Tidally driven tectonic activity as a parameter in exoplanet habitability

Context. The habitability of an exoplanet is defined by its ability to support surface liquid water. The long-term carbon cycle plays an important role in regulating planetary temperature, thus supporting the conditions for the preservation of surface liquid water and, consequently, the habitability of exoplanets. Aims. On Earth, plate tectonics play an integral role in driving the long-term carbon cycle; however, on tidally locked rocky exoplanets, alternative tectonic mechanisms driven by tidal stress and tidal heating could serve this purpose in an analogous way. Methods. We calculated tidal stress and tidal heating rates to model the likelihood of tectonic activity maintaining stable climates suitable for surface liquid water on tidally locked rocky exoplanets with radii of R-p <= 1.23 R-circle plus. Results. Applying the tidal models to our sample of 767 tidally locked rocky exoplanets reveals that similar to 10% of exoplanets, including Proxima Cen b and GJ 1061 d from the circumstellar habitable zone (CHZ), pass the tidal stress subduction threshold for mobile lid tectonic activity and simultaneously reside within the optimal tidal heating zone. This subset of exoplanets could sustain tidally induced temperate mobile lid tectonic activity that is comparable to plate tectonics on Earth, aiding in maintaining the presence of surface liquid water. Furthermore, similar to 40% of exoplanets from our sample located in the CHZ would be unable to maintain the tectonic activity needed to stabilise the climate and are unlikely to retain surface liquid water. When broadening our modelling to establish the overlap between tidal stress, tidal heating, and the CHZ to discover optimal regions to target for future observations, we determined that tidally driven tectonic activity conducive to the maintenance of surface liquid water occurs predominantly around M dwarfs. We identified intersections, where both mobile lid and optimal tidal heating could be sustained on eccentric (e > 0.1) Earth-sized exoplanets (R-p = 1.0-1.23 R-circle plus) orbiting in the CHZ of low-mass M dwarfs.

Automated summary

This study explores the role of the long-term carbon cycle on tidally locked rocky exoplanets in maintaining surface liquid water and supporting habitability. By calculating tidal stress and tidal heating rates, the study finds that about 10% of the exoplanets can sustain mobile lid tectonic activity, similar to plate tectonics on Earth, and retain surface liquid water. However, about 40% of the exoplanets in the sample would be unable to maintain the necessary tectonic activity. Furthermore, the study identifies optimal regions for future observations, with tidally driven tectonic activity predominantly occurring around M dwarfs.