Researchers propose examining the atmospheric complexity of exoplanets using assembly theory, rather than relying solely on familiar gases like oxygen and methane. The new approach could enable a less Earth-centric search for life in the universe.
Astronomers have faced a quiet but persistent challenge for decades. The standard strategy for finding extraterrestrial life is to analyze the atmospheres of exoplanets and look for gases like oxygen, methane, and ozone, which are hard to explain without biology. The idea is clever, but it has a big limitation. The checklist is based entirely on Earth, so what you’re really looking for is life similar to our own.
Meanwhile, the number of ways in which non-biological chemistry can mimic these gases, known as biosignatures, is growing rapidly. Each new false positive requires new data on the planet to rule it out, and it is questionable whether we will ever be able to gather enough information to be sure. Despite sixty years of research in astrobiology, the basic approach to biosignatures has changed very little.
Professor Sarah Walker and her colleagues are working to solve this problem.
Assembly Theory: A New Framework for Discovering Life
Assembly theory shifts the focus away from identifying specific molecules and instead looks at how difficult it is to make those molecules. Each molecule is given an assembly index, which represents the minimum number of steps needed to build it from simple chemical components. Simple molecules can form by chance, but very complex molecules that require many steps are unlikely to arise without some form of selection.
If an atmosphere contains many molecules that are very difficult to create randomly, and if these molecules show strong chemical bonds, such as sharing and reusing segments while exploring many possible combinations of bonds, then something beyond ordinary chemistry may be involved. According to the theory, this process is very likely life.
It is important to note that the theory assumes nothing about the nature of that actual life. It does not presuppose a particular metabolism, biochemistry, or molecular mechanism. In the researchers' own terms, it is agnostic about the specific realization of life. It only suggests where life might exist.
The complexity of Earth's atmosphere compared to other planets
When comparing Earth's atmosphere to Venus, Mars, and various archetypes of extrasolar planets, Earth's atmosphere stands out as the most complex by this standard, independent of any observational bias. Earth and Venus have a similar range of chemical bonds available to them, but Earth's atmosphere contains a much greater molecular diversity above any given abundance threshold. Earth's biosphere seems to allow for a much more exhaustive examination of chemical possibilities than is possible on Venus.
The framework is designed with the Living Worlds Observatory in mind, NASA’s next flagship telescope, which was specifically chosen to directly image Earth-like planets and search their atmospheres for signs of life. Rather than returning a simple dead-or-alive verdict, an analysis based on assembly theory would produce a continuous complexity score, placing planets on a spectrum from purely abiotic to richly biotic, potentially capturing the gradual transition between them rather than requiring a hard boundary.
Moreover, unlike many theoretical biosignature frameworks, this framework can be measured directly. Composition values can be calculated from EE spectroscopy, the same technique that space telescopes use to read distant atmospheres. The universe has had almost fourteen billion years to experiment with chemistry. The assumption that it has ever arrived at just one solution for life seems, on second thought, a very Earth-centric gamble.
for the scientific article DOI:10.48550/arXiv.2603.11086.
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