The detection of phosphine gas in the clouds of Venus has surprised scientists, who are now wrestling with a big question: Could it be a sign of alien life?

New research published Monday in the journal Nature Astronomy detailed the recent discovery of the gas as well as its possible origins. And while the scientists behind the research aren’t making any definitive conclusions just yet, extraterrestrial life is one of the few explanations that makes sense.

“If this signal is correct, there is a process on Venus we cannot explain that produces phosphine – and one of the hypotheses is that it’s life in the clouds of Venus,” said Janusz Petkowski, an astrobiologist at the Massachusetts Institute of Technology who worked on the research. “It’s far fetched, until it’s not.”

But don’t expect any alien visitors just yet. At best, the gas might be caused by microorganisms in the upper atmosphere of Venus, roughly 150 million miles away — the closest planet to Earth and practically next door in astronomical terms.

Still, the discovery came as something of a surprise.

“I was really shocked,” said Clara Sousa-Silva, a molecular astrophysicist at MIT who has devoted much of her career to studying phosphine as a possible “biosignature” on far-off worlds. “I put all this work into looking for phosphine everywhere else, but here it was on our nearest neighbor.”

The researchers have explored all the known chemical processes that could account for the phosphine gas on Venus – even exotic causes like volcanoes, lightning and delivery by meteorites – but they’ve found nothing to explain it.

Here on Earth, phosphine comes from microorganisms that break down decaying plants and animals without oxygen. It’s usually found with the related gas diphosphane, which smells like rotting fish.

That makes phosphine potentially what scientists call a biosignature — a chemical signal that can be detected by spectroscopic telescopes that could indicate a planet harbors at least simple forms of life.

The discovery also highlights a main way astrobiologists hope to find extraterrestrial life – not by finding spaceships or little green men, but by detecting its chemical signatures in the atmospheres of distant planets.

There are a few ways to make phosphine chemically – in the extreme temperatures and pressures inside gas giants like Jupiter and Saturn, for example – but there are no known ways to make it in the atmosphere of a rocky planet like Venus, Petkowski said.

What they have found, however, is that the gas must be continuously produced to make up for its rapid breakdown in the Venusian atmosphere, and that it’s only found in clouds near the equator, which might be expected if it has a biological cause.

The search for alien life in our solar system has focused on Mars, a small, cold rocky planet much farther from the sun than Earth. Methane gas has been detected in its atmosphere, which may be a sign of life – but methane also has known geological sources.

Venus, of a similar size to Earth, orbits nearer the sun – but it’s a scorched and inhospitable place, with surface temperatures hot enough to melt lead and a thick atmosphere unbreathable to humans filled with clouds of sulfuric acid.

Nonetheless, scientists speculate that microbes might survive in the clouds of Venus, 30 to 40 miles above the surface, where it’s cool enough for water to form – although it would be a highly acidic environment.

And that’s exactly where traces of phosphine have now been detected, and at relatively high concentrations of about 20 parts per billion – too high to be explained in any other way.

Phosphine gas was first detected on Venus in 2017 by Jane Greaves, an astrobiologist and professor of astronomy at the University of Cardiff in Wales and the lead author of the new study.

She spotted its fingerprint in the spectrum of light gathered from the Venusian clouds by the James Clerk Maxwell Telescope in Hawaii, and then confirmed it in 2019 with the ALMA radio telescope in Chile’s Atacama Desert.

Greaves explained she was looking for phosphine on Venus mainly as a theoretical test, and the discovery was a “huge surprise.”

“I thought we’d get a null result, of interest to a few other astrobiologists, and not have wasted much telescope time,” she said.

Instead, her find now has scientists scrambling to explain it – and it may only be resolved if a spacecraft eventually samples the Venusian atmosphere and tests it for life, she said.

“Chemical biosignatures are one of the primary tools we have for looking for life in the universe,” said Caleb Scharf, director of astrobiology at Columbia University in New York, who was not involved in the study. “We might not spot aliens directly, but we might be able to ‘smell’ them.”

The discovery of what could be phosphine in the atmosphere of Venus is “super intriguing,” he said. “It certainly looks like a possible biosignature.”

But he cautioned it could be the result of unknown chemical processes, which are poorly understood on Venus.

“Until we have a better handle on that, it’ll be very hard to say that this phosphine is definitely coming from living things,” he said.