An Earth-Sized Planet in a 5.4h Orbit Around a Nearby K dwarf

Abstract

We present the discovery and confirmation of the ultra-short period (USP) planet TOI-2431 b orbiting a nearby ($d\sim 36$ pc) late K star ($T_{\mathrm{eff}}$ = $4109\pm 28\mathrm{K}$) using observations from the Transiting Exoplanet Survey Satellite (TESS), precise radial velocities with the NEID and the Habitable-zone Planet Finder (HPF) spectrographs, as well as ground-based high contrast imaging from NESSI. TOI-2431 b has a period of 5 hours and 22 minutes, making it one of the shortest-period exoplanets known to date. TOI-2431 b has a radius of $1.536\pm 0.033{\mathrm{R}}_{\oplus }$, and a mass of $6.2\pm 1.2{\mathrm{M}}_{\oplus }$, suggesting it has a density compatible with an Earth-like composition and, due to its high irradiation, is likely a 'lava-world' with a $T_{\mathrm{eq}}$ = $2063\pm 30\mathrm{K}$. We estimate that the current orbital period is only 30% larger than the Roche-limit orbital period, and that it has an expected orbital decay timescale of only $\sim$31 Myr. Finally, due to the brightness of the host star ($V=10.9$, $K=7.6$), TOI-2431 b has a high Emission Spectroscopy Metric of 27, making it one of the best USP systems for atmospheric phase-curve analysis.

Submitted to A&A, 15 pages, 12 figures