Jonathan Lunine is the David C. Duncan Professor in the Physical Sciences and chair of the Department of Astronomy at Cornell University.

Is there life on Venus, Earth’s nearest planetary neighbor? This may seem like an absurd question, given that the surface temperature of Venus is above the melting point of lead. But that has not prevented 50 years of speculation that life might exist in the cool middle atmosphere of the planet, where a thick layer of sulfuric-acid droplets might provide a home. The discovery of phosphine in Venus’s atmosphere, announced in a study published on Monday, could be telling us those ideas are right.

In 1967, the yet-to-be-famous Carl Sagan wrote that life might exist in the Venusian clouds. Collaborating with biologist Harold Morowitz, the two posited that life might have evolved to live in the liquid sulfuric acid cloud layer of Venus, where temperatures are Earth-like and small amounts of water would dissolve. That would be the beginning of many papers on this possibility, culminating in a detailed study this year on how microbes might surf waves in the atmosphere. MIT professor Sara Seager and colleagues imagined a cycle where microbes live in water-sulfuric acid droplets that would grow and then sink to the hotter lower atmosphere, only to evaporate. Freed from the evaporating droplets, the microbes would ride buoyancy waves back to the cooler upper atmosphere to burrow again into the relative safety of the sulfuric-acid and water droplets.

How would we know such organisms might exist? Many chemical compounds that simple microbes produce are also made by non-biological processes. But one, phosphine or PH3, is difficult to produce on Earth abiotically (without life) and, as argued by Seager and her colleagues in another paper, could be a good “biosignature” or sign of life on planets around other stars. This isn’t always the case: The compound is found in the dense hydrogen-rich atmospheres of Jupiter and Saturn, where it is understood to be an abiotic product of simple chemistry, and will likely be found on gas giants around other stars using the James Webb Space Telescope, planned for launch next year. But Venus — which has an atmosphere in which hydrogen is extremely scarce — is a place where phosphine is a plausible biosignature.

The detection of sufficient quantities of phosphine in Venus’s atmosphere would be an intriguing pointer to the possibility of life in the sulfuric-acid clouds of our sister planet, but many questions would remain. Is it possible that planetary chemists have overlooked ways to produce phosphine on Venus in the absence of life? And if phosphine is produced by biology, where did that life originate? It is one thing to imagine life adapting to and hanging out opportunistically in the clouds of Venus. It is quite another to imagine that life could have originated there, sandwiched between the hell of the surface and the frozen realms of the thin upper atmosphere.

The high proportion of heavy (deuterium-rich) water in Venus’s atmosphere is strong evidence that the planet once had a large amount of surface water — literally an ocean. That ocean was lost by evaporation as the sun brightened, driving large amounts of carbon dioxide into the atmosphere and creating the super-greenhouse planet we see today. As the ocean was lost, microbial marine organisms might have been able to seek refuge in the clouds. Another possibility is that microbes could have been delivered in rocky material blasted off the surface of Earth or Mars (if it had life) in large impacts which we know were common during the early history of our solar system.

A third possible source of life would be bad news: contamination from space probes. In the 1970s and 1980s, multiple probes from the Soviet Union and the United States entered the Venus atmosphere and landed on the surface. One set of Soviet probes even deployed balloons that floated in the stratosphere. If these missions injected our own microbes into the clouds, it seems highly unlikely those bugs would survive the acidic conditions in the clouds’ droplets.

The detection of phosphine in Venus is so provocative that it calls for follow-up observations to confirm its presence, clever models to either rule in or rule out abiotic chemistry and, ultimately, new missions if we conclude life might be present in the clouds of Venus. Sampling those clouds would be challenging, but that can be balanced against the short trip times to Venus, which are measured in months rather than the many years it would take to more distant targets.

Venus may hold the answers to questions about the long-term livability of our own home planet: How do planetary climates change from habitable to hellish? What is the fate of Earth’s climate as our sun continues to brighten? We surely want to get a hint of how robust or fragile our home planet’s climate is from our sister planet next door. But if it turns out that Venus has its own native biota, it would radically change our perspective on what makes a planet habitable and cause us to wonder how many other exotic locales in our wildly surprising universe might be home to life.

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