Last month, astronomers announced what they believed were the most promising hints of alien life yet on a distant planet, sparking hopes that humanity might soon discover we are not alone in the universe. However, several recent studies re-examining the same data have concluded that insufficient evidence supports such ambitious claims, with one scientist even accusing the initial astronomers of “jumping the gun.”
The core of this debate centers around the exoplanet K2-18b, located 124 light-years away in the Leo constellation. This planet is considered to be within the habitable zone of its star, making it a prime candidate in the search for extraterrestrial life due to the potential for liquid water. In April, astronomers utilizing the James Webb Space Telescope made headlines by reporting the detection of traces of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the planet’s atmosphere.
These chemicals are primarily produced by living organisms on Earth, such as marine algae, leading them to be considered potential “biosignatures” indicative of life. The astronomers, led by Nikku Madhusudhan of Cambridge University, had expressed caution regarding these “hints” of a biosignature, emphasizing that they were not claiming a definitive discovery. Madhusudhan noted at the time that their detection had achieved a three-sigma level of statistical significance, meaning “there is still a three in 1,000 chance of this being a fluke.”
Biosignatures ‘Vanish’ Under Scrutiny
Two of Madhusudhan’s former students, Luis Welbanks of Arizona State University and Matthew Nixon of Maryland University, are among the researchers who have since re-analyzed the data behind the initial announcement. According to their preprint study published online late last month, when other statistical models were applied, “claims of a potential biosignature detection vanish.” Like other papers published since the April announcement, this study has not yet undergone peer review.
In one model, Welbanks and colleagues expanded the number of possible chemicals that could explain the signals detected by Webb from the original 20 to 90. Welbanks informed AFP that over 50 received a “hit.” He questioned, “When you detect everything, did you really detect anything?” Welbanks clarified that they are not asserting the planet definitively lacks DMS, but rather that more observations are necessary.
‘Arguments Are Healthy’ for Scientific Progress
Madhusudhan welcomed the robust debate, asserting that maintaining an open mind to all possibilities is a fundamental aspect of the scientific method. “These sort of arguments are healthy,” he told AFP. His team even went a step further, releasing their own preprint study last week that expanded the number of chemicals considered to an even broader 650. The three most “promising” chemicals they identified included DMS, but notably, not DMDS, which was a significant component of the team’s initial announcement in April.
The other two chemicals were diethyl sulfide and methyl acrylonitrile, the latter of which is toxic. Madhusudhan conceded that these lesser-known chemicals are likely not “realistic molecules” for a planet like K2-18b. Welbanks pointed out that “in the span of a month — with no new data, with no new models, with no new laboratory data — their entire analysis changed.”
‘Closest We Have Ever Been’ to Discovery
Telescopes observe such distant exoplanets as they transit their host star, allowing astronomers to analyze how molecules in their atmosphere block different wavelengths of light. Earlier this week, a paper led by Rafael Luque at the University of Chicago combined Webb’s observations of K2-18b in both near-infrared and mid-infrared wavelengths. This paper also found “no statistical significance for DMS or DMDS.” An earlier paper by Oxford astrophysicist Jake Taylor, employing a basic statistical test, also found no strong evidence for any biosignatures.
Madhusudhan dismissed Taylor’s paper, arguing that its simple exercise did not account for observing physical phenomena. He also stood by his own research, stating he was “just as confident” in the work as he was a month ago. Madhusudhan added that more data about K2-18b will become available over the next year, which should provide a much clearer picture.
Even if the planet does possess DMS, it is not a definitive guarantee of life—the chemical has been detected on a lifeless asteroid. However, many researchers believe that space telescopes could one day gather sufficient evidence to confidently identify alien life from afar. “We are the closest we have ever been” to such a moment, Welbanks affirmed. “But we have to use the frameworks that are in place and build up (evidence) in a reliable method, rather than using non-standard practices and jumping the gun — as has been done in this particular case,” Nixon added.
