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±0.033R⊕, 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.
@article{arxiv.2507.08464,
title = {An Earth-Sized Planet in a 5.4h Orbit Around a Nearby K dwarf},
author = {Kaya Han Taş and Gudmundur Stefansson and Syarief N. M. Fariz and Esha Garg and Juan I. Espinoza-Retamal and Elise Koo and David Bruijne and Jacob Luhn and Eric B. Ford and Suvrath Mahadevan and Sarah E. Logsdon and Caleb I. Cañas and Te Han and Mark E. Everett and Jaime A. Alvarado-Montes and Cullen Blake and William D. Cochran and Jiayin Dong and Rachel B. Fernandes and Mark R. Giovinazzi and Samuel Halverson and Shubham Kanodia and Daniel Krolikowski and Michael McElwain and Joe Ninan and Leonardo A. Paredes and Paul Robertson and Christian Schwab},
journal= {arXiv preprint arXiv:2507.08464},
year = {2026}
}