A scientific paper claims to have found an "apparent presence" of the gas phosphine in the atmosphere of Venus, but only in the barest trace amounts (20 parts per billion). Since phosphine is a gas that is mainly produced on our planet by living things, some are hailing this as evidence of life in the atmosphere of Venus. Some sources in the science press (always prone to hype and exaggerate doubtful or inconclusive research) are making this report sound like something epic. A Dr. Dena Grayson incorrectly states, "Scientists assert that something now alive is the only explanation for the chemical’s source." But the paper does not provide compelling evidence for life at Venus, and in the paper the scientists did not at all make such a claim.
There are two alternate explanations here that do not require us to believe in life on Venus.
(1) An error in interpretation could have occurred in spectral data that is hard-to-interpret because of overlapping signals from a variety of different gases in the atmosphere of Venus.
(2) There could be a non-biological reason why phosphine appeared in the atmosphere of Venus.
The first of these two possibilities is not very unlikely. For many decades scientists have used a device called a spectograph to detect elements in distant stars and planets. When light passes through a spectograph in something rather like light passing through a prism, an output visual called a spectrogram is produced. In such a spectrogram there may be particular lines that are caused by the presence of particular elements in the astronomical target. In a simple case, such lines are easy to interpret. But in a case when there are many gases and elements in a distant target, the spectrogram can be complex and hard-to-interpret. For example, here is a spectrogram obtained when the sun was the target:
When there are all those tiny little lines, the spectrogram can be very hard to interpret. We may assume that an equally complicated spectrogram was obtained using a target of Venus. In such cases there is a large possibility of misinterpreting the little lines. The authors claim to have detected phosphine after analyzing some complicated spectrogram, but they may have erred in their interpretation. Such an error is all too possible when you are merely claiming the existence of borderline traces such as 20 parts per billion.
The possibility of such an error seems real after you review a scientific case such as the BICEP2 affair. In that case a group of scientists declared they had discovered proof of primordial cosmic inflation in some hard-to-interpret readings from distant space. For weeks the science press crowed about this apparent triumph. But eventually scientists realized that the claim was not solid, and that what was observed could easily have been caused by mere dust. One of the BICEP2 scientists wrote a book called "Losing the Nobel Prize" about the missteps and premature celebrations that occurred.
We should also remember the case in which some NASA scientists in the 1990's declared that they had detected evidence of life from Mars. Their claim was based on subjective interpretations of debatable anomalies in a meteorite. Other scientists did not agree with this doubtful interpretation, and the claim is now generally regarded as no solid evidence of life on Mars. We should always remember that a scientist eager for "discovery glory" may be biased towards interpretations that suggest he has discovered something important.
The Venus paper authors refer to their "candidate discovery" of phosphine in the clouds of Venus, which does not sound like great confidence. Given their claimed detection of something existing in such marginal traces, we should not at all be surprised if some later paper by different scientists (analyzing the same data) claims that phosphine was not actually detected at Venus.
Another possibility is that phosphine was actually detected in the atmosphere of Venus, but that it is phosphine that arose because of non-biological reasons. Chemically phosphine (PH3) is a very simple compound. It consists merely of one phosphorus atom and three hydrogen atoms. So simple a compound could in theory be created by any number of non-biological processes. Since we do not understand very well the geology of Venus, we cannot exclude geological processes on Venus that might produce phosphine.
A 2014 paper claimed to have detected phosphine (PH3) in the atmosphere of a distant star. The paper states, "The detection of PH3 challenges chemical models, none of which offers a satisfactory formation scenario." Apparently phosphine can arise for non-biological reasons around a distant star, for reasons we don't understand. It is therefore not very unlikely that it might arise at Venus for non-biological reasons, for reasons we don't understand. Since it is almost infinitely easier for you to get by chance processes a simple molecule like phosphine than even the simplest living thing, a non-biological origin for phosphine at Venus would seem like a more plausible explanation. A 2006 paper also claimed to have detected phosphine in the atmosphere of a distant star. The paper suggests this non-biological origin: "In the case of hydrides such as PH3, a likely formation process is the direct hydrogenation of the heavy atom taking place on grain surfaces."
Another scientific paper claims that we should expect to find phosphine in the atmospheres of large planets and "hotter objects":
"Disequilibrium abundances of phosphine (PH3) approximately representative of the total atmospheric phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P2, PH3, PH2, PH, HCP) become increasingly important at high temperatures."
The scientists who claim to have found phosphine at Venus claim that they have wracked their brains searching for a non-biological source of phosphine, without finding one. But we should not at all be persuaded by such a failure. Scientists in their position who have a motivation not to find a non-biological source of phosphine may not think of such a source. But give me a scientist motivated to find such a source, and he may think of 100 possible sources of phosphine. We can expect to see future papers describing possible non-biological sources of phosphine in Venus, written by authors motivated to think of such sources (for the sake of publishing another paper).