Did astronomers really find a sign of life in the clouds of Venus?

Not yet, and the reasons show how hard it is to call anything a sign of life. In September 2020, astronomers reported phosphine in Venus's atmosphere, detected with the ALMA telescope. It mattered because phosphine is associated with anaerobic ecosystems on Earth, and as of 2019 no known abiotic process generates it in appreciable quantities on a rocky planet. Venus's surface, nearly 735 K at 92 times Earth's pressure, rules out water-based life, but a few scientists have speculated that acid-loving microbes might drift in the temperate upper clouds.

Then the claim came apart in pieces. A review found background calibration errors producing spurious lines, including the phosphine feature, and re-analysis found either no phosphine or about 1 ppb, 20 times below the original estimate. A January 2021 study suggested the line came from sulfur dioxide, a claim refuted in April 2021. Other observations disagree: a reanalysis of 2015 infrared data found none, SOFIA failed to detect it in 2022, while the JCMT-Venus programme reported corroboration in 2024.

For biosignatures the bar is high: since 2023 the community has agreed that a biosignature can only be explained by biological processes, with all plausible abiotic explanations eliminated. Phosphine on Venus has not cleared that bar, since the detection itself is still disputed, and a volcanic origin has been proposed. The question stays open until a probe tastes the air.