Claims of Biosignature on Exoplanet K2-18b Met with Scientific Skepticism

Skepticism Greets Claims of a Possible Biosignature on Distant Exoplanet K2-18b

This week’s news of a possible biosignature detection on a distant exoplanet has sparked headlines—and an equally swift wave of skepticism from the scientific community. Researchers using data from the James Webb Space Telescope (JWST) have identified spectral features on K2-18b, a sub-Neptune world 120 light-years away, that they tentatively attribute to dimethyl sulfide (DMS)—a chemical produced biologically on Earth.

Although some media outlets quickly jumped to conclusions about alien life, scientists are urging caution, noting that the evidence, while intriguing, is far from conclusive. As excitement mounts, experts highlight key questions that remain unanswered about the planet's composition, the reality of the signal, and its possible non-biological origins.

What’s the New Claim?

The planet at the center of attention, K2-18b, is a sub-Neptune-sized exoplanet roughly 2.5 times Earth's radius and 8.6 times its mass. It orbits within the inner edge of its host star’s habitable zone, receiving enough radiation that—under the right conditions—liquid water could exist.

Researchers used JWST’s powerful instruments to analyze starlight passing through the planet’s atmosphere during transit. By evaluating how certain wavelengths were absorbed, the team inferred the possible presence of various molecules, including methane, carbon dioxide, and notably, dimethyl sulfide or dimethyl disulfide.

On Earth, DMS is exclusively produced by biological processes, primarily marine microorganisms, making it a candidate biosignature. The team cautiously suggested the detection may be “the strongest hint yet” of life on another world—language later echoed by a University of Cambridge press release.

Why Scientists Are Skeptical

Despite the bold headlines, three major uncertainties temper the excitement surrounding this claim:

1. Is K2-18b What We Think It Is?

The team favors a “hycean” world model—a planet with a hydrogen-rich atmosphere and a global ocean beneath it. This scenario helps explain the detected chemical profile. However, the existence of such an ocean depends heavily on factors like cloud cover and atmospheric heat reflection.

JWST data showed no signs of clouds, raising doubts about whether water could remain in a liquid state. In fact, other studies suggest K2-18b might be too hot to support oceans, with competing models proposing it's more likely a gas dwarf or magma-ocean planet—both hostile to life as we know it.

2. Is the DMS Signal Real and Reliable?

The signal’s significance sits at just three sigma—enough to warrant interest, but far from a confirmed detection. Furthermore, that confidence level is measured against a featureless spectrum, and the result is only the best match among 20 molecules analyzed.

Earlier JWST studies hinted at similar spectral features, but a reanalysis of that data found no statistically significant signal. Also troubling is that our understanding of DMS spectra is based on Earth-like conditions, not the extreme temperatures and pressures expected on K2-18b. That uncertainty makes it hard to confidently match the molecule to the signal observed.

3. Could DMS Be Produced Without Life?

Even if DMS is present, it may not indicate biology. A 2024 study suggested that DMS could form through photochemical reactions—non-biological processes triggered by sunlight. While those processes may not produce DMS in large quantities, they introduce a plausible abiotic pathway that cannot be ignored.

A Starting Point, Not a Discovery

Even the researchers behind the study stress the tentative nature of their findings. The team stated that “it’s important that we’re deeply skeptical of our own results,” emphasizing that their work is a starting point for further investigations—not a definitive breakthrough.

In the search for extraterrestrial life, biosignature candidates must overcome multiple hurdles: the planet must be capable of supporting life, the chemical signal must be unambiguous, and no reasonable non-biological explanations must exist. In the case of K2-18b, none of those boxes are fully checked yet.

The Long Road Ahead

As telescope technology advances and data collection improves, biosignature claims will continue to surface. But true discovery will require scientific consensus, rigorous cross-discipline analysis, and time.

“There will be no obvious before-and-after moment of discovery when it comes to finding hints of life,” one astronomer noted. Each new observation is simply one piece in a larger conversation—one that spans astronomy, chemistry, and planetary science.

Until we can answer “yes” to the fundamental questions about habitability, signal clarity, and biological plausibility, cautious optimism must prevail.

Stay tuned to The Horizons Times for deeper insights into space science, exoplanet discoveries, and the evolving quest to find life beyond Earth.