Astronomer Jane Greaves of Cardiff University was alone in her office when she saw the signal: a fingerprint from the molecule phosphine, hanging out in data she’d taken of Venus’ atmosphere back in 2017.
You’ve probably never heard of phosphine, and so don’t know why its existence on a nearby planet would stun an astronomer, as it did Greaves. But some scientists think phosphine—a humble pyramid of three hydrogen atoms bonded to a phosphorus—may be a useful biosignature: a sign, if you see it on a solid-surface terrestrial planet, that life might live there.
It was possible, though unconfirmed, that Greaves had just caught the first glimpse of alien beings. She wandered around in a daze.
The hour was late and everyone else had gone home. “There wasn’t really anyone to tell,” she says. Cautious of jumping to conclusions, she stifled her excitement and commanded herself to do normal things, to focus on the business of living. So she left work and went to the grocery store. “Must find food, must do something sensible, must not crash the car,” she recalls telling herself on that evening in late 2018.
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“Being British, I had to buy the ingredients for curry,” she adds. A celebration.
She spent the next few days checking that she hadn’t made a mistake. The signal, which sure looked like phosphine, persisted. And today, after many more months of data gathering and analysis, she and a team of colleagues made the official announcement: There appears to be phosphine on Venus, and so far they’ve found no explanation for why it might be there—except as a result of Venusian life.
That doesn’t mean there is life on Venus. Some nonbiological process unknown on Earth might have churned the molecule into existence. Humans have, after all, cried “Aliens!” because of suspicious chemistry before. But while there’s much followup work to be done—many complications, confirmations, or denials that could and probably will come—it’s also possible today’s the day humans are introduced to the ultimate other.
On its hot, horrid face, the idea that Venus might make a nice home sounds absurd. Its surface is more than 800 degrees Fahrenheit. Its pressure is akin to that found 3,000 feet under the earthly ocean. It’s very metal there. So metal you could melt lead. So metal that human-sent spacecraft melt and crumple within hours of landing.
But scientists have speculated about strange life on Venus for decades. In part that’s because the planet was not always thus: It may once have boasted an ocean. Life could have arisen back when the neighborhood was nicer and evolved as conditions transformed the planet into a literal hellscape. Evolved, that is, to live in a chiller part of the planet: the clouds, more than 30 miles above the surface, where the pressure eases up and the temperature dips into the 80s.
None other than Carl Sagan wrote in Nature, “It is by no means difficult to imagine an indigenous biology in the clouds of Venus.” He went on to imagine one version of such life: a being that looks and acts like a “float bladder,” a skin sac of sorts filled with hydrogen, bobbing through the clouds, collecting water and minerals.
Greaves had been intrigued by the possibility of atmospheric life. “I had this old idea that there could be an aerial habitat on Venus,” she says. She’d heard that phosphine was a marker of life on Earth, specifically of anaerobic life, which doesn’t need oxygen. Maybe, she thought, she could check for its presence on this oxygen-starved neighbor planet, using the James Clerk Maxwell Telescope. She would collect the light coming from Venus and look for a characteristic dip in its spectrum: a lack of light around a particular frequency, indicating that phosphine molecules had absorbed some photons as they passed through the atmosphere.
She hadn’t expected to actually see it. But there it seemed to be: a flattish line on the plot, followed by a deep, V-shaped drop and another flattish line.
Results in hand, curry consumed, she asked her colleague Paul Rimmer to meet her for coffee. “I’ve got this crazy thing to do with an atmosphere,” she told him. He agreed to help build atmospheric simulations to understand the data, and pointed her in the direction of a friend: Clara Sousa-Silva, a researcher at MIT and, Greaves says now, “the real world expert on phosphine.” She’s the one who helped establish phosphine as a promising biosignature on other planets, namely exoplanets. So when Greaves emailed her about the Venus data, Sousa-Silva's first thought was, "Is she sure?”
She wrote back, “That’s extraordinary!” Then, of course, she freaked out.
