How probable is Gaia?

Abstract Astronomers are now able to peer into the atmospheres of distant planets and search for signs of life. With the large diversity in exoplanets being found, confidently identifying remote signs of life is a complex process. E.g. while the high level of oxygen in Earth’s atmosphere is the product of life, different planetary scenarios can produce oxygen rich atmospheres without any life-processes. Therefore we will need multiple lines of evidence to confidently declare a planet inhabited. Earth has been dramatically shaped by life for over 4 billion years and life plays a key role in regulating Earth’s surface chemistry and climate, providing multiple detectable signals a remote observer would be able to detect from afar. The biosphere-Earth co-evolving system is known as ‘Gaia’. Understanding the likelihood of Gaian-systems emerging on inhabited planets will inform us on the probability of confidently identifying an inhabited planet.