TOI-1438: A rare system with two short-period sub-Neptunes and a tentative long-period Jupiter-like planet orbiting a K0V star

¹ Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory, 439 92 Onsala, Sweden
² Chalmers University of Technology, Department of Space, Earth and Environment, 412 93, Gothenburg, Sweden
³ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁴ Instituto de Astrofísica de Canarias (IAC), C. Via Lactea S/N, 38205 La Laguna, Tenerife, Spain
⁵ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁶ Max-Planck Institut für Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany
⁷ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Toruri, Poland
⁸ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁹ INSA Lyon – Institut National des Sciences Appliquées, France
¹⁰ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
¹¹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
¹³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁴ Observatoire astronomique de l’Université de Genéve, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁵ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹⁶ NHFP Sagan Fellow
¹⁷ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹⁹ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
²⁰ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
²¹ Center for Astrophysics l Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²² McDonald Observatory, The University of Texas, Austin, TX, USA
²³ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
²⁴ NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 10022, 1200 E. California Blvd., Pasadena, CA 91125, USA
²⁵ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁶ 501 Campbell Hall, University of California at Berkeley, Berkeley, CA 94720, USA
²⁷ Institute of Space Research, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
²⁸ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁰ Department of Astronomy, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
³¹ U.S. Naval Observatory, Washington, DC 20392, USA
³² Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
³³ NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
³⁴ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁵ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁶ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³⁷ Department of Physics & Astronomy, Vanderbilt University, Nashville, TN 37235, USA
³⁸ SETI Institute, Mountain View, CA 94043, USA
³⁹ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴⁰ Mullard Space Science Laboratory, University College London, Dorking, Surrey, RH5 6NT, UK
⁴¹ Department of Physics and Astronomy, University of Notre Dame, Notre Dame, IN 46556, USA

^★ Corresponding author: carina.persson@chalmers.se

Received: 28 April 2025
Accepted: 31 July 2025

Abstract

We present the detection and characterisation of the TOI-1438 multi-planet system discovered by the Transiting Exoplanet Survey Satellite (TESS). To confirm the planetary nature of the candidates and determine their masses, we collected a series of followup observations including high-spectral resolution observations with HARPS-N and HIRES over a period of 5 years. Our combined modelling shows that the K0V star hosts two transiting sub-Neptunes with R_b = 3.04 ± 0.19 R_⊕, R_c = 2.75 ± 0.14 R_⊕, M_b = 9.4 ± 1.8 M_⊕, and M_c =10.6 ± 2.1 M_⊕. The orbital periods of planets b and c are 5.1 and 9.4 days, respectively, corresponding to instellations of 145 ± 10 F_⊕ and 65 ± 4 F_⊕. The bulk densities are 1.8 ± 0.5 g cm⁻³ and 2.9 ± 0.7 g cm⁻³, respectively, suggesting a volatile-rich interior composition. By combining the planet and stellar parameters, we were able to compute a set of planet interior structure models. Planet b presents a high-metallicity envelope that can accommodate up to 2.5% in H/He in mass, while planet c cannot have more than 0.2% as H/He in mass. For any composition of the core considered (Fe-rock or ice-rock), both planets would require a volatile-rich envelope. In addition to the two planets, the radial velocity (RV) data clearly reveal a third signal, likely coming from a non-transiting planet, with an orbital period of 7.6_−2.4^+1.6 years and an RV semi-amplitude of 35₋₅⁺³ m s⁻¹. Our best-fit model finds a minimum mass of 2.1 ± 0.3 M_J and an eccentricity of 0.25_−0.11^+0.08. However, several RV activity indicators also show strong signals at similar periods, suggesting this signal might (partly) originate from stellar activity. More data over a longer period of time are needed to conclusively determine the nature of this signal. If it is confirmed as a triple-planet system, TOI-1438 would be one of the few detected systems to date characterised by an architecture with two small, short-period planets and one massive, long-period planet, where the inner and outer systems are separated by an orbital period ratio of the order of a few hundred.