPIST-1 campaign continues with closely spaced transit observations designed to disentangle planetary atmospheres from stellar noise. Each new dataset brings us closer to a definitive answer about whether the habitable-zone planet TRAPPIST-1 e holds an atmosphere at all.
Beyond individual targets, JWST is building something larger: a statistical picture of rocky planet atmospheres. With each new transmission spectrum, the catalogue grows. We are learning not just about specific worlds, but about how common atmospheres are on small planets, the conditions under which they survive, and the chemical inventories they contain. The planets being born in stellar nurseries across the galaxy will one day be studied the same way; JWST is writing the methods.
Further ahead, NASA’s proposed Habitable Worlds Observatory would carry the search forward with a coronagraph designed to directly image Earth-like planets orbiting Sun-like stars, bypassing the transit method entirely. That telescope remains in its concept phase, likely a generation away. For now, JWST is what we have. And it is, by a considerable margin, the most powerful tool humanity has ever pointed at this question.
The light that passed through K2-18 b’s atmosphere left that world 124 years ago. We are only now learning to read what it carried. Whether the answer turns out to be chemistry or biology, the fact that we can ask the question at all, from the surface of one small planet orbiting one ordinary star, is its own kind of extraordinary.
