The GAPS programme at TNG

LXXI. A sub-Neptune suitable for atmospheric characterization in a multiplanet and mutually inclined system orbiting the bright K dwarf TOI-5789 (HIP 99452)^★

¹ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁶ Space Research Institute, Austrian Academy of Sciences, Schmiedl-strasse 6, 8042 Graz, Austria
⁷ Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland
⁸ Université Aix Marseille, CNRS, CNES, LAM, Marseille, France
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹¹ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹² INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹³ Sternberg Astronomical Institute Lomonosov Moscow State University, Universitetskii prospekt, 13 Moscow, Russia
¹⁴ Hazelwood Observatory, Churchill, Victoria 3840, Australia
¹⁵ Dept. of Physics, Engineering and Astronomy, Stephen F. Austin State Univ., 1936 North St, Nacogdoches, TX 75962, USA
¹⁶ Fundación Galileo Galilei - INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja (TF), Spain
¹⁷ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁸ Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy

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

Received: 12 October 2025
Accepted: 13 January 2026

Abstract

Sub-Neptunes with planetary radii of R_p ≃ 2–4 R_⊕ are the most common planets around solar-type stars in short-period (P < 100 d) orbits. It is still unclear, however, what their most likely composition is, that is whether they are predominantly gas dwarfs or water worlds. The sub-Neptunes orbiting bright host stars are very valuable because they are suitable for atmospheric characterization, which can break the well-known degeneracy in planet composition from the planet bulk density, when combined with a precise and accurate mass measurement. Here we report on the characterization of the sub-Neptune TOI-5789 c, which transits in front of the bright (V = 7.3 mag and K_s = 5.35 mag) and magnetically inactive K1 V dwarf HIP 99452 every 12.93 days, thanks to TESS photometry and 141 high-precision radial velocities obtained with the HARPS-N spectrograph. We find that its radius, mass, and bulk density are R_c = 2.86_−0.15^+0.18 R_⊕, M_c = 5.00 ± 0.50 M_⊕, and ρ_c = 1.16 ± 0.23 g cm⁻³, respectively, and we show that TOI-5789 c is a promising target for atmosp^-heric characterization with both JWST and, in the future, Ariel. By analyzing the HARPS-N radial velocities with different tools, we also detected three additional non-transiting planets, namely TOI-5789 b, d, and e, with orbital periods and minimum masses of P_b = 2.76 d, M_b sin i = 2.12 ± 0.28 M_⊕, P_d = 29.6 d, M_d sin i = 4.29 ± 0.68 M_⊕, and P_e = 63.0 d, M_e sin i = 11.61 ± 0.97 M_⊕. TOI- 5789 is a mutually inclined system as the difference between the orbital inclinations of planets b and c must be higher than ~4 deg. Nevertheless, from sensitivity studies based on both the HARPS-N and archival HIRES radial-velocity measurements, we can exclude the possibility that these relatively high mutual inclinations are due to the perturbation by an outer gaseous giant planet.