TOI-1438: A rare system with two short-period sub-Neptunes and a tentative long-period Jupiter-like planet orbiting a K0V star
Journal article, 2025
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 Rb = 3.04 ± 0.19 R⊕, Rc = 2.75 ± 0.14 R⊕, Mb = 9.4 ± 1.8 M⊕, and Mc =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 MJ 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.
planets and satellites: interiors
planets and satellites: composition
planets and satellites: dynamical evolution and stability
planets and satellites: general
planets and satellites: fundamental parameters
planets and satellites: detection
Author
Carina Persson
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Emil Knudstrup
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Aarhus University
Ilaria Carleo
Instituto de Astrofísica de Canarias
University of La Laguna
Lorena Acuña-Aguirre
Max Planck Society
G. Nowak
Nicolaus Copernicus University (SGMK)
Instituto de Astrofísica de Canarias
University of La Laguna
Alexandra Muresan
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
D. Jankowski
Nicolaus Copernicus University (SGMK)
K. Goździewski
Nicolaus Copernicus University (SGMK)
R. A. García
University Paris-Saclay
S. Mathur
University of La Laguna
Instituto de Astrofísica de Canarias
Dinil B. Palakkatharappil
University Paris-Saclay
Lina Borg
Institut National des Sciences Appliquees de Lyon
University Paris-Saclay
A. J. Mustill
Lund University
Rafael Barrena
Instituto de Astrofísica de Canarias
University of La Laguna
Malcolm Fridlund
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Leiden University
D. Gandolfi
University of Turin
A. Hatzes
Thüringer Landessternwarte Tautenburg
Judith Korth
University of Geneva
R. Luque
NHFP Sagan Fellow
University of Chicago
Eduardo L. Martín
University of La Laguna
Instituto de Astrofísica de Canarias
T. Masseron
Instituto de Astrofísica de Canarias
University of La Laguna
Giuseppe Morello
Spanish National Research Council (CSIC)
Istituto nazionale di astrofisica (INAF)
F. Murgas
University of La Laguna
Instituto de Astrofísica de Canarias
J. Orell-Miquel
The University of Texas at Austin
Instituto de Astrofísica de Canarias
University of La Laguna
Enric Palle
Instituto de Astrofísica de Canarias
University of La Laguna
Simon Albrecht
Aarhus University
Allyson Bieryla
Harvard-Smithsonian Center for Astrophysics
William D. Cochran
The University of Texas at Austin
I. J.M. Crossfield
University of Kansas
H. Deeg
Instituto de Astrofísica de Canarias
University of La Laguna
Elise Furlan
National Aeronautics and Space Administration (NASA)
E. W. Guenther
Thüringer Landessternwarte Tautenburg
S.B. Howell
National Aeronautics and Space Administration (NASA)
H. Isaacson
University of California
K. W.F. Lam
German Aerospace Center (DLR)
J.H. Livingston
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
National Institutes of Natural Sciences
Rachel A. Matson
US Naval Observatory
Elisabeth Matthews
Max Planck Society
S. Redfield
Wesleyan University
Joshua E. Schlieder
National Aeronautics and Space Administration (NASA)
S. Seager
Massachusetts Institute of Technology (MIT)
A. M.S. Smith
German Aerospace Center (DLR)
Keivan G. Stassun
Vanderbilt University
J. D. Twicken
SETI Institute
National Aeronautics and Space Administration (NASA)
Vincent Van Eylen
University College London (UCL)
Cristilyn N. Watkins
Harvard-Smithsonian Center for Astrophysics
Lauren M. Weiss
University of Notre Dame
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 702 A69Exoplanets from space -CHEOPS and PLATO ESA's next two projects (Phase 2)
Swedish National Space Board (177/19), 2020-01-01 -- 2022-12-31.
Swedish National Space Board (65/19), 2020-01-01 -- 2022-12-31.
Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202555318