Here we go again. A team of scientists (led by Professor Jane Greaves of Cardiff University) has discovered possible evidence of life on another planet.

In this case, the planet is Venus and the ‘evidence’ is the detection of phosphine (PH₃) — a gas associated with microbial activity.

In theory, the chemistry of PH₃ is such that a biological process would be required to produce it naturally in detectable quantities. In other words, it is a ‘biosignature’ gas — or, less politely, a fart.

Just as there is no smoke without fire, there are no farts without life, which is why detecting phosphine in the Venusian atmosphere is such a big deal.

Of course, all this assumes that the theory is right and that the gas can only have had a biotic origin. So it’s worth pointing out that phosphine has already been detected on Jupiter (and Saturn) without anyone pointing the finger at extraterrestrials. However, unlike an ultra-cold gas giant, Venus is hot and rocky — and thus some hitherto unsuspected chemical process would be required to produce the phosphine abiotically (i.e. without life).

So is life the simpler explanation? No, because we’d need some hitherto unsuspected biological process to explain its presence on Venus.

The planet was named after the Roman goddess of love, but it is anything but lovely. The mean surface temperature is a toasty 464°C and the atmospheric pressure 92 times that of Earth. A day lasts longer than a year; and, to cap it all, it rains sulphuric acid. Not the most promising place to look for life, then. The upper atmosphere is less hellish, so maybe microbes could exist there; but that would still require an explanation of how life came to evolve in such an unlikely environment. Bear in mind that we still don’t know how it evolved on Earth.

The Venus phosphine is an amazing discovery — and one that requires explaining. But if we apply Occam’s razor to the question, a purely chemical explanation is still more likely than a biological one.

And don’t forget we’ve been here before. In 1996, there was the supposed discovery of ‘microfossils’ in a Martian meteorite. That didn’t come to very much — and it’s unlikely that this discovery, as interesting as it is, will either. Those who want to believe are setting themselves up for more disappointment.

By the way, don’t confuse phosphine with phosphene. The latter is refers to a kind of visual hallucination — i.e. seeing something that isn’t actually there.