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

Kaya Han Tas; Gudmundur Stefansson; Syarief N. M. Fariz; Esha Garg; Juan I. Espinoza-Retamal; Elise Koo; David Bruijne; Jacob K. 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

doi:10.1051/0004-6361/202556365

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

Kaya Han Tas^*, Gudmundur Stefansson, Syarief N. M. Fariz, Esha Garg, Juan I. Espinoza-Retamal, Elise Koo, David Bruijne, Jacob K. 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. GiovinazziSamuel Halverson, Shubham Kanodia, Daniel Krolikowski, Michael Mcelwain, Joe Ninan, Leonardo A. Paredes, Paul Robertson, Christian Schwab

^*Corresponding author for this work

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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 ± 28K) 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.

Original language	English
Article number	A238
Pages (from-to)	1-16
Number of pages	16
Journal	Astronomy and Astrophysics
Volume	707
DOIs	https://doi.org/10.1051/0004-6361/202556365
Publication status	Published - 17 Mar 2026

Bibliographical note

Keywords

Planets and satellites: detection
Planets and satellites: general
Planets and satellites: terrestrial planets
Techniques: photometric
Techniques: radial velocities

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Tas, K. H., Stefansson, G., Fariz, S. N. M., Garg, E., Espinoza-Retamal, J. I., Koo, E., Bruijne, D., Luhn, J. K., Ford, E. B., Mahadevan, S., Logsdon, S. E., Cañas, C. I., Han, T., Everett, M. E., Alvarado-Montes, J. A., Blake, C., Cochran, W. D., Dong, J., Fernandes, R. B., ... Schwab, C. (2026). An Earth-sized planet on a 5.4 h orbit around a nearby K dwarf. Astronomy and Astrophysics, 707, 1-16. Article A238. https://doi.org/10.1051/0004-6361/202556365

@article{224103248ed74e01bd68250255788ee1,

title = "An Earth-sized planet on a 5.4 h orbit around a nearby K dwarf",

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 (Teff = 4109 ± 28K) 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 Teq = 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.",

keywords = "Planets and satellites: detection, Planets and satellites: general, Planets and satellites: terrestrial planets, Techniques: photometric, Techniques: radial velocities",

author = "Tas, \{Kaya Han\} and Gudmundur Stefansson and Fariz, \{Syarief N. M.\} and Esha Garg and Espinoza-Retamal, \{Juan I.\} and Elise Koo and David Bruijne and Luhn, \{Jacob K.\} and Ford, \{Eric B.\} and Suvrath Mahadevan and Logsdon, \{Sarah E.\} and Ca{\~n}as, \{Caleb I.\} and Te Han and Everett, \{Mark E.\} and Alvarado-Montes, \{Jaime A.\} and Cullen Blake and Cochran, \{William D.\} and Jiayin Dong and Fernandes, \{Rachel B.\} and Giovinazzi, \{Mark R.\} and Samuel Halverson and Shubham Kanodia and Daniel Krolikowski and Michael Mcelwain and Joe Ninan and Paredes, \{Leonardo A.\} and Paul Robertson and Christian Schwab",

note = "{\textcopyright} The Authors 2026. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.",

year = "2026",

month = mar,

day = "17",

doi = "10.1051/0004-6361/202556365",

language = "English",

volume = "707",

pages = "1--16",

journal = "Astronomy and Astrophysics",

issn = "0004-6361",

publisher = "EDP Sciences",

}

Tas, KH, Stefansson, G, Fariz, SNM, Garg, E, Espinoza-Retamal, JI, Koo, E, Bruijne, D, Luhn, JK, Ford, EB, Mahadevan, S, Logsdon, SE, Cañas, CI, Han, T, Everett, ME, Alvarado-Montes, JA, Blake, C, Cochran, WD, Dong, J, Fernandes, RB, Giovinazzi, MR, Halverson, S, Kanodia, S, Krolikowski, D, Mcelwain, M, Ninan, J, Paredes, LA, Robertson, P & Schwab, C 2026, 'An Earth-sized planet on a 5.4 h orbit around a nearby K dwarf', Astronomy and Astrophysics, vol. 707, A238, pp. 1-16. https://doi.org/10.1051/0004-6361/202556365

TY - JOUR

T1 - An Earth-sized planet on a 5.4 h orbit around a nearby K dwarf

AU - Tas, Kaya Han

AU - Stefansson, Gudmundur

AU - Fariz, Syarief N. M.

AU - Garg, Esha

AU - Espinoza-Retamal, Juan I.

AU - Koo, Elise

AU - Bruijne, David

AU - Luhn, Jacob K.

AU - Ford, Eric B.

AU - Mahadevan, Suvrath

AU - Logsdon, Sarah E.

AU - Cañas, Caleb I.

AU - Han, Te

AU - Everett, Mark E.

AU - Alvarado-Montes, Jaime A.

AU - Blake, Cullen

AU - Cochran, William D.

AU - Dong, Jiayin

AU - Fernandes, Rachel B.

AU - Giovinazzi, Mark R.

AU - Halverson, Samuel

AU - Kanodia, Shubham

AU - Krolikowski, Daniel

AU - Mcelwain, Michael

AU - Ninan, Joe

AU - Paredes, Leonardo A.

AU - Robertson, Paul

AU - Schwab, Christian

PY - 2026/3/17

Y1 - 2026/3/17

N2 - We present the discovery and confirmation of the ultrashort 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 (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 Teq = 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.

AB - We present the discovery and confirmation of the ultrashort 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 (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 Teq = 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.

KW - Planets and satellites: detection

KW - Planets and satellites: general

KW - Planets and satellites: terrestrial planets

KW - Techniques: photometric

KW - Techniques: radial velocities

U2 - 10.1051/0004-6361/202556365

DO - 10.1051/0004-6361/202556365

M3 - Article

AN - SCOPUS:105033334005

SN - 0004-6361

VL - 707

SP - 1

EP - 16

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

M1 - A238

ER -

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

Abstract

Bibliographical note

Keywords

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