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Astronomers have found a potential sign of life high in the atmosphere of neighboring Venus: hints there may be bizarre microbes living in the sulfuric acid-laden clouds of the hothouse planet.

Two telescopes in Hawaii and Chile spotted in the thick Venusian clouds the chemical signature of phosphine, a noxious gas that on Earth is only associated with life, according to a study in Monday鈥檚 journal Nature Astronomy.

Several outside experts 鈥� and the study authors themselves 鈥� agreed this is tantalizing but said it is far from the first proof of life on another planet. They said it doesn鈥檛 satisfy the 鈥渆xtraordinary claims require extraordinary evidence鈥澛� standard established by the late Carl Sagan, who speculated about the possibility of life in the clouds of Venus in 1967.

鈥淚t鈥檚 not a smoking gun,鈥� said study co-author David Clements, an Imperial College of London astrophysicist. 鈥淚t鈥檚 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.鈥�

As astronomers plan for 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. After three astronomers met in a bar in Hawaii, they decided to look that way at the closest planet to Earth: Venus. They 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 chemical warfare agent in World War I.) The other way is as part of some kind of poorly understood function in animals and microbes. Some scientists consider it a waste product, others don鈥檛.

Phosphine is found in 鈥渙oze at the bottom of ponds, the guts of some creatures like badgers, and perhaps most unpleasantly associated with piles of penguin guano,鈥� Mr. Clements said.

Study co-author Sara Seager, an MIT planetary scientist, said researchers 鈥渆xhaustively went through every possibility and ruled all of them out: volcanoes, lightning strikes, small meteorites falling into the atmosphere. ... Not a single process we looked at could produce phosphine in high enough quantities to explain our team鈥檚 findings.鈥�

That leaves life.

The astronomers hypothesize a scenario for how life could exist on the inhospitable planet where temperatures on the surface are around 800 degrees with no water.

鈥淰enus is kind of Earth鈥檚 evil twin,鈥� Mr. Clements said. 鈥淐learly something has gone wrong, very wrong, with Venus. It鈥檚 the victim of a runaway greenhouse effect.鈥�

But that鈥檚 on the surface.

Ms. Seager said all the action may be 30 miles above ground in the thick carbon-dioxide layer cloud deck, where it鈥檚 about room temperature or slightly warmer. It contains droplets with tiny amounts of water but mostly sulfuric acid that is a billion times more acidic than what鈥檚 found on Earth.

The phosphine could be coming from some kind of microbes, probably single-cell ones, inside those sulfuric acid droplets, living their entire lives in the 10-mile-deep clouds, Ms. Seager and Mr. Clements said. When the droplets fall, the potential life probably dries out and could then get picked up in another drop and reanimate, they said.

Life is definitely a possibility, but more proof is needed, several outside scientists said.

Cornell University astronomer Lisa Kaltenegger said the idea of this being the signature of biology at work is exciting, but she said we don鈥檛 know enough about Venus to say life is the only explanation for the phosphine.

鈥淚鈥檓 not skeptical, I鈥檓 hesitant,鈥� said Justin Filiberto, a planetary geochemist at the Lunar and Planetary Institute in Houston who specializes in Venus and Mars and isn鈥檛 part of the study team.

Mr. Filiberto said the levels of phosphine found might be explained away by volcanoes. He said recent studies that were not taken into account in this latest research suggest that Venus may have far more active volcanoes than originally thought. But Mr. Clements said that explanation would make sense only if Venus were at least 200 times as volcanically active as Earth.

David Grinspoon, a Washington-based astrobiologist at the Planetary Science Institute who wrote a 1997 book suggesting Venus could harbor life, said the finding 鈥渁lmost seems too good to be true.鈥�

鈥淚鈥檓 excited, but I鈥檓 also cautious,鈥� Mr. Grinspoon said. 鈥淲e found an encouraging sign that demands we follow up.鈥�

NASA hasn鈥檛 sent anything to Venus since 1989, though Russia, Europe, and Japan have dispatched probes. The American space agency is considering two possible Venus missions. One of them, called DAVINCI+, would go into the Venusian atmosphere as early as 2026.

Mr. Clements said his head tells him 鈥渋t鈥檚 probably a 10% chance that it鈥檚 life,鈥� but his heart 鈥渙bviously wants it to be much bigger because it would be so exciting.鈥�

This story was reported by The Associated Press.聽