In Search of Future Earths: Assessing the possibility of finding Earth analogues in the later stages of their habitable lifetimes

Abstract

Earth will become uninhabitable within 2-3 Gyr as a result of the moving boundaries of the habitable zone caused by the increasing luminosity of the Sun. Predictions about the future of habitable conditions on Earth include a decline in species diversity and habitat extent, ocean loss and changes in the magnitudes of geochemical cycles. However, testing these predictions on the present-day Earth is difficult. The discovery of a planet that is a near analogue to the far future Earth could provide a means to test these predictions. Such a planet would need to have an Earth-like biosphere history, requiring it to have been in its system's habitable zone (HZ) for Gyr-long periods during the system's past, and to be approaching the inner-edge of the HZ at present. Here we assess the possibility of finding this very specific type of exoplanet and discuss the benefits of analysing older Earths in terms of improving our understanding of long-term geological and bio-geological processes. As an illustrative example, G stars within 10 parsecs are assessed as potential old-Earth-analogue hosts. Surface temperature estimates for hypothetical inner-HZ Earth analogues are used to determine whether any such planets in these systems would be at the right stage in their late-habitable lifetimes to exhibit detectable biosignatures. Predictions from planet formation studies and biosphere evolution models suggest that only 0.36% of G stars in the solar neighbourhood could host an old-Earth-analogue. However, if the development of an Earth-like biosphere is assumed to be rare, requiring a sequence of low-probability events to occur, then such planets are unlikely to be found in the solar neighbourhood - although 1000s could be present in the galaxy as a whole.