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Venus is often called Earth’s sister planet, a neighboring twin of similar density and size. But the resemblance stops there. As the hottest planet in our solar system, the choking Venusian atmosphere is full of heat-trapping carbon dioxide and clouds thick with sulfuric acid that shroud its dry, volcanic terrain.

So it’s one of the last places anyone might think to look for life beyond our planet.

That’s why it came as such a shock last September when a group of scientists, led by Jane Greaves of Cardiff University, announced that they’d found a possible sign of alien life in the Venusian atmosphere. In the study, published in Nature Astronomy, they reported the detection of a colorless, toxic gas called phosphine in the planet’s clouds and concluded that no known chemical or geological processes could explain its presence. Phosphine could indicate life, they argued, noting recent work by astrophysicist Clara Sousa-Silva of MIT who suggests the gas could be a biosignature. On Earth, phosphine is often found in places that host anaerobic life, including lakes, marshes, paddy fields and in the sludge of landfills.

But when the news reached Jonathan Lunine, an astronomer at Cornell University, he and graduate student Ngoc Truong were immediately skeptical. “It’s problematic to invoke phosphine as a biosignature on Venus, simply because the environment on Venus is totally different from the environment on Earth,” says Truong. Even on our own planet, he says, there is some confusion as to whether phosphine is associated with life, and he believes that this should be confirmed before extrapolating these observations to environments so unlike our own. 

Truong and Lunine weren’t alone in their doubt: After the phosphine announcement, the Internet exploded with discussions about the discovery. Scientists weighed in on Twitter threads, argued on Facebook posts, and flocked to, a preprint server for scientific research, to lay out other theories for what non-biological processes might be producing the phosphine.

Truong, who until that point had been studying the oceans on Saturn’s moons, convinced Lunine that they needed to further explore one potential source of phosphine in particular: volcanoes. Their research culminated in a new study published Monday in the journal Proceedings of the National Academy of Sciences. In it, Truong and Lunine paint a picture of how phosphine might make it into Venus’ atmosphere. Trace amounts of phosphides (negatively charged phosphorus ions attached to metals like iron) found deep in the mantle of Venus could be pulled up to the surface by volcanic activity. When the volcanoes erupt, these phosphides could be thrust into the atmosphere and chemically react with sulfuric acid in the clouds to form phosphine.

“Our study only suggests a roadmap to assessing the level of volcanic eruptions” on Venus, Truong says. Two conditions are needed for this to be a viable explanation. First, the planet must be volcanically active. (While thousands of volcanoes have been spotted in radar images of Venus, scientists lack the data to confirm recent eruptions, since so far, landers can only withstand the raging heat and crushing pressure of the Venusian surface for about an hour.) “And not just active in the sense of ‘Hawaiian-style volcanism,’” Lunine says, which typically produces lava flows without much explosivity. Explosive volcanism is key, because there needs to be a mechanism for the phosphides to be ejected into the atmosphere.

Second, scientists would need to verify that the phosphine is actually there—and that’s currently a huge point of contention. Without this proof, Lunine says, the volcano theory “becomes an empty postulate rather than a hypothesis.”