SETI Talks: Is Oxygen Really a Biosignature?

Biosignatures, the footprint of life on other planets, or exoplanets, could be the easiest way for astronomers to prove that we are not alone, and oxygen has been for decades the top contenders in proving that life exists elsewhere. But how truly useful is oxygen as a biosignature?

In Earth’s history, oxygen is the byproduct of photosynthesizers such as plants, algae and cyanobacteria. With the fast growth of the field of exoplanets and the arrival of future extremely large telescopes and space-based telescopes that could soon find oxygen signatures, or its light-modified form, ozone, are we on the verge of finding life on an exoplanet?

Scientists have been busy modeling exoplanet formations, chemistry and their atmosphere, and as a byproduct of this work, they succeeded in poking holes in the atmospheric oxygen-means-life scenario. We invited two scientists to discuss the discovery of potential false positives for atmospheric oxygen as a biosignature. Jade Checlair is a planetary scientist at the University of Chicago who models the atmosphere and climate of habitable exoplanets and will show how the observation of oxygen on a large number of exoplanets would allow astronomers to discover true Earth-like exoplanets. Joshua Krissansen-Totton, an astrobiologist at the University of California, Santa Cruz, models the atmospheric evolution of rocky planets to anticipate exoplanet biosignatures and their false positives.

Hypothetical scenarios have been proposed for non-biological oxygen accumulation on exoplanets, especially around M-dwarf stars, as oxygen remains behind as hydrogen escapes the atmosphere of their planets. The researchers will also discuss numerous other ways that planet atmospheres can be filled with oxygen from a non-biological source.

In this bleak environment, there is still hope. Oxygen false positives could, in principle, be distinguished from inhabited planets using contextual clues and corroborating signs of life. But the task may not be straightforward, and proving beyond reasonable doubt that there is life on exoplanets could require telescopes much more advanced than anything currently being built.

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