Astronomers at the University of Cambridge have detected chemical traces in the atmosphere of a distant planet — signs that, on Earth, are produced solely by living organisms. The findings don’t confirm the existence of alien life, but they offer the strongest evidence yet that the exoplanet K2-18b may have the right conditions to support biology.
Using data from National Aeronautics and Space Administration’s (NASA) James Webb Space Telescope (JWST), the researchers found compelling evidence of two sulphides in the atmosphere of exoplanet K2-18b. The findings have been published in the journal The Astrophysical Journal Letters.
On Earth, these two sulphides — dimethyl sulphide (DMS) and and its chemical cousin, dimethyl disulphide (DMDS) — don’t last long in the atmosphere without being constantly replenished. Seeing them at such high levels around K2-18 b is very difficult to explain without some form of life, or at least something we don’t yet understand.
K2-18 b is a planet 124 light-years away, slightly bigger and heavier than Earth. The exoplanet orbits a cool red dwarf star in just 33 days. The planet is more than twice the size of Earth and about 8.6 times as massive, placing it in a category known as “Hycean” worlds — planets that may host liquid oceans beneath hydrogen-rich atmospheres.
What did JWST find?
The study was built on earlier JWST observations from 2023, which identified methane and carbon dioxide in the planet’s atmosphere. This latest study offers a second, independent line of evidence for DMS and DMDS using JWST’s Mid-Infrared Instrument (MIRI), which operates in a different wavelength range from previous observations.
Methane and carbon dioxide are consistent with conditions that could support life. The 2023 research also hinted at the possible presence of DMS.
The new data supported the initial suspicion: The signal was “strong and clear” and consistent with the presence of DMS or DMDS, or both. While the findings have reached a “three-sigma” level of statistical significance — meaning there is only a 0.3 per cent chance they occurred randomly — scientists say further data are required to reach the gold standard of five-sigma confidence.
Abiotic chemistry (non-life processes) could possibly produce these gases, but not at the levels detected — unless something strange is happening.
Lead author Nikku Madhusudhan urged caution in a press statement. “It’s important that we’re deeply sceptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them,” he said.
DMS and DMDS concentrations in K2-18b’s atmosphere differ greatly from those on Earth, which are typically less than one part per billion by volume. They are estimated to be thousands of times stronger on K2-18b, with concentrations exceeding ten parts per million.
Such levels, scientists say, are hard to explain without biological activity, although they cannot yet rule out unknown chemical processes. “Either we are looking at a new chemical process that we haven’t seen before… or we’re witnessing the first signs of biological activity outside of Earth,” study co-author Måns Holmberg, researcher at Space Telescope Science Institute in Baltimore, told American daily The Washington Post.
DMS and DMDS both leave distinct fingerprints in the light from a planet’s atmosphere as it passes in front of its star, a process known as a transit. JWST can detect tiny changes in the starlight during such events, which scientists analyse to identify gases in the atmosphere .
NASA also remained cautious. In a statement quoted by The Washington Post, the agency said that detecting gases produced only by life is difficult with Webb and requires long observation times. Even then, “detection of a single potential biosignature would not constitute discovery of life,” it said .
The team believes further observations — between 16 and 24 hours more — could help reach the statistical threshold needed to strengthen their claim.
