In the past few decades, several phenomena have led to excited speculation in the scientific community that they might indeed be indications that there is extraterrestrial life. It will no doubt happen again.
Recently, two very different examples sparked excitement. In 2017, it was the mystery interstellar object ‘Oumuamua. And in 2021, it was the possible discovery of the gas phosphine in the clouds of Venus.
In both cases, it seemed possible that the phenomenon indicated some kind of extraterrestrial biological source. Notably, physicist Avi Loeb from Harvard University argued in favour of the oddly shaped ‘Oumuamua being an alien spaceship.
And phosphine in the atmosphere of a rocky planet is proposed to be a strong signature for life, as it is continuously produced by microbes on Earth.
These are just two of the latest cases of a long list of examples of such initially promising phenomena. But although a few of the examples are still controversial, most have turned out to have other explanations (it wasn’t aliens).
So how can we be sure we’ve come to the right conclusion for something as subtle as the presence of a certain gas or a strange looking space rock?