Tidally driven tectonic activity as a parameter in exoplanet habitability^★

¹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2601, Australia
e-mail: sarah.mcintyre@anu.edu.au
² Research School of Earth Sciences, Australian National University, Canberra, ACT 2601 Australia

Received: 18 April 2021
Accepted: 31 March 2022

Abstract

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_⊕.

Results. Applying the tidal models to our sample of 767 tidally locked rocky exoplanets reveals that ~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, ~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_⊕) orbiting in the CHZ of low-mass M dwarfs.