The Ability of Significant Tidal Stress to Initiate Plate Tectonics

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.Comment: 34 pages, 3 figures, 3 Tables, accepted to Icaru

A Tale of Two Circularization Periods

We re-analyze the pristine eclipsing binary data from the $\textit{Kepler}$ and TESS missions, focusing on eccentricity measurements at short orbital periods to emperically constrain tidal circularization. We find an average circularization period of $~$6 days, as well as a short circularization period of $\sim$3 days for the $\textit{Kepler}$/TESS field binaries. We argue previous spectroscopic binary surveys reported longer circularization periods due to small sample sizes, which were contaminated by an abundance of binaries with circular orbits out to $\sim$10 days, but we re-affirm their data shows a difference between the eccentricity distributions of young ($<$1 Gyr) and old ($>$3 Gyr) binaries. Our work calls into question the long circularization periods quoted often in the literature.Comment: Main text 9 pages, 13 including appendix, 8 Figures, submitted to ApJ

Sweeping Secular Resonances and Giant Planet Inclinations in Transition Discs

The orbits of some warm Jupiters are highly inclined (20$^\circ$-50$^\circ$) to those of their exterior companions. Comparable misalignments are inferred between the outer and inner portions of some transition discs. These large inclinations may originate from planet-planet and planet-disc secular resonances that sweep across interplanetary space as parent discs disperse. The maximum factor by which a seed mutual inclination can be amplified is of order the square root of the angular momentum ratio of the resonant pair. We identify those giant planet systems (e.g. Kepler-448 and Kepler-693) which may have crossed a secular resonance, and estimate the required planet masses and semimajor axes in transition discs needed to warp their innermost portions (e.g. in CQ Tau). Passage through an inclination secular resonance could also explain the hypothesized large mutual inclinations in apsidally-orthogonal warm Jupiter systems (e.g. HD 147018).Comment: 13 pages, 13 figures, submitted to MNRA