Asked by Stuart Rowe

Biological processes are thought to be a major contributing factor to the levels of atmospheric oxygen on Earth. We therefore often think of oxygen as a biosignature – a chemical marker in the atmosphere that’s closely associated with the presence of life.

Oxygen is highly reactive, so a large amount of the element in an exoplanet’s atmosphere suggests that there’s likely a significant oxygen producing process that’s ongoing. Although this could be attributed to life, there are a number of abiotic processes that can produce oxygen. As such, studying oxygen levels alone is unlikely to be enough unless combined with other gases. Scientists think that finding high levels of both oxygen and another biosignature, such as methane, may be a much better indicator.