Astronomers using the James Webb Space Telescope have detected chemical traces in the atmosphere of a distant exoplanet that may point to the presence of microbial life. The discovery centres on K2-18b, a planet located 124 light-years from Earth, and could mark a major step in the search for extraterrestrial biology.
The research, led by Professor Nikku Madhusudhan at the University of Cambridge’s Institute of Astronomy, identified molecules known as dimethyl sulfide (DMS) and dimethyl disulfide (DMDS).
On Earth, both gases are only produced by marine microorganisms such as phytoplankton. Their detection on another planet has led scientists to consider the possibility of biological activity beyond Earth.
The findings were published in The Astrophysical Journal Letters. The Cambridge team used Webb’s Mid-Infrared Instrument to confirm earlier suspicions, following an initial signal detected last year using a different set of instruments. The presence of carbon dioxide and methane had already been observed in the planet’s atmosphere.
The latest analysis suggests the concentrations of DMS and DMDS could be thousands of times higher than those found in Earth’s atmosphere. “If the association with life is real, then this planet will be teeming with life,” Madhusudhan said.
K2-18b is believed to be a “Hycean” world, a term coined by Madhusudhan’s team to describe planets with global oceans beneath hydrogen-rich atmospheres. The planet orbits a small red dwarf star within the habitable zone, where temperatures allow for liquid water.
The telescope observed light from the host star as it passed through the planet’s atmosphere, picking up faint chemical signatures. This method, known as transmission spectroscopy, allows researchers to identify gases based on how they absorb specific wavelengths of light.
Although the signals for DMS and DMDS were detected independently and with strong intensity, the research team has not claimed definitive evidence of life. The current analysis has reached a statistical confidence of three sigma, equivalent to a 99.7% certainty. Scientific convention requires a five-sigma threshold, or a 99.99994% probability, before a discovery is accepted as definitive.
The findings have prompted both excitement and caution. Professor Catherine Heymans, Astronomer Royal for Scotland and an astrophysicist at the University of Edinburgh, said that the origin of the detected gases remains uncertain. “Even with perfect data we can't say for sure that this is of a biological origin,” she told the BBC.
Other scientists not involved in the study have raised alternative explanations. Some researchers suggest the gases may arise from unknown geological or chemical processes unrelated to life. Others question the nature of K2-18b itself. Proposed models describe it as a mini-Neptune, a Hycean world, or even a molten rocky planet, each scenario implying different atmospheric and surface conditions.
Dr Subhajit Sarkar, part of the Cambridge research group, noted that the absence of ammonia supports the ocean hypothesis, as ammonia could be absorbed by a deep liquid body. But other interpretations remain under discussion.
Independent research teams are now expected to reanalyse the data. The Cambridge group plans to release its full dataset to facilitate external verification. Madhusudhan said that with additional telescope time — estimated between 16 and 24 hours — they could achieve the level of statistical certainty required for a discovery.
Dr Eddie Schwieterman, an astrobiologist at the University of California, Riverside, said he found the results “intriguing” but noted that confirming the presence of DMS would require further testing. His own models suggest that if DMS is present in the quantities proposed, it would typically be accompanied by ethane, a gas that has not been detected.
Others, such as Dr Nicolas Wogan at NASA’s Ames Research Center, suggest K2-18b may have no solid surface at all and could be a small gas giant. This would significantly reduce the likelihood of any form of life as currently understood.
The debate underscores the challenges of interpreting remote signals in planetary atmospheres. The data gathered by Webb is highly sensitive but limited to spectral imprints, meaning scientists must infer a planet’s properties through indirect observation.
Since the launch of Webb, researchers have targeted planets that lie in the habitable zones of their stars. The detection of potential biosignatures on K2-18b is the strongest result so far in that effort, but it is far from conclusive.
Madhusudhan acknowledged the limitations of the findings. “We want to be really thorough, and make more observations,” he said. “It’s a big claim if true, so we have to be careful.”
The research team estimates that confirmation of the detected signals could be achieved within one to two years. Until then, K2-18b remains a candidate for life — one that represents both a significant step in scientific progress and a reminder of the uncertainties that come with exploring distant worlds.
HT
