One of the groups used archival observations to search for signs of gas in the planet’s clouds, and could not find it.
“They’re saying they don’t see phosphine. It’s really problematic,” says Connor Nixon, a planetary scientist at NASA’s Goddard Space Flight Center, who did not participate in the analysis. The study has been peer-reviewed and Accepted for publication Astronomy and Astrophysics.
Two other groups reproduced the original data from the discovery team and also found no evidence for phosphine.
But detecting a faint signal from a specific molecule on another planet is a complex process, and the original study authors are not surprised that other scientists are taking a closer look at their work.
“This is normal. It looks like science. If it was data that you could just see with the naked eye and look at phosphine, it would have been discovered long ago,” a Harvard-Smithsonian of the authors Clara Susa-Silva of the Center for Astrophysics says. “I’m so relieved that people are finally looking at this data and it’s not just us.”
A shocking discovery
The team identified phosphine using two devices that inspected radio waves. First, in 2017, Jane Greaves of Cardiff University and her colleagues discovered what phosphine could be with the James Clerk Maxwell Telescope (JCMT) in Hawaii. But that observation needed to be confirmed, and in 2019, the team turned to a more powerful tool: the Atacama Large Millimeter / Submillimeter Array (ALMA), a network of 66 radio dishes in the high desert of Chile.
Still some scientists were suspicious. At the time, ALMA Observatory scientist John Carpenter questioned the way the original team’s scientists analyzed the data, suggesting that their process could produce serious signals.
Additionally, astronomers typically look for multiple spectral lines created by a single molecule to confirm their presence — something the team did not have.
“What exactly is the spectral line, and is it important?” Nixon Miracle. “If there’s a line, is it phosphine?” And if so, is this life? “
Double check with another telescope
Those observations, from 2015, do not show a strong signal from phosphine. Published by Theres Encras of the Paris Observatory, the study’s authors conclude that the data placed an upper limit on the level of phosphine in the Venus atmosphere which is one-fourth of the originally known amount. Observations also suggest that any phosphine would need to be at an altitude above the planet’s clouds, which astronomers think would be unlikely because the gas would break down quickly.
Susa-Silva points to several possible explanations for phosphine deficiency in infrared observations. The amount of phosphine may have varied over time, or infrared observations would not have examined the clouds in depth so that the gas could be detected at the reported levels. Even now, the team does not necessarily consider what elevation infrared observations measure.
“I believe that Enkrenaz’s work, and therefore there is no phosphine -” Sosa-Silva says. “It’s just, where is it?” What is the height we are seeing? And this means that we are investigating quite deeply, and there is no phosphine because it never was? Does this mean that there is no phosphine because it is variable? Or does it mean that we didn’t investigate as deep as we thought? “
Another look at the data
The first group, which included more than two dozen researchers, failed to find evidence for phosphine in both JCMT and ALMA data. The JCMT detected a spectral line at the correct frequency, but the team suggests that this could be explained by the sulfur dioxide gas in the Venusian atmosphere at the same location to generate the spectral line.
“It’s a famous gas on Venus,” Nixon says. “Not controversial at all.”
The data from ALMA, which produces extremely high-resolution observations, were more complex to analyze. Bright, nearby objects such as Venus can cause problems for ultra-sensitive telescope arrays such as ALMA. To draw a signal from Venus observations, astronomers had to remove radio noise produced by the Earth’s atmosphere, Venus, and even the instrument in the observatory.
“This is a very difficult data deduction,” says Brian Butler of the National Radio Astronomy Observatory, which studies solar system objects using ALMA and works on new analysis. “Venus is a very bright object, it is large, and even their detection of the line, whether it was real, is still a faint line.”
To make matters more complicated, the ALMA Observatory recently identified an error in its calibration system, which produced a spectrum of Venus with too much noise to work with Greaves and his colleagues. “This data is messy, and noisy, and delicate,” Susa-Silva says. (ALMA has removed the original Venus data from the archive and is now reproducing it.)
using the A technique called polynomial fittingThe original search team mathematically observed background noise around the region in the spectrum for the spectral line of phosphine, where phosphate should be. In theory, this type of analysis allows astronomers to infer which parts of observations are noisy and which actual signals are. Once the team smoothed the spectrum to remove excess noise, astronomers concluded that the phosphine signal was important enough to allow it to be detected.
But other astronomers suspect the team’s data processing. To pull the phosphene signal from the mess data set, the team subtracted it using background noise A higher order polynomial, Meaning that more variables were used to make the data specific. In addition, the team created background noise by looking at portions of the spectrum outside the region where they expected to receive a signal from phosphine – a method that usually prevents unknown noise from obscuring the potential signal. However, combining a higher-order polynomial with a noisy data set makes it possible To artificially create a false signal Where there will be phosphine.
“You can always improve a dataset by adding a variable, but you have to define a metric that tells you when to stop,” says Meredith McGregor, an astronomer at the University of Colorado, Boulder. “At some point, you.” End fitting noise and bloating signals that are not real. “
Butler downloaded the ALMA data and started from scratch, redrawed some initial fragments, and then processed the data as it normally would. He did not find any evidence of phosphine in the planet’s spectrum.
Butler says, “I have just used in my experience the best practice to reduce this kind of data.” “If you don’t do that then they do it, you don’t get this facility [of phosphine]. “Additionally, when they treated the data using the same methods as the original search team, they found that polynomial fitting produced false spectral lines.
Another analysis of ALMA data, led by Ignas Snellen and colleagues from the Leiden Observatory, also failed to find any signs of phosphine. Similarly, that team pointed out that higher-order polynomial fitting can create many spectacular spectral lines.
“They’ve shown that this fitting process can be really problematic,” Nixon says. “It is very capricious and it can produce features as easily as it can remove them. In the end, you are not really sure what you are seeing. “
Greaves and his team declined to comment on the new analysis of the ALMA observations until the observatory had a chance to reproduce the data.
A constant search
These attempts to confirm the discovery of phosphine are exactly about how science works, which many astronomers say. Independent replication is not as common as it should be, although it is an important part of verifying searches. A final determination of the presence of phosphine on Venus will have to wait for new analyzes to be peer-reviewed and published – and then scrutinize itself – and perhaps for additional observations of the planet.
“We need follow-up comments so we’re not relying on some, very noisy data sets,” Sosa-Silva says. “The lesson is a push for more analysis and more data.”
Over time, researchers are confident that they will get to the bottom of the phosphine secret. “I think science is self-correcting, and ideally, in this day and age, the Internet and so on, we don’t take years to self-correct things,” Nixon says.
Extraordinary claims require extraordinary evidence. “If this result is incorrect,” Butler says, “it will not be the first.”