A landmark study published this week found a chemical suspended in the clouds of Venus that could be early proof of life on a planet previously thought to be uninhabitable.
The study is making waves in the scientific community, raising questions about where -- and how -- life could reside on a planet written off for its barren surfaces, high pressures and temperatures of nearly 900 degrees Fahrenheit. Skeptics say the study is intriguing, but it will take a lot more research to make a compelling case for life on Venus.
The study, published Monday, used two telescopes on earth to observe what appears to be phosphine gas in Venus' atmosphere. Phosphine is considered a biosignature, meaning its presence indicates the presence of life, and researchers have scratched their heads trying to figure out what could be producing the gas other than some kind of living microbes.
The discovery is a big surprise, in part because Venus had been mostly written off as a candidate for extraterrestrial life, said Seth Shostak, a senior astronomer at the SETI institute in Mountain View. NASA has spent billions of dollars sending high-tech hardware to Mars, in no small part because the planet was believed to be a candidate for life, only for a potential biological discovery on Venus. It's not clear whether the researchers were even originally looking for phosphine or expected a biomarker to appear.
While the "life" in this case is likely something like anaerobic bacteria suspended in the Venusian atmosphere, Shostak said the implications are huge. You can make a compelling case that life outside of Earth is not only probable but extremely likely if Earth's next-door neighbor has living microbes hanging out in the atmosphere.
"It means life got started on Venus, not just Earth but the nearest planet to us," he said. "You can at least safely conclude that life is extremely common. There are 1 trillion planets in the Milky Way, and this suggests we're not that special."
It also raises some interesting theories about where the microbes came from. The current belief is that Venus previously had oceans that have since boiled off, Shostak said, and it's possible that microbes from a previously hospitable world were able to evacuate and take refuge in the mild temperatures of the planet's clouds.
Some researchers say the study, while interesting, isn't yet compelling proof of life. During a Facebook Live broadcast hosted by SETI, astrobiologist Nathalie Cabrol said there are nonbiological sources of phosphine out there, including volcanic activity. She wondered whether the unique temperature and pressure of Venus could interact with magma in a way that results in phosphine gas.
"I am just like anybody else, I am extremely excited about the prospect of a biosignature on Venus," Cabrol said. "But I have to look at the body of evidence, and phosphine has been produced in very, very low amounts on earth by volcanism."
Cabrol also questioned the idea that the atmosphere of Venus is a stable environment with conditions suited for life to thrive.
"It would take something absolutely special for life to be suspended there," she said. "I'm not saying it's not possible, but I need a lot more observation to start believing that we have a stable, habitable environment in the atmosphere."
Chris McKay, an astrobiologist at NASA Ames in Mountain View, said the results are interesting but only the first step. There needs to be more proof that phosphine is actually present in the atmosphere -- bolstering the evidence collected by the two telescopes used in the study -- and there needs to be a workable theory that any part of Venus is inhabitable.
While the case does a good job ruling out non-biological sources of phosphine, McKay said there's an equally compelling argument that the atmosphere is anything but suitable for life. The purportedly habitable cloud layers have low water activity and are composed of high concentrations of sulfuric acid, creating an environment in which nothing can live.
"Basically we have no coherent theory for how phosphine could be present on Venus," he said. "So if phosphine is confirmed, it is interesting. It would mean that something unexpected is happening on Venus even if it's not biological."