If we discovered evidence of alien life, would we even realise it? Life on other planets could be so different from what we’re used to that we might not recognise any biological signatures that it produces.

Recent years have seen changes to our theories about what counts as a biosignature and which planets might be habitable, and further turnarounds are inevitable. But the best we can really do is interpret the data we have with our current best theory, not with some future idea we haven’t had yet.

This is a big issue for those involved in the search for extraterrestrial life. As Scott Gaudi of Nasa’s Advisory Council has said: “One thing I am quite sure of, now having spent more than 20 years in this field of exoplanets … expect the unexpected.”

But is it really possible to “expect the unexpected”? Plenty of breakthroughs happen by accident, from the discovery of penicillin to the discovery of the cosmic microwave background radiation left over from the Big Bang. These often reflect a degree of luck on behalf of the researchers involved. When it comes to alien life, is it enough for scientists to assume “we’ll know it when we see it”?

Many results seem to tell us that expecting the unexpected is extraordinarily difficult. “We often miss what we don’t expect to see,” according to cognitive psychologist Daniel Simons, famous for his work on inattentional blindness. His experiments have shown how people can miss a gorilla banging its chest in front of their eyes. Similar experiments also show how blind we are to non-standard playing cards such as a black four of hearts. In the former case, we miss the gorilla if our attention is sufficiently occupied. In the latter, we miss the anomaly because we have strong prior expectations.

Read more: The Conversation