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

We present the discovery and confirmation of the ultra-short period (USP) planet TOI-2431 b orbiting a nearby (d ∼ 36 pc) late K star (Teff = 4109 ± 28K) 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 ± 0.033 R⊕, and a mass of 6.2 ± 1.2M⊕, suggesting it has a density compatible with an Earth-like composition and, due to its high irradiation, is likely a ‘lava-world’ with a Teq = 2063 ± 30K. 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 ∼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.