A team of astronomers made a blockbuster claim in the fall. They said they had discovered compelling evidence pointing to life floating in the clouds of Venus.

If true, that would be stunning. People have long gazed into the cosmos and wondered whether something is alive out there. For an affirmative answer to pop up on the planet in the orbit next to Earths would suggest that life is not rare in the universe, but commonplace.

The astronomers, led by Jane Greaves of Cardiff University in Wales, could not see any microscopic Venusians with their telescopes on Earth. Rather, in a paper published in the journal Nature Astronomy, they reported the detection of a molecule called phosphine and said they could come up with no plausible explanation for how it could form there except as the waste product of microbes.

Five months later, after unexpected twists and nagging doubts, scientists are not quite sure what to make of the data and what it might mean. It might spur a renaissance in the study of Venus, which has largely been overlooked for decades. It could point to exotic volcanism and new geological puzzles. It could indeed be aliens. Or it could be nothing at all.

Dr. Greaves and her colleagues remain certain about their findings even as they have lowered their estimates of how much phosphine they think is there. I am very confident there is phosphine in the clouds, Dr. Greaves said.

Clara Sousa-Silva, a research scientist at the Center for Astrophysics in Cambridge, Mass., and one of the authors of the Nature Astronomy paper, said, I think the team in general still feels pretty confident that its phosphine, that the signal is real and that there are no real abiotic explanations.

But, Dr. Sousa-Silva added, theres a lot of uncertainty in all of us.

In the wider circle of planetary scientists, many are skeptical, if not disbelieving. Some think that the signal is just a wiggle of noise, or that it could be explained by sulfur dioxide, a chemical known to be in the Venus atmosphere. For them, there is so far no persuasive evidence of phosphine let alone microbes that would make it at all.

Whatever it is, its going to be faint, said Ignas Snellen, an astronomer at Leiden University in the Netherlands who is among the skeptics. If the signal is faint, he said, its not clear whether its real, and, if its real, whether its going to be phosphine or not.

The debate could linger, unresolved, for years, much like past disputed claims for evidence of life on Mars.

When the observation came out, I was like, Oh, thats interesting, said Martha S. Gilmore, a professor of geology at Wesleyan University in Middletown, Conn. Dr. Gilmore is the principal investigator of a study that has proposed to NASA an ambitious flagship robotic mission to Venus that would include an airship flying through the clouds for 60 days.

I think were skeptical, Dr. Gilmore said. But I dont personally feel yet that we want to throw out this observation at all.

The surface of Venus today is a hellish place where temperatures roast well over 800 degrees Fahrenheit. But early in the history of the solar system, it could have been much more like Earth today, with oceans and a moderate climate. In this early era, Mars, which is now cold and dry, also appears to have had water flowing across its surface.

Potentially, four billion years ago, we had habitable environments on Venus, Earth and Mars all three of them, said Dirk Schulze-Makuch, a professor at the Technical University Berlin in Germany. And we know that there is still a viable, thriving biosphere on our planet. So on Venus, it got too hot. On Mars, it got too cold.

But life, once it arises, seems to stubbornly hold on, surviving in harsh environs. You could have potentially, in environmental niches, microbial life hanging on, Dr. Schulze-Makuch said.

For Mars, some scientists think it is possible that life persists today underground, in the rocks. But the subsurface of Venus is too hot, said Dr. Schulze-Makuch, who two decades ago scrutinized whether any parts of that planet were still habitable.

Instead, he said, Venusian life could have moved up, to the clouds. Thirty miles up are short-sleeve temperatures about 85 degrees Fahrenheit. Microbes in that part of the atmosphere would stay aloft at that altitude for several months, more than long enough to reproduce and maintain a viable population.

But even the clouds are not a serene, benign place. They are filled with droplets of sulfuric acid and bathed in ultraviolet radiation from the sun. And it is dry, with only smidgens of water, an essential ingredient for life as we know it.

