In 1960, the first survey dedicated to the Search for Extraterrestrial Intelligence (SETI) was mounted at the Green Bank Observatory in West Virginia. This was Project Ozma, which was the brainchild of famed astronomer and SETI pioneer Frank Drake (for whom the Drake Equation is named). Since then, the collective efforts to find evidence of life beyond Earth have coalesced to create a new field of study known as astrobiology.

The search for extraterrestrial life has been the subject of renewed interest thanks to the thousands of exoplanets that have been discovered in recent years. Unfortunately, our efforts are still heavily constrained by our limited frame of reference. However, a new tool developed by a team of researchers from the University of Glasgow and Arizona State University (ASU) could point the way towards life in all of its forms!

The study that describes their findings, which recently published in the journal Nature Communications, was conducted by Prof. Leroy Cronin and his team from the School of Chemistry at the University of Glasgow, UK. They were joined by members of the Beyond Centre for Concepts in Fundamental Science at Arizona State University (ASU), and the Astrobiology Analytical Laboratory at NASA’s Goddard Space Flight Center.


Chemical space, visualized. Credit: Naomi Johnson, Lee Cronin/ASU

Central to this new tool is a concept known as assembly theory, which was developed by Prof. Leroy Cronin – a Regius Professor of Chemistry – and his colleagues at Glasgow’s School of Chemistry, with the assistance of scientists from ASU. This theory describes how living systems can be distinguished from non-living ones by identifying complex molecules that are produced in abundance (and cannot form randomly).

Applied to molecules, assembly theory identifies molecules as biosignatures based on what life does, not what it is. As Cronin explained in an ASU press release:

“Our system is the first falsifiable hypothesis for life detection and is based on the idea that only living systems can produce complex molecules that could not form randomly in any abundance, and this allows us to sidestep the problem of defining life.”

The next step was to come up with a way to quantify this complexity, which the team did by developing an algorithm that would assign a score to a given molecule. This is what is known as a “molecular assembly” (MA) number, which is based on the number of bonds needed to make the molecule. Naturally, large biogenic molecules would have a higher MA than smaller ones, or molecules that are not biogenic (large or small).

To test their method, the team used their algorithm to assign MA numbers to a database containing about 2.5 million molecules. They then used a sample subject of about 100 small molecules and small protein fragments (peptides) to verify the expected correlation between the MA number and the number of peptides a molecule would generate once exposed to a mass spectrometer – which breaks samples into pieces and analyzes the number of unique parts.


This artist’s impression shows the view from the planet in the TOI-178 system found orbiting furthest from the star. Credit: ESO/L. Calçada/spaceengine.org

In collaboration with NASA, the team also examined samples from around the globe and some extraterrestrial samples. These included a fragment of the Murchison meteorite, a carbonaceous chondrite meteorite rich in
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