Shedding Light on Desert Dwellers

Abstract

The ``sub-Jovian desert" ($2{\lesssim}R_{\rm p}{\lesssim}10 \ R_{\oplus}$, periods $\lesssim$3 days) is sparsely populated but no longer empty. Recent surveys have revealed that planets residing in the desert are dense (${\rho}{\gtrsim}1$ g/cm$^{3}$), massive ($\sim$10${-}$50 $M_{\oplus}$), and orbit metal-rich stars that are indistinguishable from those hosting hot Jupiters. However, their origins remain mysterious. In this work we adopt and test the hypothesis that tidal destruction of hot Jupiters can populate the sub-Jovian desert with stripped remnant planets. We first show that stars hosting desert dwellers exhibit Galactic kinematics indicative of an older population descended from those hosting hot Jupiters. We highlight that tidally-driven Roche lobe overflow (RLO) can indeed populate the desert with planets similar to those observed, but only if angular momentum transfer during RLO is inefficient (``lossy" RLO). The entire width of the sub-Jovian desert can be backfilled with the remnants of hot Jupiters that possessed their empirically inferred spread in entropy. In this picture, current desert dwellers such as LTT 9779b should be tidally decaying at an observationally testable rate of ${\sim}0.5$ ms/yr. Our theory also predicts that desert dweller host stars may rotate up to an order of magnitude more rapidly than field stars; rotation period differences may persist ${\sim}$Gyr after RLO. Lossy RLO may also manifest as a burst of IR excess that could outshine the host star for up to ${\sim}10^{3}$ yr. If these predictions are confirmed by observations, our theory indicates that desert dwellers can be leveraged to study the interiors of giant planets in exquisite detail.