Doubt has been cast on the supposed discovery of phosphine in the atmosphere of Venus after several papers were published on the arXiv preprint server challenging the result. The discovery had been announced in September when a team of researchers led by Jane Greaves of Cardiff University, UK, claimed it had observed the spectral fingerprint of phosphine (PH3) in the clouds of Venus. If true, the paper would have been our strongest evidence yet of life beyond Earth, but the tone of some of the resulting criticism – as well as a surprising statement from an international body over the press coverage of the work – has outraged astronomers.
Phosphine – a potential biosignature – is created in the high temperatures and pressures within the interiors of Jupiter and Saturn, but on Earth it is only produced by anaerobic microbial life. To detect phosphine on Venus, the researchers used the James Clerk Maxwell Telescope (JCMT) in Hawaii and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
As John von Neumann once said: with four parameters I can fit an elephant and with five I can make him wiggle his trunk
Mark Thompson
Shortly after the announcement, however, the organizing committee of the International Astronomical Union (IAU) Commission F3 on Astrobiology released a statement lambasting Greaves’ team for the resulting press coverage of the claimed discovery. “It is an ethical duty for any scientist to communicate with the media and the public with great scientific rigour and to be careful not to overstate any interpretation which will be irretrievably picked up by the press,” they wrote, adding that the commission “would like to remind the relevant researchers that we need to understand how the press and the media behave before communicating with them”.
The IAU statement was met with scorn from many quarters, including the commission’s own members, many of whom said the organizing committee did not speak for them. The statement was then swiftly retracted by the IAU executive, who insisted that it did not reflect the view of the organization. In its own statement, the executive added that the organizing committee of Commission F3 had “been contacted to retract their statement and to contact the scientific team with an apology”. The IAU said it will now produce a procedure for future public communication that all members will be advised to follow.
Wait and see
The dust had barely settled from that brouhaha when a paper entitled “No phosphine in the atmosphere of Venus” was submitted to Nature Astronomy’s “Matters Arising” section. It argued that Greaves’ team had misidentified the absorption line of sulphur dioxide in Venus’ atmosphere as that of phosphine. Written by a group led by Geronimo Villanueva of NASA’s Goddard Space Flight Center, the team ended its abstract with the “suggestion” that Greaves’ team retract its original paper – seen by some as unduly aggressive.
The furore around this paper led to an apology by Villanueva’s team. “We agree that the sentence calling for retraction was inappropriate and we apologise for harm caused to the Greaves et al. team,” the team notes in a statement. It adds that the specific language had been used because of a misinterpretation of the guidelines issued by Nature Astronomy, which had encouraged Villanueva’s team to post the preprint for public discussion.
Mark Thompson, an astrophysicist from the University of Hertfordshire who has written his own critique of the phosphine discovery, agrees that the statement from the organizing committee of Commission F3 and the abstract to Villanueva team’s paper overstepped the mark. However, he thinks that the IAU statement has a point, agreeing that the fallout “comes in large part from the over-hyping of the result by some parts of the press”.
The key argument of Thompson’s paper is how Greaves and colleagues calibrated their data. The absorption line of phosphine appears against a bright continuum of thermal emission from Venus, which forms a baseline in the spectrum. This baseline has to be removed, which is normally a simple process of subtraction, but when the baseline emission is complex, as is the case with Venus, the baseline needs to be fitted as a higher-order polynomial before being subtracted.
However, the higher the polynomial, the more parameters and assumptions there are. Greaves’ team used a 12th-order polynomial, which is considered exceptionally high and often comes with unintended consequences, potentially including the creation of false positives. “As John von Neumann once said, ‘with four parameters I can fit an elephant and with five I can make him wiggle his trunk’,” says Thompson. Has evidence of life been found in the clouds of Venus?
Although Greaves’ team justified its choice of using a high-order polynomial in order to address “ripples” – instrumental artefacts in the data that become apparent when observing an object as bright as Venus – Thompson says that he does “worry that the resulting absorption feature may be the elephant wiggling its trunk instead of being a true detection”.
Similarly, a team led by Ignas Snellen of Leiden Observatory has also come to the same conclusion about the baseline calibration. Whereas Greaves’ team reports the phosphine absorption line as being 15 times stronger than the surrounding noise, “What we see is that the strength of the feature is about a factor of 2 higher than the noise, which is below statistical significance,” Snellen told Physics World.
For now, the phosphine paper is the only set of results that has undergone peer review. Thompson encourages discussion to now wait until the arXiv papers have been similarly vetted and Greaves’ team have conducted their own re-analysis. If, after all that, the results are still in dispute, then it may be that new observations at different frequencies are required. Phosphine is difficult to detect from the ground but NASA’s airborne Stratospheric Observatory For Infrared Astronomy telescope, which flies at an altitude of over 13.7 km on board a modified Boeing 747SP, could confirm or deny the finding.