Phosphine is, in a lot of ways, Sousa-Silva’s molecule. When she entered grad school at University College London, her job was to simulate the spectra of molecules—the signatures they produce across the electromagnetic spectrum, from radio waves to visible light on up. Every atom and molecule has a unique spectrum, absorbing and emitting very particular shades of light. Scientists sometimes call a molecule’s spectrum its fingerprint. Mapping them in the lab can be expensive and often dangerous, so scientists lean on simulations to get the full picture.
Sousa-Silva was contributing to an astronomical project called ExoMol, a database of spectra that scientists can use to understand the chemicals found in space. It comes in especially handy when astronomers build models of interesting stars, brown dwarfs, or exoplanets. Her approach was to focus on a compound’s quantum characteristics, which arise from its subatomic particles and the counterintuitive laws of quantum mechanics. But which molecule should she tackle first?
“I could do methane,” she suggested to her PhD advisor. Nah, he said.
“Ammonia?” she recalls. Also no.
He suggested phosphine instead. Scientists barely knew what its main features were, and she could be the first to decipher its details. “So I Googled ‘phosphine,’” she says. It was a start.
If you zoom way in on a phosphine molecule, it resembles a phone tripod, its hydrogen legs propping up a single phosphorus. But that cute picture belies its toxic nature: According to the CDC, the gas “has an odor of garlic or decaying fish,” and, more importantly, it can kill you—in the pilot episode of the fictional television show Breaking Bad, Walter White murders a meth dealer by ginning up some PH3.
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In addition to finding phosphine in drug dens, you can find it in swamps, bogs, animal intestines (including your own), and some fumigants. The bacteria involved in decomposition produce it. In earthly biological systems, Sousa-Silva says, “all reports of its detection were near anaerobic life.” Before Sousa-Silva was born, in the 1970s, scientists discovered phosphine floating around Jupiter and Saturn, where it had formed deep in their atmospheres, where it’s hot and the hydrogen pressure is high—conducive to nonbiological making of phosphine. Those kinds of extreme conditions don’t exist in the same way on terrestrial planets like Earth or Venus. But most people still didn’t care much about the compound.
Soon, Sousa-Silva was a leading expert in this little-characterized molecule. She identified 16.8 billion features across the full spectrum, greatly expanding on the mere thousands anyone knew about before. She remembers the first time she put her theoretical quantum calculations to the test, to see if they matched up with how phosphine behaved in the nontheoretical world.
Turns out, they did: She could take tiny particles, and physical laws fundamentally based on uncertainty, and predict what would occur with a very concrete gas you could buy on the internet. Suddenly, her scientific knowledge was clicking into place. She was finally seeing for herself that the quantum world does indeed tug at our macroscopic existence. “I realized that until that moment I hadn’t believed in quantum physics, really,” she says. “I mean, I knew intellectually it was real. But in my heart, I totally didn’t believe.”
She teared up. She felt powerful. “It was a moment that changed how I saw myself as a scientist,” she says. But the feeling was complicated. “It was a very lonely, weird place to be, because no one really cared for it, and no one had really asked for phosphine information,” she says. “I was queen of a hill no one could see.”
With phosphine’s full fingerprint established, Sousa-Silva could then learn how to pick it out in a planet’s atmosphere. It would manifest in the little gaps in the light a telescope saw, where the killer gas had absorbed the photons. She and Greaves eventually converged, independently, on the same idea: If oxygen-hating organisms on Earth produce phosphine, maybe oxygen-hating organisms on other planets do too. Perhaps humans could look for them. Sousa-Silva started to muse about the distant planets she might one day explore through the eye of a telescope.
“I’d imagine tropical planets covered in sewage,” Sousa-Silva says. “And what would an intestine planet look like?”
She’d never daydreamed that way about Venus. Until Greaves emailed her.
As a researcher at MIT, Sousa-Silva had continued simulating the signatures of molecules, using her work on phosphine as the template. Creating a program called RASCALL, she now has rough spectra for around 16,000 molecules that interest astrobiologists. The basic questions underlying this chemical analysis, though, aren’t all that complicated: They mirror the ones we all ask. “Like most people on Earth, I do wonder about things like ‘Are we alone?’ ‘Is there other life?’” Sousa-Silva says. “I am not unusual in hoping that we’re not alone, that there is life out there somewhere.”
