Scientists are excited because the only way they know to produce the gas, other than in a laboratory, is by living microbes. The gas is therefore regarded as a “biosignature” – something which scientists consider to be persuasive evidence of life.
The international team, which includes researchers from the USA and Japan, estimates that phosphine – which consists of hydrogen and phosphorus – exists in Venus’s clouds at a small concentration, only about 20 molecules in every billion. The team says non-biological processes could not make anything like as much as they observed.
The research was led by Professor Jane Greaves, of the University of Cardiff. Her team made the detection of phosphine using the James Clerk Maxwell Telescope in Hawaii (which once faced closure), and their result was confirmed by the Atacama Large Millimeter Array (ALMA) in Chile.
Both facilities observed Venus at a wavelength of about 1 millimetre, much longer than the human eye can see, and only effectively detectable by telescopes at high altitude.
“When we got the first hints of phosphine in Venus’s spectrum, it was a shock,” said Professor Greaves. “In the end, we found that both observatories had seen the same thing – faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below.”
Venus is a rocky world, almost the same size as Earth, and is the next planet between us and the Sun. But it became a different world that has been compared to a vision of hell.
The surface temperature is twice the maximum in a kitchen oven, it is covered by thousands of volcanoes, and the atmosphere is largely made up of poisonous sulphuric acid. On top of that, the air pressure at the surface is so great that anything reaching it would be instantly crushed.
Related: Read our guide to Venus
However, this dense atmosphere, which completely hides the surface from normal view, is much cooler in its upper layers. Temperatures and pressure there are similar to those found on Earth, with some water vapour and oxygen too.
It has therefore long been speculated that a simple microbial form of life could exist and be blowing around in the clouds of Venus.
Another member of the discovery team was Professor Sara Seager, of the Massachusetts Institute of Technology, who has a special interest in planetary atmospheres and the search for life.
She told the Royal Astronomical Society press conference announcing the discovery: “We are not claiming we have discovered life on Venus.” But she said it was “an exciting possibility”.
Professor Seager said she wanted to see the discovery tested with observations at other wavelengths, and for space missions to follow up on the findings.
She added that it had been speculated for decades that there could be life in the clouds of Venus. Microbial life is found in Earth’s clouds, surviving for around a week.
Professor Seager suggested that perhaps life originated in the past when Venus was cooler with liquid water oceans, and later migrated into the clouds.
Dr William Bains, also of MIT, led the work on assessing natural ways to make phosphine. Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of these could make anywhere near enough of it. Natural sources were found to make at most one ten thousandth of the amount of phosphine that the telescopes saw.
In March, 2018, a team of NASA scientists reported seeing mysterious dark patches in the clouds of Venus, which they said resembled the light-absorbing properties of some bacteria on Earth.
NASA, the European Space Agency, and Japan have all sent space probes into orbit around Venus. The former Soviet Union even landed some craft in the 1970s, taking photos of the rocky surface, though they survived just a short time before being destroyed by the inhospitable conditions.
The Soviets also dropped balloons into the clouds, in 1985, which floated at an altitude of about 30 miles for nearly two days.
Venus scientists are keen to send more balloons or airships to fly around Venus. One advanced concept being drawn up by Northrop Grumman is for a delta-winged aircraft called VAMP – the Venus Atmospheric Maneuverable Platform – which would be dropped by a space probe and spend up to a year flying about in the atmosphere.
Professor Abel, of the University of Puerto Rico, sounded a note of caution about the new claim for life on Venus, by suggesting there would be little for microbes to feed on.
He tweeted: “I don’t think that the cloud environments of Venus could be currently habitable, mostly due to low nutrient fluxes, at least for life as we know it.”
Space scientists have previously been focusing on Mars in their search for ET. Detections of plumes of methane on Mars have been interpreted as compelling evidence for life on the Red Planet.
