  Plate tectonics is a geophysical process currently unique to Earth, has an
important role in regulating the Earth's climate, and may be better understood
by identifying rocky planets outside our solar system with tectonic activity.
The key criterion for whether or not plate tectonics may occur on a terrestrial
planet is if the stress on a planet's lithosphere from mantle convection may
overcome the lithosphere's yield stress. Although many rocky exoplanets closely
orbiting their host stars have been detected, all studies to date of plate
tectonics on exoplanets have neglected tidal stresses in the planet's
lithosphere. Modeling a rocky exoplanet as a constant density, homogeneous,
incompressible sphere, we show the tidal stress from the host star acting on
close-in planets may become comparable to the stress on the lithosphere from
mantle convection. We also show that tidal stresses from planet-planet
interactions are unlikely to be significant for plate tectonics, but may be
strong enough to trigger Earthquakes. Our work may imply planets orbiting close
to their host stars are more likely to experience plate tectonics, with
implications for exoplanetary geophysics and habitability. We produce a list of
detected rocky exoplanets under the most intense stresses. Atmospheric and
topographic observations may confirm our predictions in the near future.
Investigations of planets with significant tidal stress can not only lead to
observable parameters linked to the presence of active plate tectonics, but may
also be used as a tool to test theories on the main driving force behind
tectonic activity.
