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.
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.