Astronomers have found a potential sign of life high in the atmosphere of Venus with hints there may be bizarre microbes living in the sulphuric acid-laden clouds of the hothouse planet.
Telescopes in Hawaii and Chile spotted the chemical signature of phosphine, a noxious gas that on Earth is only associated with life, according to a study in the scientific journal Nature Astronomy.
Experts have said it is far from the first proof of life on another planet, pointing out it does not satisfy the standard established by the late astronomer Carl Sagan that "extraordinary claims require extraordinary evidence".
Mr Sagan speculated about the possibility of life in the clouds of Venus in 1967.
"It's not a smoking gun," said study co-author David Clements, an Imperial College of London astrophysicist. "It's not even gunshot residue on the hands of your prime suspect, but there is a distinct whiff of cordite in the air which may be suggesting something."
As astronomers plan searches for life on planets outside our solar system, a major method is to look for chemical signatures that can only be made by biological processes, called biosignatures.
In the case of Venus, astronomers searched for phosphine, which is three hydrogen atoms and a phosphorous atom.
On Earth, there are only two ways phosphine can be formed, study authors said. One is in an industrial process - the gas was produced for use as a chemical-warfare agent in World War I.
The other is as part of a poorly understood function in animals and microbes. Some scientists consider it a waste product.
Phosphine is found in "ooze at the bottom of ponds, the guts of some creatures like badgers and, perhaps most unpleasantly, it's associated with piles of penguin guano," Mr Clements said.
Study co-author and MIT planetary scientist Sara Seager said researchers exhaustively went through every possibility and ruled all of them out, including volcanoes, lightning strikes and small meteorites falling into the atmosphere.
"Not a single process we looked at could produce phosphine in high enough quantities to explain our team's findings," she said.
The astronomers suggest life could exist on the inhospitable planet where temperatures on the surface are around 425C with no water.
"Venus is hell. Venus is kind of Earth's evil twin," Mr Clements said.
"Clearly something has gone wrong, very wrong, with Venus.
"It's the victim of a runaway greenhouse effect."
But that's on the surface.
Seager said all the action may be 50 kilometres above ground in the thick carbon-dioxide layer cloud deck, where it's about room temperature or slightly warmer.
It contains droplets with tiny amounts of water but mostly sulphuric acid that is a billion times more acidic than what's found on Earth.
The phosphine could be coming from some kind of microbes - probably single-cell - inside those sulphuric acid droplets, living their entire lives in the 16-kilometre-deep clouds.
When the droplets fall, the potential life probably dries out and could then get picked up in another drop and reanimate, they said.
David Grinspoon, a Washington-based astrobiologist at the Planetary Science Institute who wrote a 1997 book suggesting Venus could harbour life, said the finding "almost seems too good to be true".
"I'm excited, but I'm also cautious," he said. "We found an encouraging sign that demands we follow up."