Astronomers have assumed for some time that the best places to hunt for extraterrestrial life in our solar system are on Jupiter and Saturn's moons, or on Mars. But new research suggests we should check out our other planetary neighbour, too. 

A study published Wednesday in the journal Nature Astronomy reveals that Venus' clouds contain traces of phosphine, an extremely flammable, poisonous, and corrosive gas also found on Earth. On Earth, phosphine is produced either by people in labs or by anaerobic bacteria. So the phosphine on Venus seems most likely to have come from a living organism.

The authors of the study weren't able verify the origin of the phosphine, but none of the other potential sources they investigated, like volcanic activity on Venus, could explain the amount of the gas they found.

"We really went through all possible pathways that could produce phosphine on a rocky planet," Janusz Petkowski, a co-author of the new study, told MIT News. "If this is not life, then our understanding of rocky planets is severely lacking."

Phosphine's mysterious origin

Venus is the hottest planet in our solar system. Its average surface temperature is a blistering 470 degrees Celsius – hot enough to melt lead.

That, coupled with the planet's carbon-dioxide-clogged atmosphere, would seem to make Venus quite inhospitable to life.

But some researchers, including famed astronomer Carl Sagan, have suggested that although Venus' surface may not be habitable, the band of clouds that surrounds the planet could host microbes.

The new discovery lends further credence to Sagan's idea. 

The phosphine gas scientists found is in the upper reaches of Venus' cloud deck, up to 60 kilometres above the surface. The gas, PH3, consists of phosphorus and hydrogen atoms linked together.

Petkowski's team discovered it thanks to observations of Venus from the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter Array observatory in Chile. Based on that data, they calculated that Venus' clouds have about 20 parts per billion of phosphine gas.  

An artist's impression of the Solar Orbiter, a sun-observing satellite developed by the European Space Agency, flying around Venus.

The scientists didn't stop there, however. They then attempted to model how the phosphine might have accumulated in Venus' cloud deck.

First they modelled geological and chemical scenarios, like tectonic plates moving, volcanic activity, or meteor strikes. They also tested whether chemical processes or intense lightning in the clouds could have produced the gas.

None of those calculations yielded the amount of phosphine they'd detected.

"If no known chemical process can explain PH3 within the upper atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions. This could be unknown photochemistry or geochemistry, or possibly life," the study authors wrote.

Other scientists say further observations are needed to confirm the new findings, however.

Astrobiologist Dirk Schulze-Makuch, who was not involved in the study, told National Geographic that a biological explanation for the phosphine is still possible, as are so-far-undiscovered geologic or light-induced chemical reactions.

"Venus is basically still an alien planet," Schulze-Makuch said. "There are a lot of things we don't understand."

Venus' surface and atmosphere are vastly different

Conditions on Venus' surface contrast starkly with those in its atmosphere.

The surface of Venus on September 16, 2010.

The planet's surface "is very, very corrosive," Gustavo Costa, a chemist and materials scientist at NASA's Glenn Research Center, previously told Business Insider, adding, "it's like hell on Earth."

The pressure on Venus' surface, Costa said, would feel like being being 900 meters underwater.

But high in the planet's atmosphere, about 60 kilometres above the surface, temperatures range from 30 to 70 degrees Celsius. Pressures there are equivalent to sea-level on Earth.

That's the part of the atmosphere where Petkowski's team found phosphine.

"This phosphine signal is perfectly positioned where others have conjectured the area could be habitable," Petkowski told MIT News

Scientists have suggested before that Venus' clouds may be habitable

In the 1960s, Carl Sagan wrote two papers discussing the possibility of life on Venus. He concluded that the planet's surface, on average, is too hot for proteins and biological processes to hold up.

"The overall chances of life on the surface of Venus remain bleak," Sagan wrote

But he added that "while the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether."

Sagan and Harold Morowitz, a molecular biophysicist at Yale, postulated in 1967 that there could be a liveable layer in Venus' clouds where churning air would keep microbes aloft. They even worked out the math for an organism with a balloon-like, hydrogen-filled "float bladder" that could keep it alive. 

In the decades since then, astronomers have continued to investigate this possibility. A study published in 2018 suggested that temperate Venusian clouds could harbour extremophiles: microbes that can inhabit environments that are extremely hot, acidic, or toxic.

"On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid," one of the co-authors of that study, Rakesh Mogul, said in a press release.

Mogul's team mentioned as an example living microbes that have been found in our own atmosphere, 40 kilometres above Earth's surface.

There's no reason similar microorganisms couldn't thrive on other planets.

"Venus could be an exciting new chapter in astrobiology exploration," Mogul said in 2018.

Future missions to Venus could tell us more about its clouds

To get a clear look inside Venus' clouds, astronomers need to send a probe.

NASA has researched and partly funded conceptual missions to Venus for decades, but the agency hasn't launched a dedicated mission to the planet since 1989. Europe and Japan, meanwhile, put spacecraft into orbit around Venus in 2006 and 2015, respectively, but those weren't equipped to seek out signs of life.

Peter Beck, the CEO of private launch company Rocket Lab has said he'd like to change that. On August 31, the company launched its first Photon spacecraft, which is designed to carry technology into space. 

Beck said Photon could eventually reach Venus, Mars, and the moon.

One of Rocket Lab's carbon-fiber Electron launch vehicles lifts off from New Zealand in August 2020.

"I'm madly in love with Venus," Beck said in an August video, adding that he's working to launch a mission to the planet in 2023 using Photon. "We're going to learn a lot on the way there, and we're going to have a crack at seeing if we can discover what's in that atmospheric zone. And who knows? You may hit the jackpot."