on the surface [@Fole16], which will reduce the magnitude of $F_{t}$ and lower the likelihood of plate tectonics on these planets. To date, there are no definitive observations of terrestrial exoplanet atmospheres and thus inference of mobile versus stagnant-lid states through atmospheric observation is currently beyond the reach of exoplanetary science.

Together, $F_{c}+F_{t}$ for Earth’s subducting plates contribute a net buoyancy of $\sim-(2-3)* 10^{13}$ N per meter length of subducted plate downward [recalculated from @Kird14]. While the processes responsible for the initiation of subduction are not well-understood on Earth, plates having sufficient negative buoyancy at the depth of the basalt-eclogite transition is a necessary precondition for incipient subduction and prevention of a stagnant lid [@vanH02]. These upper mantle forces, together with the shear traction and induced flow of a slab inciting downward mantle motion, constitute one of two major driving forces of plate tectonics [@Conr02].

An exoplanet’s composition and mineralogy are not directly observable and are instead inferred from mass-radius relations [e.g. @Seag07; @Weis14; @Wolf16; @Unte16]. Given only a planet’s mass and radius, there is considerable degeneracy in planet composition and structure. As noted by @Dorn15, this degeneracy is reduced when the stellar composition of terrestrial planet-building elements (Mg, Si, 