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

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ Millennium Institute for Astrophysics, Santiago, Chile
⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁵ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Center for Astrostatistics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁹ U.S. National Science Foundation National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
¹⁰ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ Australian Astronomical Optics, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
¹² Astrophysics and Space Technologies Research Centre, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
¹³ School of Mathematical and Physical Sciences, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
¹⁴ Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
¹⁵ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹⁶ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
¹⁷ Department of Physics and Astronomy, Amherst College, Amherst, MA 01002, USA
¹⁸ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
¹⁹ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
²⁰ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
²¹ Department of Physics & Astronomy, The University of California, Irvine, Irvine, CA 92697, USA
²² McDonald Observatory and Center for Planetary Systems Habitability, The University of Texas at Austin, Austin, TX 78730, USA
²³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA

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Received: 11 July 2025
Accepted: 15 December 2025

Abstract

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.