👽 Assembly Theory: a new path to detecting extraterrestrial life

The search for life on other planets relies, for lack of a better alternative, on what we know on Earth. However, this situation could make us blind to radically different forms of life, thus limiting our possibilities of discovery.

For decades, astronomers have been analyzing the atmospheres of exoplanets in search of gases like oxygen or methane. These molecules, when present in large quantities, are considered potential signs of biological activity. However, this strategy rests on the assumption that extraterrestrial life functions in a manner similar to our own. Furthermore, non-biological chemical processes can sometimes produce these same gases, making interpretations delicate and subject to controversy.


To go beyond these limits, a team of researchers led by Sara Walker is proposing a radically new approach based on "Assembly Theory". This theory, developed in astrobiology, does not focus on the presence of specific molecules, but on the overall richness of atmospheric chemistry. The idea is to quantify how difficult it is to form the diversity of observed molecules, thus offering a more universal criterion less tied to our own terrestrial bias.

Each molecule is assigned an assembly index, which corresponds to the minimum number of steps required to build it from simple chemical components. Simple molecules can appear by chance, but those that are very elaborate and require many steps are unlikely without a selection process.

If an atmosphere contains a great diversity of molecules with a high assembly index, and if these molecules show close chemical connections, this could indicate the presence of a form of life, or even technology, without presuming its exact nature.

Applying this method has allowed scientists to compare Earth's atmosphere to those of Venus, Mars, and models of exoplanets. They found that Earth's atmosphere exhibits a far greater chemical richness, independent of any observational bias. For example, although Earth and Venus have access to a similar range of chemical bonds, Earth displays a much more diverse chemical environment, likely due to its active biosphere, clearly distinguishing it.


This approach is particularly suited to future space missions, like NASA's Habitable Worlds Observatory, which aims to directly image Earth-like planets. Instead of giving a binary answer, Assembly Theory would provide a chemical richness score, placing planets on a spectrum ranging from abiotic to biotic. This would help avoid overly simplistic interpretations, relying on techniques like infrared spectroscopy already used by space telescopes.

Assembly Theory, by freeing itself from terrestrial preconceptions, thus paves the way for a more inclusive search for life. It assumes that the Universe, nearly fourteen billion years old, could have experimented with many chemical pathways leading to life. It considerably broadens our horizons in the quest for life among our cosmic neighbors, without imposing a single model based on our own Earth.