Still, if that was the environment that Venus microbes had to survive in, it was possible that they had evolved to do just that.

Phosphine is a simple molecule a pyramid of three atoms of hydrogen attached to one phosphorus atom. But it takes considerable energy to push the atoms together, and conditions for such chemical reactions do not seem to exist in the atmosphere of Venus.

Phosphine could be created in the heat and crushing pressure of the interior of Venus. Even with the lower amounts of phosphine that Dr. Greavess group now estimates, it would be unexpected and surprising if Venuss volcanic eruptions turned out to be so violently voluminous that they spewed out enough phosphine to be detected where Dr. Greavess team said it was: in the clouds, more than 30 miles up.

We cant easily rule in or out volcanism to explain this new, lower phosphine abundance, said Paul Byrne, a professor of planetary science at North Carolina State University in Raleigh, who pointed to the many unknowns about the planet and its geological system. Its probably not volcanism. But we cant say for sure.

On Earth, phosphine is produced by microbes that thrive without oxygen. It is found in our intestines, in the feces of badgers and penguins, and in some deep sea worms.

In 2017, Dr. Greaves found indications of phosphine using the James Clerk Maxwell Telescope in Hawaii. Different molecules absorb and emit specific wavelengths of light, and these form a fingerprint that enables scientists to identify them from far away. The measurements found what scientists call an absorption line at a wavelength that corresponded to phosphine. They calculated that there were 20 parts per billion of phosphine in that part of Venuss air.

Follow-up observations in 2019 used the Atacama Large Millimeter Array, or ALMA, a radio telescope in Chile that consists of 66 antennas. Those again turned up the same dark line corresponding to phosphine, although at lower concentrations, about 10 parts per billion.

But other scientists like Dr. Snellen did not find the analysis by the scientists, and the suggestions of a biological source, nearly as convincing.

The ALMA data, which recorded the brightness of light from Venus over a range of wavelengths, contained many wiggles and the one corresponding to phosphine was not particularly larger than any of the others. Dr. Greaves and her colleagues used a technique called polynomial fitting to subtract out what they believed was noise and pull out the phosphine signal. The technique is common, but they also used a polynomial with an unusually large number of variables 12.

That, critics said, could generate a false signal seeing something when there was nothing there.

If your signal is not stronger than your noise, then you just cannot succeed, Dr. Snellen said.

Other scientists contend that even if there was a signal, it was much more likely to come from sulfur dioxide, which absorbs light at nearly the same wavelength.

Dr. Greaves argued that the critics did not understand the precautions taken to rule out fake lines. She said the specific shape of the absorption line was too narrow to match that of sulfur dioxide.

As the scientists debated back and forth, there was an unexpected surprise in October: the ALMA observatory had provided incorrectly calibrated data to Dr. Greaves, and it contained spurious noise. For weeks, the Venus researchers waited in limbo.

When the reprocessed ALMA data became available in November, the noisy wiggles around the phosphine absorption line were diminished, but there now also appeared to be less phosphine about 1 part per billion over all, with places that might be as high as 5 parts per billion.

The line weve got now is much nicer looking, Dr. Greaves said, even though it was not as pronounced. But it is what it is. We now have a better result.

Bryan Butler, an astronomer at the National Radio Astronomy Observatory in Socorro, N.M., said he and others had looked at the same ALMA data, both the original and reprocessed versions, and failed to see any sign of phosphine.

They claim they still see it, and we still claim that its not there, Dr. Butler said. From a purely data scientists viewpoint, nobody is backing them up because nobodys been able to reproduce their results.

A new paper by a team of astronomers, led by Victoria S. Meadows at the University of Washington, says that a more detailed model of Venuss atmosphere developed in the 1990s shows that phosphine in the cloud layer would not even create an absorption line detectable from Earth. The team found that the phosphine would have to be some 15 miles higher in order to absorb the light. The research will be published in The Astrophysical Journal Letters.

