An Earth-sized planet on a 5.4 h orbit around a nearby K dwarf

We present the discovery and confirmation of the ultrashort period (USP) planet TOI-2431 b orbiting a nearby (d ~ 36 pc) late K star (T_eff = 4109 ± 28 K) using observations from the Transiting Exoplanet Survey Satellite (TESS), precise radial velocities (RVs) with NEID and 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 1.534 ± 0.033 R_⊕ and a mass of 6.2 ± 1.6 M_⊕, where the exact mass precision shows a slight dependence on the choice of prior. This suggests TOI-2431 b has a density compatible with an Earth-like composition and due to its high irradiation, it is likely to be a “lava-world” with a T_eq = 2063 ± 30 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 ~31 Myr. Finally, due to the brightness of the host star (V = 10.9, K = 7.6), we find that TOI-2431 b has a high emission spectroscopy metric (ESM) of 27, making it one of the best USP systems for atmospheric phase-curve analyses.