Scientists have hailed the discovery of a gas known as phosphine in the clouds above Venus. It's the first time the rare molecule has been found on a rocky planet other that Earth, which has sparked excitement that it could lead a step closer to proving the existence of extraterrestrial life.
The atmosphere of Venus contains a gas that on Earth "can be attributed to living organisms", scientists said in a report published in the science journal Nature Astronomy.
Phosphine is a flammable gas that can only come from the breakdown of organic matter or man-made processes.
"The detection of phosphine molecules, which consist of hydrogen and phosphorus, could point to this extraterrestrial 'aerial' life," says the report.
The research provided evidence "for anomalous and unexplained chemistry" which may be a stepping stone towards other projects on Venus.
Lead author Jane Greaves, from Cardiff University's School of Physics and Astronomy told AFP that the presence of phosphine alone was not proof of life on one of Earth's closest neighbours.
"I don't think we can say that - even if a planet was abundant in phosphorus, it might lack something else important to life - some other element, or conditions might be too hot, too dry," she said.
However it is the first time phosphine has been found on a rocky planet other than Earth, and the only way to know more would be to send a spacecraft to collect samples, Greaves added.
To collect the data, the astronomers used telescopes in Hawaii and Chile's Atacama Desert to observe Venus' upper cloud levels, around 60 kilometres from the surface, where temperatures are around 20-30 degrees Celsius.
Venus' daytime temperatures of up to 400 degrees Celsius are hot enough to melt lead and its atmosphere is comprised almost entirely of carbon dioxide.
The clouds of the planet are incredibly acidic - around 90 percent sulphuric acid - posing major issues for microbes to survive there, astronomers said.
Scientists at the Massachusetts Institute of Technology in the United States provided some extra research into how phosphine is produced in nature to better understand the research, as explained in this video.
However, the group warns that some information is lacking - and that the only other study of phosphorus on Venus came from one lander experiment, carried by the Soviet Vega 2 mission in 1985.
NASA administrator Jim Bridenstine reacted on social media, calling the Cardiff group study "the most significant development yet in building the case for life off Earth," and "it's time to prioritise Venus."
Will Venus steal Mars's thunder?
Until now, Earth's other closest neighbour Mars has attracted more attention in terms of space exploration, with several unmanned probes launched by the US and China in the bid to find proof of life.
In one of the latest projects, Nasa at the end of July launched the 'Perseverance' rover which is due to land in February 2021.
But Alan Duffy, an astronomer from Swinburne University and Lead Scientist of The Royal Institution of Australia, said the Venus find was "one of the most exciting signs of the possible presence of life beyond Earth I have ever seen".
But he admitted: "We have to rule out all possible other non-biological means of producing it.
Professor Emma Bunce, President of the Royal Astronomical Society of the UK said the breakthrough "makes a strong case for a return space mission to Venus".