But, of course, as queen of the hill no one could see, Sousa-Silva always kept her eye on phosphine. It seemed like it could make an exceptional biosignature, which was a frequent topic of conversation in her group at MIT. “We established this trifecta,” she says: Life has to produce it in abundance; it has to survive in detectable amounts in a planet’s atmosphere and be distinguishable from other molecules; and it shouldn’t easily trick scientists by popping into existence in nonbiological, hard-to-track ways.
The more work the team—which grew beyond MIT to include UK and California collaborators—did on phosphine, the more promising it seemed: On Earth, the molecule only appeared in the presence of living things. Work led by colleague William Bains showed that planetary processes (on rocky planets, rather than gas giants) couldn’t make much of the gas, even on an extreme world, such as those Sousa-Silva jokingly describes as “all volcanoes everywhere all day every day.” But while some phosphine could get mixed up that way, it would be negligible compared to what you’d expect from living beings, allowing the group to distinguish the two routes of production.
The group simulated oxygen-starved fictional planets, to see if their biospheres would produce phosphine that would then accumulate in their atmospheres and stick around in a way telescopes could see. It could, and powerful instruments like the planned James Webb Space Telescope would be sensitive enough to detect whiffs of it light-years away.
It finally felt to Sousa-Silva like a perfect biosignature to seek. “I changed my twitter handle to @DrPhosphine,” she says.
After Greaves saw what looked like phosphine on Venus, the handle seemed even more apt. Along with their colleagues, the two scientists continued to probe the idea. But not too quickly, or without caution: The first thing to do if you think you might have found aliens is determine all the not-alien things it could be instead. You also have to double-check that the signal exists at all, and that it is not a measurement error or some other scientific fake-out.
One way to get confirmation is to look for the signature with a different telescope. So they turned to the Atacama Large Millimeter/Submillimeter Array, and got approved to use it for three hours in March 2019. If they also found phosphine’s fingerprint there, they’d have some confirmation the signal at least existed. “Half of me was saying ‘It will just turn out we were a little overly optimistic,’” Greaves says.
But the signal seemed to etch itself into the data there, too. It was definitely something. Something that matched phosphine.
Even that confirmation, though, needs to be taken with grains of sodium chloride. In both cases, the telescopes were uncovering just one feature in the fingerprint. “That means that the observations could be confused by instrumental effects, or another type of molecule absorbing at or near that position, even though the observers considered those possibilities and tried to rule them out,” says Victoria Meadows, an astrobiologist at the University of Washington, who was not involved in the research.
To exclude nonbiological scenarios, the MIT team, led by Bains, tuned their simulations to Venus’s conditions and continued to look at other processes that might give rise to phosphine: sunlight, minerals swept up from the ground, volcanic and tectonic activity, lightning, or sprinklings from meteorites and comets. “Most chemical compounds, there’s lots of way to make them,” Greaves says.
The team looked at every possibility they could think of. None could account for the amount of phosphine they saw. The only one they were left with? Life.
They are not the first to feel this way: On multiple occasions, scientists have thought they detected signs of life on or from Mars. In the 1970s, some researchers thought the Viking landers had found evidence of microbial respiration, but the consensus points toward unusual soil chemistry. Two decades later, they got excited about what looked like microscopic fossils in a martian meteorite. More recently, methane claimed the spotlight when scientists found spikes in its presence on that same Red Planet. Yet, here we are, still, with no known-for-sure aliens. “These initial claims all required rigorous follow-up over a long period of time,” Meadows says. “One of Carl Sagan’s most famous quotes was that ‘extraordinary claims require extraordinary evidence,’ and that is certainly true here.”
