TOI-4552 b: A new ultra-short-period rocky world revealed by NIRPS and TESS

¹ Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
² Observatoire du Mont-Mégantic, Québec, Canada
³ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Department of Physics & Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada (Madrid), Spain
⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN, 59072-970, Brazil
⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁰ Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
¹¹ Department of Physics, University of Toronto, Toronto, ON M5S 3H4, Canada
¹² Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
¹³ Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 0E8, Canada
¹⁴ Centre Vie dans l’Univers, Faculté des sciences de l’Université de Genève, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland
¹⁵ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁷ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁸ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
¹⁹ Bishop’s University, Dept of Physics and Astronomy, Johnson-104E, 2600 College Street, Sherbrooke, QC J1M 1Z7, Canada
²⁰ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, 99 University Avenue, Kingston, ON K7L 3N6, Canada
²¹ Department of Physics and Space Science, Royal Military College of Canada, 13 General Crerar Cres., Kingston, ON K7P 2M3, Canada
²² Center for astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²³ American Association of Variable Star Observers, Cambridge, USA
²⁴ Department of Physics, University of Oxford, Oxford OX13RH, UK
²⁵ LESIA, Observatoire de Paris, CNRS, Université Paris Diderot, Universite Pierre et Marie Curie, 5 place Jules Janssen, 92190 Meudon, France
²⁶ AIM, CEA, CNRS, Université Paris-Saclay, Universite de Paris, 91191 Gif-sur-Yvette, France
²⁷ Department of Physics and Astronomy, The University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA
²⁸ SETI Institute, Mountain View, CA 94043, USA NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁹ Department of Physics, The University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³¹ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
³² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³³ Astrobiology Research Unit, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 December 2025
Accepted: 16 March 2026

Abstract

Context. A particularly intriguing subclass of rocky exoplanets are ultra-short-period worlds, which orbit their host stars in less than a day. These planets are particularly rare around M dwarf stars, and only ten of them have a constrained mass and radius so far.

Aims. We present the validation and characterisation of the ultra-short-period (0.3 days) Earth-sized planet TOI-4552 b orbiting a nearby (27.26 pc away) M4.5V dwarf.

Methods. We complemented TESS photometry ground-based transit observations from LCO, ExTrA, and SPECULOOS to validate the planetary radius, and we cleared the field of any contaminants. Speckle imaging with Zorro (Gemini-S) rules out false-positive scenarios caused by eclipsing binary sources. Spectroscopic observations with NIRPS and HARPS were used to obtain stellar abundances, constrain the planetary mass, and estimate the orbital parameters in conjunction with the transit observations.

Results. TOI-4552 is a quiet star. It lacks short-term stellar variations in photometric or radial velocity data that might be associated with stellar rotation. Long-term photometric data from ASAS-SN also suggest a lack of activity signals. TOI-4552 b (M_p = 1.83 ± 0.47 M_⊕, R_p = 1.11 ± 0.04 R_⊕) lies between the Earth-like and iron-rich composition tracks on the mass-radius diagram. The exopie interior structure model, without constraints from refractory abundance ratio, yields a core mass fraction of 0.54_−0.25^+0.17 and a bulk density of 7.74±2.14g/cm³. Since the core mass fraction spans a wide range because the uncertainty on the mass is high, the definitive interior composition cannot be determined with the current dataset.

Conclusions. TOI-4552 b probably is marginally richer in iron than the Earth, but confirmation of its status requires additional precise radial velocity measurements. Combined with its high emission spectroscopic metric (ESM = 19.5), negligible stellar activity, and short orbital period, TOI-4552 b emerges as a compelling target for atmospheric and surface composition studies with JWST.