Astronomers have faced a quiet but persistent challenge for decades. The usual strategy for finding life beyond Earth is to analyze exoplanet atmospheres for gases such as oxygen, methane, and ozone, which are hard to explain without biology. The idea is smart, but it has a major limitation. This checklist is based entirely on Earth, so it effectively searches for life that resembles our own.
Meanwhile, the number of ways non-biological chemistry can imitate these so-called biosignature gases is growing quickly. Each new false positive requires additional planetary data to rule it out, raising doubts about whether we can ever gather enough information to be certain. Despite sixty years of research in astrobiology, the basic approach to biosignatures has changed very little.
Sara Walker, a professor of astrobiology at Arizona State University, and her colleagues are working to address this issue. Their solution is based on assembly theory, which takes a fundamentally different approach.
Assembly theory shifts the focus away from identifying specific molecules. Instead, it considers how difficult those molecules are to form. Each molecule is assigned 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 highly complex ones that require many steps are unlikely to appear without some form of selection.
If an atmosphere contains many molecules that are extremely difficult to produce randomly, and if those molecules show strong chemical connections, such as sharing and reusing fragments while exploring many possible bond combinations, then something beyond standard chemistry may be involved. According to the theory, that process is very likely life.
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