A telescope stares at a planet 124 light-years away and sees gases in its atmosphere that on Earth are produced by microbes. The immediate thought: life. But it's not that simple.
Planet K2-18b is a sphere wrapped in a thick layer of hydrogen. In such a layer, like in a giant chemical flask, reactions take place. Two sulfur-smelling gases, dimethyl sulfide and dimethyl disulfide, can form there without any bacteria, simply from starlight and heat. The authors tested previously proposed pathways for their formation and found that one pathway does yield a noticeable amount of these gases. But it only works if one step in the reaction chain is crossed easily enough. And no one has yet measured the height of that step in the lab.
Much more reliably, hydrocarbons (ethane, for example) form in such an atmosphere. Through a telescope, ethane looks almost identical to the sulfur gases and could fool an observer. So the signal that was taken as a sign of life is most likely ethane.
Another finding: hydrogen sulfide, under the action of light, gives rise to haze. Even a tiny trace of hydrogen sulfide is enough to shroud the atmosphere in a veil. Different planets have varying amounts of this haze, and now we understand why: it all comes down to how much sulfur is in their air.
So K2-18b remains a chemical laboratory, not a home for life.
as written by the authors arXiv:2603.18923
The organosulfur biosignature gases dimethylsulfide (DMS) and dimethlydisulfide (DMDS) have recently been claimed to be present in the atmosphere of sub-Neptune exoplanet K2-18b, leading to the suggestion of possible extraterrestrial life. Abiotic formation pathways for DMS and DMDS in reducing atmospheres have also been proposed, raising concern over the use of DMS and DMDS as biosignature gases more generally. In this paper we independently test and contrast the proposed abiotic formation pathways for DMS and DMDS using K2-18b as a case study, and explore the wider implications for the atmospheric carbon and sulfur chemistry of hydrogen-rich sub-Neptunes. We demonstrate that one proposed formation pathway is capable of producing observable abundances of abiotic DMS and DMDS, however it depends sensitively on the energy barrier of the limiting step, which remains unmeasured experimentally. The formation of hydrocarbons including C2H6, however, occurs abundantly and offers a plausible alternative explanation to the reported suggestions of organosulfur compounds on K2-18b, having previously been shown to share similar spectral features with DMS and DMDS at near-IR wavelengths. Finally, we demonstrate that sulfur hazes form via the photochemistry of H2S and condense in the atmosphere of K2-18b even at trace abundances. We propose that variation in atmospheric sulfur abundance can explain the diversity of haziness observed across the sub-Neptune population so far with JWST.
Hydrogen sulfide smells like rotten eggs, while dimethyl sulfide smells like the sea: it's the scent we catch at the ocean shore.
In Stanisław Lem's novel "Solaris," the planet's ocean itself creates strange structures — scientists still debate whether that counts as life.