The complexity of molecules can determine unambiguously whether they were made by living organisms or not, say researchers – a discovery that could have profound implications for the search for alien life.

The team led by Lee Cronin of the University of Glasgow, UK, has devised a measure of molecular complexity called the molecular assembly index (MA), and used it to study a variety of samples, including one from a meteorite that fell to Earth in the 1960s.

Their research suggests that complex molecules created by biological organisms have an MA of 15 or greater, and can be distinguished from molecules created by non-living processes, such as volcanoes or atmospheric chemistry, which typically have an MA of 12 or lower. The method could be used to analyse material samples from the solar system or the light from distant exoplanets to tell unambiguously which contain evidence of biosignatures – chemicals produced by living organisms – regardless of the chemistry they are based on.

More: Molecular complexity index could help find proof of alien life