Phosphine, she points out, is a simple molecule, one that could come from planetary processes we’re not yet privy to. In this case, that means processes beyond those the team simulated. That would be the simplest explanation for seeing phosphine on a planet like Venus. “Invoking an unknown life form that has to overcome multiple known challenges in an amazingly hostile environment would be a less likely explanation,” Meadows says. There’s a lot more work to do, she continues, before the broader astronomical community is ready to believe phosphine, here, equals life.
Greaves and Sousa-Silva agree the work has just begun. “There’s lots of missing information about both Venus’s surface and its atmosphere,” Greaves says. What with the clouds and the spacecraft-crushing conditions, it’s a difficult planet to get to know. Beneath its shroud of clouds, a process might occur that doesn’t happen on Earth and that the scientists haven’t anticipated. It’s also possible, but less likely, that the team has in fact detected some other molecule that just happens to mimic this part of phosphine’s fingerprint. “It’s really hard to be sure that we’re looking at life,” in any extraterrestrial circumstance, Sousa-Silva says.
She’s been preparing for that ambiguity her whole career. “It fits with my expectations,” she says. “It also fits with my wildest dreams.”
In finding a glimmer of evidence that we may not be alone in the universe, she’s also found herself less alone in her molecular obsession. Today, phosphine is the word on every astronomer’s lips.
This story isn’t over, and its ending remains uncertain, a blurry scene in a movie we haven’t yet finished watching. Or a kind of Schroedinger’s cat: We have to hold in our heads the possibilities that Venus is alive and not-alive, at once.
But that was always going to be the case. The discovery of alien life likely won’t happen with either a bang or a whimper, but with a series of mid-volume conversations spread over spacetime and expensive scientific-journal PDFs.
We’ve all experienced many shares of uncertainty in the past eight or so months, as a tiny organism tears through our own planet, protests for racial justice and against police brutality roil American cities, and wildfires turn western US skies the color of Mars. Our whole future, and the shape of our collective story, have grown increasingly blurry.
But Greaves hopes this Venusian mystery provides some respite, however fractional, from the uncertain, scary circumstances on this planet. “I hope this is just something nice,” she says. “I hope it’s a good feeling.”
At least the path toward answers is straightforward. Outside researchers can confirm or refute or expand their data analysis—a task that, Sousa-Silva notes, scientists could have been doing earlier if her team hadn’t kept their discovery quiet until its publication today, as is customary. “I think it’s bad for science to keep it a secret,” she says. “The scientific community would have been better off if they had had access to this discovery early on.”
That community needs to drill deeper into potential nonbiological explanations for phosphine. Sousa-Silva and colleague Jason Dittmann plan to look at Venus using telescopes that sense infrared light, to detect (or not) that press of phosphine’s fingerprint, and to see if other biosignatures pop up.
They were supposed to do some of that work earlier this year, but, you know, Covid. The missed opportunity has felt frustrating to Sousa-Silva. Lately she’s been going outside and staring at Venus, its light wobbling through our atmosphere. She feels its photons going into her eye—unquantified, uncaptured—and it pains her. “They’re just going to waste,” she says. “Every night, Venus is sending us all the information we need to prove this discovery, and we’re just not analyzing it.”
Someday, the scientists hope they can do the ultimate experiment: sending a spacecraft to Venus. Just a simple, little one, Greaves says. One that can fall through those strange clouds and send back data as it whizzes by. Ideally, that project could come together more quickly than a typical, large space mission. But if it takes awhile, so be it. “I can wait 10 years if I have to,” she says.
It may be harder for the rest of us, not so inured to astronomical timescales, to keep our heads around the ambiguity for that long. To hold both possibilities as possible. But if this detection is phosphine, and if this phosphine comes from life, then it would feel kind of poetic. You find love when you least expect it. You find the word you’re looking for when you stop thinking about it. You remember what you wanted to say when the person you wanted to say it to is gone.
You find alien life not in a nice Earth-like place, with a nice ocean and plentiful oxygen, but on a hostile, hot planet, because it’s leaking toxic gas into that toxic world. But there it is, carrying on, in the face of all that.
If that’s the case, it’s very 2020.