In 2020, scientists detected a gas called phosphine in the atmosphere of an Earth-size rocky planet. Knowing of no way that phosphine could be produced except through biological processes, “the scientists assert that something now alive is the only explanation for the chemical’s source,” the New York Times reported. As “biosignature gases” go, the phosphine seemed like a home run.
Until it wasn’t.
The planet was Venus, and the claim about a potential biosignature in the Venusian sky is still mired in controversy, even years later. Scientists can’t agree on whether phosphine is even present there, let alone whether it would be strong evidence of an alien biosphere on our twin planet.
What turned out to be hard for Venus will only be harder for exoplanets many light-years away.
NASA’s James Webb Space Telescope (JWST), which launched in 2021, has already beamed back data on the atmospheric composition of a midsize exoplanet dubbed K2-18 b that some have interpreted — controversially — as possible evidence of life. But even as hopes for a biosignature detection soar, some scientists are starting to openly ask whether gases in the atmosphere of an exoplanet will ever be convincing evidence for aliens.
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