Look alive: The James Webb Space Telescope (JWST) may have picked up signs of a potential biosignature on a steamy, ocean-covered exoplanet called K2-18b—a biosignature that, on Earth, is produced by marine life.

The main character here is dimethyl sulfide, a molecule produced by many ocean denizens, but especially plankton. If the molecule is really floating around in the atmosphere of K2-18b, it raises the tantalizing possibility that something on the world might be alive. Or at least emitting suspiciously life-like chemical signals.

K2-18b, located 120 light-years away, has been on scientists’ radar since NASA’s Kepler space telescope spotted it in 2015. It’s about 8.6 times the mass of Earth and orbits within the habitable (or “Goldilocks”) zone of a red dwarf star.

Earlier observations from Hubble hinted that K2-18b had water vapor in its atmosphere, a claim later shown to be in error. But JWST has taken matters several steps further, doubling down on an earlier finding of dimethyl sulfide in the planet’s atmosphere. The team behind the discovery, led by Nikku Madhusudhan from the University of Cambridge, includes researchers from five institutions.

The finding suggests that K2-18b may indeed be a Hycean world, or a water-covered planet with a hydrogen-rich atmosphere. But the team’s observations—made using JWST’s Mid-Infrared Instrument (MIRI) and earlier data from NIRISS and NIRSpec—revealed methane and carbon dioxide, but very little ammonia, on the planet.

The curious mix suggests K2-18b hosts a watery, potentially habitable environment. Crucially, the team also found more evidence of dimethyl sulfide, along with a related molecule, dimethyl disulfide. The team’s work repeats the 2023 detection and adds further credence to the possibility of life on the relatively nearby exoplanet—assuming the findings are sound and that dimethyl sulfide is produced and behaves on exoplanets the same way it does on Earth. The team’s findings are published in The Astrophysical Journal Letters.

Speaking to the BBC, Madhusudhan said his team detected a surprising amount of gas during the single observation. “The amount we estimate of this gas in the atmosphere is thousands of times higher than what we have on Earth,” he explained, saying that, if the link to life is valid, “then this planet will be teeming with life.” And should scientists confirm the presence of life on K2-18b, “it should basically confirm that life is very common in the galaxy,” Madhusudhan told the British broadcaster.

In its landmark decadal survey on astronomy and astrophysics, the National Academies made one thing clear: finding habitable worlds should be a top scientific priority. The James Webb Space Telescope is front and center in that mission—and while NASA already has plans for its eventual successor, the Habitable Worlds Observatory, that next-gen telescope won’t launch for at least another decade. Until then, it’s up to Webb—and the ever-reliable Hubble—to carry the torch in our search for life beyond Earth.

The detection isn’t a done deal yet—it comes with a statistical confidence of around 3-sigma (about 99.73%), which makes it interesting, but not definitive. A 5-sigma detection (roughly 99.99994% confidence) is typically the gold standard for confirming a discovery. And even at 5-sigma, that would merely confirm the presence of dimethyl sulfide in the planet’s atmosphere—not that the dimethyl sulfide has biotic origins. There’s still a chance that abiotic (non-biological) processes or instrumental quirks could be responsible. This past Sunday, a separate team posted a paper on the preprint server arXiv suggesting the planet may not be Hycean at all, but instead a rocky world covered in magma, with hydrogen-rich skies—and virtually no chance of life.

Further observations will help to validate the team’s findings. But to be clear, if life does exist on K2-18b, it’s likely microbial given the apparent evidence, and not a sign of alien intelligence. As an important aside, microbial life—like plankton—existed on Earth for a billion years, a long but crucial chapter that paved the way for more complex organisms to emerge. Regardless, life has never been found beyond Earth, so confirming even a single amoeba on a distant world would be nothing short of revelatory.

At minimum, K2-18b is shaping up to be one of the most promising places to search for life beyond Earth. And at maximum—if further studies validate the recent findings—we may be getting our first chemical whiffs of a living ocean on another world.