The James Webb Space Telescope has done it again. In a paper published in The Astrophysical Journal Letters, an international team of astronomers reports the detection of dimethyl sulphide (DMS) in the atmosphere of K2-18b, a sub-Neptune exoplanet 120 light-years from Earth. On Earth, DMS is produced exclusively by marine phytoplankton. No known abiotic process produces it in significant quantities.

The finding is preliminary, the signal is at the edge of statistical significance, and the authors are careful to note that it does not constitute evidence of life. But it is the most tantalising hint yet that JWST — the $10 billion telescope launched on Christmas Day 2021 — may be capable of detecting biosignatures in exoplanet atmospheres.

What Is K2-18b?

K2-18b orbits a red dwarf star in the habitable zone — the region where liquid water can exist on a planetary surface. It was first detected by the Kepler space telescope in 2015 and has been one of the most studied exoplanets ever since. It is about 8.6 times the mass of Earth and 2.6 times its radius, placing it in a class of planets called "sub-Neptunes" or "Hycean worlds" — planets that may have hydrogen-rich atmospheres overlying vast liquid water oceans.

In 2023, JWST detected carbon dioxide and methane in K2-18b's atmosphere — a combination that, on Earth, would be rapidly destroyed by chemical reactions unless continuously replenished by biological activity. The new DMS detection adds to a growing list of atmospheric anomalies that are difficult to explain without invoking biology.

The Caveats

The scientific community is appropriately cautious. The DMS signal has a statistical significance of 3.2 sigma — above the conventional threshold for a "detection" in astronomy (3 sigma) but well below the 5 sigma standard used in particle physics for a "discovery." There is approximately a 0.1% chance the signal is a statistical fluctuation.

More importantly, K2-18b is a very different planet from Earth, and our understanding of the chemistry of Hycean world atmospheres is limited. It is possible — though currently unexplained — that some abiotic process on K2-18b produces DMS. "We need to be very careful about assuming that what's true on Earth is true everywhere," said Dr. Nikku Madhusudhan of Cambridge University, the lead author of the paper. "But we also need to take this signal seriously."

What Comes Next

The team has been awarded 50 additional hours of JWST observation time to confirm or refute the DMS detection. Those observations, scheduled for late 2026, will use a different instrument configuration to cross-check the signal. If the DMS detection is confirmed at higher significance, it will trigger a global scientific response — and a public conversation about the implications that will dwarf anything in the history of astronomy.

JWST has fundamentally changed the field of exoplanet atmospheric science. Before its launch, detecting molecules in the atmospheres of small rocky or sub-Neptune planets was essentially impossible. Now it is routine. The telescope has already characterised the atmospheres of dozens of exoplanets, and its sensitivity is improving as the science team refines its data reduction techniques.

The Bigger Question

The detection of life beyond Earth — even microbial life in a distant ocean — would be the most significant scientific discovery in human history. It would tell us that life is not a fluke, that the universe is almost certainly teeming with it, and that we are not alone. The fact that we are now, for the first time, building instruments sensitive enough to ask this question observationally is one of the most remarkable developments in the history of science.

We may not have the answer yet. But we are, finally, asking the right question with the right tools.

The Implications for the Search for Life

The K2-18b findings have reinvigorated the debate about how to prioritise the search for extraterrestrial life. The traditional focus has been on Mars and the icy moons of our solar system — places where we can send spacecraft and collect samples. JWST has opened a new front: atmospheric biosignature detection on exoplanets hundreds of light-years away. We cannot visit these worlds, but we can read their atmospheric chemistry from Earth.

The next generation of ground-based telescopes — the Extremely Large Telescope (ELT) under construction in Chile, and the Thirty Meter Telescope (TMT) planned for Hawaii — will have the resolution to characterise the atmospheres of Earth-sized planets in the habitable zones of nearby stars. If JWST's K2-18b findings are confirmed, the ELT and TMT will be the instruments that either find definitive biosignatures or rule them out. The 2030s may be the decade when humanity finally answers the question it has been asking since it first looked up at the stars.

Further Reading