Is there really phosphine on Venus?

EXTRAORDINARY CLAIMS require extraordinary evidence. So goes the dictum, usually credited to Carl Sagan, a celebrated astronomer, on the need for caution when interpreting radical new ideas in science. And there are few claims more extraordinary than that of the discovery of life beyond Earth.Jane Greaves of Cardiff University, in Britain, has not actually made that claim. But she came close to it when, in September, she and her colleagues published research that appeared to show the existence of a gas called phosphine in the clouds of Venus. This substance, a compound of phosphorus and hydrogen, should be able to survive only briefly in an atmosphere like that of Venus. But Dr Greaves’s team reported that it actually seemed to be persistent there, at a concentration of 20 parts per billion. This turned heads because, on Earth, the minuscule amounts of phosphine around have only two sources: chemists and microbes. The former are surely absent from Venus, so the question became whether there was a plausible, natural, but non-biological explanation for the gas being there. Neither Dr Greaves nor anyone else has yet come up with one, so that leaves open the tantalising possibility that it is a sign of life on the planet.