The molecule phosphine had been found in the atmosphere of Venus, our nearest neighboring planet. At that time — admittedly — most of us had never heard of phosphine, but it’s a toxic, corrosive, smelly gas made up of one phosphorus and three hydrogen atoms (PH3).

On Earth, there are small amounts of phosphine in our own atmosphere — which may be produced by microbes — and it is produced industrially as a fumigant. Additionally, it’s found naturally in places like swamps where anaerobic (low oxygen-using) bacteria produce phosphine as a waste product. Occasionally it shows up in the intestinal bacteria of some animals, such as penguins.

But there it was, apparently floating amidst the middle layer of the atmosphere of Venus. How and why the phosphine was there has yet to be explained. The initial detection was based on observations by two Earth-based radio telescopes, the James Clerk Maxwell Telescope (JCMT) in Hawai’i saw it in 2017, and the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in northern Chile in 2019. 

The detection surprised many Venus experts, including the team leader of the study, astronomer Jane Greaves from Cardiff University in the UK. The phosphine showed up in very low quantities, about 20 parts per billion – which is a thousand times more than the amount of phosphine gas in Earth’s atmosphere. And the signal seemed undeniable in the data.  

Moreover, that amount of phosphine — plus the fact that it was detected twice nearly two years apart — meant that the phosphine was too abundant and persistent to exist without a replenishing source. Sunlight or sulfuric acid in the clouds should have annihilated the gas before it could accumulate. 

Greaves and her team — which had grown to 19 scientists from around the world, experts in various disciplines — modeled every possibility, from tectonic to volcanic activity, meteor strikes, lightning or other known chemical processes, but none of their calculations yielded the amount of phosphine detected. The only plausible source would be some type of unknown Venusian chemistry, or most intriguing, some type of microbes living in the clouds.  

However, when it comes to places considered habitable, Venus isn’t usually on that list. The hot, greenhouse-effect-gone-mad planet with toxic sulfuric acid clouds certainly isn’t friendly to life as we know it. And the planet’s crushing surface pressure has doomed the few spacecraft that have attempted to reach the planet’s mysterious landscape. 

But about 40 to 60 km (25 to 37 miles) above the surface is a different story. The atmosphere of Venus is the most Earth-like of any other place in the Solar System. There, Venus has air pressure of approximately 1 bar and temperatures in the 0°C to 50°C range. Except for the acidic clouds and lack of oxygen, it would be quite livable by human standards. Several scientists, including Carl Sagan have theorized that anaerobic life could possibly exist in those conditions. 

At a Zoom press briefing in September 2020, Greaves and her team first made the announcement of their discovery of phosphine. Their words were measured and they took pains to say that they had NOT detected life in the cloud tops of Venus. 

"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," they wrote in the paper, published in Nature Astronomy. "This could be unknown photochemistry or geochemistry, or possibly life."

Unusual for a paper published in Nature, the team requested their data and work be freely available and open to the public.

The news made a splash in the press, which not surprisingly focused on the aspect of potential life on Venus. The skeptical scientific community quickly began poring over the open data, where some problems arose, along with some unexpected backlash. First came grumblings from within the scientific community, expressing concerns the announcement was sensationalized and made too quickly — even though the team had spent three years analyzing the data before publishing their paper. 

A statement posted on the International Astronomical Union’s website voiced such criticisms, but was quickly taken down, with apologies issued. Another rebuttal paper included a condescending tone towards Greaves and her team in the summary; it was quickly revised. 

In just a few weeks, three different studies using the Greaves’ team data questioned the evidence of phosphine in Venus’ atmosphere. And then the ALMA observatory found an error in its calibration system used to collect the data Greaves and her team used. A re-working with corrected data still showed phosphine, but less of it. 

However, other groups have since confirmed finding evidence for phosphine, while another group looked at old data from NASA’s 1978 Pioneer Venus Multiprobe. Their look back in time showed data consistent with phosphine, even though the probe’s mass spectrometer was not designed to look for that specific gas. 

More recently, at the American Astronomical Society meeting this summer, Greaves had news to share in a keynote lecture. New data from observations in 2020 show that the signal for phosphine was still present. The observations were conducted again with JCMT, but with a new instrument and new technique. 

“So, we have 2017 and 2020 data with the same telescope, but with different instruments and different [tools for] tackling the processing issues, and we got the same results,” Greaves said in the lecture. She also shared two other interesting confirmations: The ALMA data her team used previously has been reprocessed independently, confirming their findings, and the SOFIA airborne telescope has shown a hint of detection of phosphine when it turned its flying spectrometer towards Venus. 

Now, in a recent radio interview, Greaves said her team has made a fourth detection of phosphine, again using the JCMT. 

“[In 2021,] they allowed us to do what's called a legacy survey where we can use far, far more telescope time and collect a whole slew of data,” she told listeners to Planetary Radio. “I finally teased out the third detection of phosphine from the JCMT just this morning. In fact, your listeners are the first to know that because I haven't had time to email my colleagues yet.” 

We can expect a new paper detailing the results in the near future. 

Some have asked, why not unleash the power of the new James Webb Space Telescope to peer into Venus’ atmosphere to tease out even more data? While Webb’s infrared vision is perfect for looking at planetary atmospheres, especially those of distant exoplanets, it can’t risk looking at something as close to the Sun as Venus. Because the telescope and instruments have to be kept incredibly cold, Webb’s sunshield will be blocking the inner solar system from view. This means that the Sun, Earth, Moon, Mercury, and Venus, as well as any sun-grazing comets cannot be observed.

While the debate and data still churn, the unexpected finding of phosphine offered enough intrigue and wonder to change the tide of opinions and interest in our sister planet. Venus has been considered the forgotten planet, as it has been 30 years since NASA has sent a space mission to Venus, but that is about to change. At least six spacecraft are scheduled to visit Venus in the next ten years, including two NASA missions that could launch as early as 2028: DAVINCI will explore Venus' atmosphere and VERITAS will use radar to map Venus’ surface. India’s Shukrayaan-1 is scheduled to launch as soon as 2024 and will include an instrument that will be able to detect phosphine. A private company, Rocket Lab is hoping to send their own spacecraft to hunt for the source of phosphine in Venus’ atmosphere. 

“An open science approach led to many new connections and colleagues,” Greaves said in her AAS lecture. “The phosphine project has stimulated curiosity about our neighbor planet. Venus is a planet we can benchmark as we work towards understanding a myriad of worlds.”

However the phosphine story eventually turns out, the road to its understanding tells the tale of the scientific method; about the collaboration of different fields and instruments that are required to address a problem, and the messy human interactions that are inevitably part of science.

How does Greaves hope the phosphine story turns out?  

“I think I’d love for it to turn out to be life,” she said in response to a question after her lecture, “but what we really need is more and varied observations. In the end, I’d just be happy that we added something to the knowledge about Venus.”