Earth will become uninhabitable within 2-3 Gyr as a result of the movingboundaries of the habitable zone caused by the increasing luminosity of theSun. Predictions about the future of habitable conditions on Earth include adecline in species diversity and habitat extent, ocean loss and changes in themagnitudes of geochemical cycles. However, testing these predictions on thepresent-day Earth is difficult. The discovery of a planet that is a nearanalogue to the far future Earth could provide a means to test thesepredictions. 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-longperiods during the system's past, and to be approaching the inner-edge of theHZ at present. Here we assess the possibility of finding this very specifictype of exoplanet and discuss the benefits of analysing older Earths in termsof improving our understanding of long-term geological and bio-geologicalprocesses. As an illustrative example, G stars within 10 parsecs are assessedas potential old-Earth-analogue hosts. Surface temperature estimates forhypothetical inner-HZ Earth analogues are used to determine whether any suchplanets in these systems would be at the right stage in their late-habitablelifetimes to exhibit detectable biosignatures. Predictions from planetformation studies and biosphere evolution models suggest that only 0.36% of Gstars in the solar neighbourhood could host an old-Earth-analogue. However, ifthe development of an Earth-like biosphere is assumed to be rare, requiring asequence of low-probability events to occur, then such planets are unlikely tobe found in the solar neighbourhood - although 1000s could be present in thegalaxy as a whole.
展开▼
