Super-Earth LHS3844b is tidally locked

TL;DR

This study analyzes the tidally locked state of super-Earth LHS 3844b using thermal modeling and phase curve data, providing empirical evidence supporting tidal locking and highlighting the role of space weathering in surface darkening.

Contribution

It offers the first empirical constraints on the rotation state of LHS 3844b, ruling out non-synchronous rotation and suggesting space weathering influences its surface.

Findings

LHS 3844b is likely tidally locked with very low eccentricity.

Rapid non-synchronous rotation is ruled out by lack of tidal heating evidence.

Surface darkening by space weathering is a plausible explanation for phase curve observations.

Abstract

Short period exoplanets on circular orbits are thought to be tidally locked into synchronous rotation. If tidally locked, these planets must possess permanent day- and nightsides, with extreme irradiation on the dayside and none on the nightside. However, so far the tidal locking hypothesis for exoplanets is supported by little to no empirical evidence. Previous work showed that the super-Earth LHS 3844b likely has no atmosphere, which makes it ideal for constraining the planet's rotation. Here we revisit the Spitzer phase curve of LHS 3844b with a thermal model of an atmosphere-less planet and analyze the impact of non-synchronous rotation, eccentricity, tidal dissipation, and surface composition. Based on the lack of observed strong tidal heating we rule out rapid non-synchronous rotation (including a Mercury-like 3:2 spin-orbit resonance) and constrain the planet's eccentricity to less than 0.001 (more circular than Io's orbit). In addition, LHS 3844b's phase curve implies that the planet either still experiences weak tidal heating via a small-but-nonzero eccentricity (requiring an undetected orbital companion), or that its surface has been darkened by space weathering; of these two scenarios we consider space weathering more likely. Our results thus support the hypothesis that short period rocky exoplanets are tidally locked, and further show that space weathering can significantly modify the surfaces of atmosphere-less exoplanets.