The Indicators of Alien Life on Exoplanet K2-18b Have Nearly Vanished

Hope for discovering alien life on K2-18B is diminishing. Recent observations have not revealed clear evidence of biomolecules suggested by earlier studies. While many scientists concur that this casts doubt on past claims, one researcher involved in those earlier findings argues that the new data may actually bolster their conclusions.

In April, Nick Madhusudan at Cambridge University and his team proposed that K2-18B, a rocky planet larger than Earth and located around 124 light years away, shows signs of atmospheric molecules dimethyldimethylsulfide (DMS) and dimethyldisulfide (DMD). On Earth, these compounds are exclusively produced by living organisms. At the time, Madhusudhan described these as “the first hints we see about this alien world, possibly a home for life.”

However, subsequent analyses of the same JWST data by other researchers using different statistical methods found no substantial evidence for these molecules. Madhusudhan later stated that his team conducted a more in-depth reanalysis of their data. In an interview with New Scientist, he expressed “increased confidence” that DMS could be the best explanation for the findings. Without further observations of K2-18B, astronomers remain divided on the potential for life on the planet.

Recently, Renyu Hu from Caltech and his colleagues collaborated with Madhusudhan’s team to examine new JWST observations of K2-18B. They found no statistical evidence supporting the detection of DMS. “This paper does not present conclusive evidence regarding the presence of this molecule in the atmosphere,” remarked Hu.

Madhusudhan, Hu, and their teams utilized JWST’s near-infrared camera to observe the light from K2-18B’s star. This camera captured light at wavelengths differing from the mid-infrared measurements used in the earlier analysis conducted in April. The researchers then assessed their findings using various atmospheric models for K2-18B. Each model had different assumptions regarding the molecular composition and the presence of water vapor in its atmosphere.

Some models incorporating DMS provided slightly better explanations for the data than those without, but this does not necessarily indicate a firm detection, falling short of the threshold for statistical evidence. “The models suggest that if there is a signal, it is quite weak,” stated Hu. “I remain cautious.”

Madhusudhan acknowledged the lack of strong evidence for detection, but contended that it is more reasonable to compare this data with recent observations from JWST’s near-infrared cameras taken in 2023, as opposed to April’s mid-infrared results. “Statistically, the data does seem to show slightly stronger evidence for DMS than what we discussed in our paper,” said Madhusudhan.

“There could still be other molecules mimicking DMS,” he added, but he believes that signals produced by unknown compounds exhibit specific traits best accounted for by DMS. “However, we cannot make confident assertions.”

“This study clearly states that there is no evidence of dimethyl sulfide. No statistical support exists for these gases,” stated Lewis Wellbanks of Arizona State University. Sarah Seager from MIT remarked that her team views the statistical significance presented by the researchers as “not enough to be considered a detection.”

“It seems we may be nearing the end of the discussion regarding whether DMS can be detected at viable levels. [K2-18b] conditions are not conducive enough for more critical detection,” remarked Jake Taylor from Oxford University.

In a further setback for biosignature proponents, Hu and his team discovered that specific hydrogen-rich atmospheres in planets like K2-18b can generate DMS through chemical pathways devoid of biological influence. “This helps narrow down which molecules may serve as biosignatures in exoplanetary atmospheres, implying that DMS is not exclusively indicative of life,” noted Taylor.

However, he emphasized that additional mid-infrared observations with JWST, akin to those undertaken in April, could yield more specific data, as DMS characteristics and other intricate molecules target distinct light regions that are detectable.

What is widely accepted by astronomers is that K2-18B is rich in water. Hu and his team have gathered robust evidence for the existence of methane and carbon dioxide, suggesting the presence of water. However, it remains unclear whether this water exists as oceans, atmospheric vapor, or is confined inside the planet.

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