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An Earth-Sized Planet In A 5.4h Orbit Around A Nearby K Dwarf

By Keith Cowing
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astro-ph.EP
July 14, 2025
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An Earth-Sized Planet In A 5.4h Orbit Around A Nearby K Dwarf
Best fit results from a joint transit and RV fit using a quasi-periodic GP model to account for the excess RV scatter in the RVs. a) Top: NEID (yellow) and HPF (green) RVs as a function of time. The best-fit Keplerian model is shown in red (appears as a solid band due to the short period orbit). The median of the GP component is shown in blue. The joint best-fit model is shown in grey. Bottom: Residuals from the best fit model. b) Top: Phase-folded RVs on the period of TOI-2431 b. Best fit Keplerian model after subtracting the GP model is shown in red. Bottom: RV residuals as a function of phase. — astro-ph.EP

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.

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,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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