What were showing is that the gas above basically doesnt cool to the point that it can absorb until it gets to about 75 or 80 kilometers, Dr. Meadows said. Which is well above the cloud deck.

Other scientists delved into older observations of Venus to see whether there might be signs of phosphine hidden there.

In 1978, a NASA spacecraft, Pioneer Venus, dropped four probes in the planets atmosphere. One of them even continued sending back data from the surface for more than an hour after impact.

Going back through the Pioneer Venus data, Rakesh Mogul, a professor of chemistry at California State Polytechnic University-Pomona, spotted telltale signs for the element phosphorous in Venuss clouds. There is a chemical, most likely a gas, that contains phosphorus, Dr. Mogul said. The data does support the presence of phosphine. Its not the highest amounts, but its there.

However, scientists looking at data from Venus Express, a European Space Agency spacecraft that orbited Venus from 2006 to 2014, came up empty for phosphine.

So did astronomers including Dr. Greaves and Dr. Sousa-Silva who were trying to identify a different absorption line of phosphine in infrared observations from a NASA telescope in Hawaii.

Dr. Greaves said the Venus Express and the infrared observations in Hawaii did not peer as deeply into the Venus atmosphere, and thus it should not be a surprise that they did not detect phosphine.

The levels of phosphine, if it is there, could also be changing over time.

That would make it more difficult to come up with definitive answers, much like the enduring mystery of methane on Mars. More than a decade ago, telescopes on Earth and an orbiting European spacecraft reported the presence of methane in the Martian air. On Earth, most methane is produced by living organisms, but it can also be produced in hydrothermal systems without any biology involved.

But the methane readings were faint, and then subsequent observations failed to confirm it. Perhaps the readings were misinterpreted noise. When NASAs Curiosity rover arrived on Mars in 2012, it carried an instrument that could measure minute amounts of methane. The scientists looked and looked and measured none.

But then, Curiosity did detect a burst of methane that persisted for weeks before dissipating. Later, it detected an even stronger outburst, but then it was gone again.

Mars scientists remain at a loss as to the quick appearance and disappearance of the methane.

The Venus phosphine debate will remain a stalemate until there are further observations. But the coronavirus pandemic shut down ALMA as well as NASAs Stratospheric Observatory for Infrared Astronomy, or SOFIA, a telescope aboard a modified 747 that can study infrared light from high in Earths atmosphere (The plane resumes flying this month).

The balloon that would be part of Dr. Gilmores flagship Venus mission could resolve the uncertainties by directly collecting samples of air. It would be able to find not only the phosphine but also carbon-based molecules of any microbes.

We really need to be in the clouds, Dr. Gilmore, of Wesleyan University, said, because that is the habitat that is hypothesized to support life.

Planetary scientists are in the process of putting together their once-a-decade recommendations to NASA about their priorities. There are many intriguing places to study, and NASA usually undertakes only one costly flagship mission at a time. A flagship mission also takes longer to build and one for Venus would not be scheduled to launch until 2031 at the earliest.

NASA is also considering a couple of smaller Venus missions for its Discovery program, a competition in which scientists propose missions that fit under a $500 million cost cap.

One of them, DAVINCI+, would be a 21st century version of one of the Pioneer Venus probes. It would be able to look for phosphine, although just at one place and one-time.

The second proposal, VERITAS, would send an orbiter that would produce high-resolution images of the surface. Although it does not include a phosphine-detecting instrument, one could be added.

And at least one private company, Rocket Lab, wants to send a small probe to study Venus in the coming years.

Further observations are warranted, said Dr. Butler of the National Radio Astronomy Observatory. Theres nothing you can point to that says, Oh, yeah, we absolutely see phosphine on Venus. But, you know, its tantalizing.

But he also said, I would not bet my life savings that its not there.

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Life on Venus? The Picture Gets Cloudier - The New York Times