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 (T_eff = 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 T_eq = 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.

Kaya Han Taş, Gudmundur Stefansson, Syarief N.M. Fariz, Esha Garg, Juan I. Espinoza-Retamal, Elise Koo, David Bruijne, Jacob Luhn, Eric B. Ford, Suvrath Mahadevan, Sarah E. Logsdon, Caleb I. Cañas, Te Han, Mark E. Everett, Jaime A. Alvarado-Montes, Cullen Blake, William D. Cochran, Jiayin Dong, Rachel B. Fernandes, Mark R. Giovinazzi, Samuel Halverson, Shubham Kanodia, Daniel Krolikowski, Michael McElwain, Joe Ninan, Leonardo A. Paredes, Paul Robertson, Christian Schwab

Comments: Submitted to A&A, 15 pages, 12 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.08464 [astro-ph.EP] (or arXiv:2507.08464v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.08464
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Submission history
From: Kaya Han Taş
[v1] Fri, 11 Jul 2025 10:13:19 UTC (8,952 KB)
https://arxiv.org/abs/2507.08464
Astrobiology,