"Signature of life" detected on another world by the Webb telescope, study suggests

A narrow slice of starlight has raised suspense about the possibility of company in the cosmos. While the James Webb Space Telescope (JWST) watched the exoplanet K2-18b glide across its parent star, sulfur‑bearing gases appeared in the planet’s airborne haze.

Dimethyl sulfide and dimethyl disulfide – molecules tied to biological activity on Earth – leave unmistakable fingerprints in the data.

No telescope had ever hinted at these compounds beyond our neighborhood, so the finding instantly climbed toward the top of every astrobiologist’s wish list.

The signal meets the three‑sigma bar, which means there is only a 0.3 percent chance it popped out of random noise.

Scientists need five sigma – less than 0.00006 percent probability – to declare a discovery. Yet getting this close with a planet 124 light‑years, or roughly 730 trillion miles, away stands out as today’s sharpest clue that life might thrive elsewhere.

Those numbers alone for K2-18b promise a lively debate in laboratories around the globe for months to come. They also focus attention on the interplay between ocean chemistry and atmospheric circulation on watery worlds that orbit small, dim suns.

K2‑18b has room for an ocean

Discovered in 2015, K2‑18b weighs about 8.6 times as much as Earth and spans 2.6 times our planet’s diameter – big enough to keep a thick hydrogen atmosphere yet small enough to support a surface ocean.

The world sits inside the habitable zone of a cool red dwarf, where sunshine could keep water liquid and the hydrogen atmosphere could retain heat.

“We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,” said Professor Nikku Madhusudhan of Cambridge’s Institute of Astronomy, who led the research.

Sulfur signals in stardust

Astronomers tease out atmospheric chemistry by watching a planet dim its star during transit. As K2‑18b moves across the stellar face, a sliver of light filters through the gas envelope, and JWST shows which wavelengths are absorbed.

Earlier passes with its NIRISS and NIRSpec cameras suggested sulfur compounds; the fresh visit used the Mid‑Infrared Instrument, which covers longer wavelengths and overlaps with neither camera.

“This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations,” said Madhusudhan. “The signal came through strong and clear.”

“It was an incredible realization seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests,” added co‑author Måns Holmberg of the Space Telescope Science Institute in Baltimore, USA.

Why dimethyl sulfide matters

On Earth, marine phytoplankton exhale dimethyl sulfide, giving seaside air its faint whiff of cabbage. Levels rarely rise above one part per billion here, yet the telescope sees more than ten parts per million on K2‑18b – thousands of times stronger.

At that concentration, even a thin soup of microbes churning beneath global clouds could recharge the atmosphere in short order.

“Earlier theoretical work had predicted that high levels of sulfur‑based gases like DMS and DMDS are possible on Hycean worlds,” Madhusudhan said.

“And now we’ve observed it, exactly as predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”

K2-18b and the five‑sigma milestone

The team’s analysis already meets the community’s three‑sigma standard, but only five sigma counts as confirmation.

Simulations show that between 16 and 24 hours of additional JWST time should push the statistics over the line, provided the telescope secures enough clear transits in the months ahead.

“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them,” said co‑author Subhajit Sarkar of Cardiff University.

“Our work is the starting point for all the investigations now needed to confirm and understand the implications of these exciting findings,” added co‑author Savvas Constantinou, also of Cambridge’s Institute of Astronomy.

Overcoming skepticism

Researchers remain cautious because exotic photochemistry or interior geologic vents could, in principle, forge the same molecules without biology.

Laboratory experiments, improved climate simulations, and careful comparisons with other exoplanets will probe those alternative pathways while new telescope time gathers stronger data.

“It’s important that we’re deeply skeptical of our own results, because it’s only by testing and testing again that we’ll reach the point where we’re confident in them,” Madhusudhan said. “That’s how science has to work.”

What happens next for K2-18b?

Webb’s sensitive eye was built for this kind of measurement, yet even sharper tools lie on the horizon.

Planned observatories such as the Habitable Worlds Telescope and the European Extremely Large Telescope will study smaller, cooler planets with richer spectra, expanding the growing catalog of possible biospheres and giving scientists the ability to cross‑check signs of life across many worlds.

“Decades from now, we may look back at this point in time and recognize it was when the living universe came within reach,” Madhusudhan said.

“This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”

These sulfur‑scented hints from K2‑18b remind us how quickly the search for life is moving from speculation to evidence.

If the signal holds, K2‑18b will stand as the first concrete clue that life can bloom under unfamiliar skies, turning a distant red dwarf’s faint light into a global ocean’s worth of metabolism.

The full study was published in The Astrophysical Journal Letters.

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