Last night, a group of astronomers from the University of Cambridge announced a potentially groundbreaking discovery regarding signs of life beyond our Solar System. They’ve identified certain atmospheric molecules on the distant planet K2-18 b, which they believe might be produced by living organisms.

However, this announcement has been met with considerable skepticism from the scientific community that examines biosignatures in exoplanet atmospheres.

“It’s not compelling evidence,” comments Stephen Schmidt, an astronomer at Johns Hopkins University in Baltimore. “It’s almost certainly not life,” adds Tessa Fisher, an astrobiologist at the University of Arizona in Tucson.

Supporting Science Journalism

If you find this article engaging, please consider supporting our award-winning journalism by subscribing. Your subscription helps sustain future reporting on significant discoveries and ideas that are shaping our understanding of the world.

Here, Nature delves into the high-profile claim—and explains why many experts are cautious about considering it proof of alien life.

Discoveries on K2-18 b

The Cambridge research team utilized the James Webb Space Telescope (JWST) to detect traces of dimethyl sulfide (DMS), a compound with a distinctive odor often produced by bacteria, in the atmosphere of K2-18 b—a planet smaller than Neptune, located about 38 parsecs from Earth. Their analysis involved studying how starlight changes as it passes through the planet’s atmosphere, revealing possible signs of DMS or a related compound, dimethyl disulfide (DMDS). These substances are notable because they are generated by life forms like marine phytoplankton on Earth.

In their 2023 findings, the team observed these molecules under different wavelengths, providing a clearer and more reliable indication of their presence.

This capability to analyze the chemical makeup of distant planets represents a significant advancement in exoplanet research, according to team leader Nikku Madhusudhan, an astronomer at Cambridge. He shared these insights during a live-streamed colloquium on April 17 but was unavailable for an interview prior to this article’s publication.

The Significance of These Findings

For centuries, scientists have pursued the question of life beyond Earth. If indeed DMS and DMDS are found on K2-18 b and are products of biological processes, it could mark a historical breakthrough in the quest for extraterrestrial life.

Additionally, understanding planets like K2-18 b, which are among the most abundant types discovered in the universe so far, could deepen our knowledge of potentially habitable environments. Madhusudhan’s team speculates that some of these planets might be water worlds enveloped in hydrogen-rich atmospheres, making them prime candidates for hosting life.

Skepticism Among Scientists

The debate begins with the basic conditions on K2-18 b, such as the presence of water or a solid surface—factors that are essential for life as we know it. Studies suggest that the planet is likely inhospitable. “The most straightforward explanation is that it’s a barren mini-Neptune,” states Joshua Krissansen-Totton, a planetary scientist at the University of Washington in Seattle.

Moreover, the validity of the DMS/DMDS detection itself is under question. “The measurements really test the limits of JWST’s capabilities,” notes Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany. Schmidt and his team reevaluated the 2023 findings and found no conclusive evidence of the molecules. They describe the signal as “quite noisy,” which could mean that the detected features might just be random statistical variations. Despite this, the Cambridge researchers maintain that there’s only a 0.3% chance the signals are due to random chance.

Even if the signals are genuine, numerous questions must be addressed before attributing them to biological sources. For example, lab studies have shown that DMS can be synthesized through non-biological processes. “Our understanding of these planetary atmospheres is still very limited,” explains Eleanor Browne, a chemist at the University of Colorado Boulder who led recent studies on the topic. It’s also noted that DMS has been found on a comet surveyed by the European Space Agency, which certainly harbors no life.

“Context is key,” asserts Edward Schweiterman, an astrobiologist at the University of California Riverside. If these molecules are indeed present, he says, “we need to think creatively about how they could be abundantly produced by non-biological processes and evaluate these possibilities before claiming them as evidence of life.”

Next Steps in Research

Madhusudhan and his team hope to secure additional observation time with JWST to confirm the significance of their findings. “Ideally, you’d want confirmation from several independent groups,” says Schweiterman.

Regardless of the outcome, this research underscores the importance of studying planets like K2-18 b, according to Kreidberg, who emphasizes, “This planet offers a rich environment for learning about planetary atmospheres.”

This article is reproduced with permission and was originally published on April 17, 2025.