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)

Abstract

Sub-Neptunes with planetary radii of $R_p\simeq 2-4R_{\oplus }$ 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 K1V 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 $R_c=2.86_{-0.15}^{+0.18}{R}_{\oplus }$, $M_c=5.00\pm 0.50M_{\oplus }$, and $\ddot{I}{}_c=1.16\pm 0.23$ g cm${}^{-3}$, and we show that TOI-5789 c is a promising target for atmospheric 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, $M_b\sin i=2.12\pm 0.28M_{\oplus }$, $P_d=29.6$ d, $M_d\sin i=4.29\pm 0.68M_{\oplus }$, and $P_e=63.0$ d, $M_e\sin i=11.61\pm 0.97M_{\oplus }$. TOI-5789 is a mutually inclined system as the difference between the orbital inclinations of planets b and c must be higher than $\sim 4$ deg. Nevertheless, from sensitivity studies based on both the HARPS-N and archival HIRES radial-velocity measurements, we can exclude that these relatively high mutual inclinations are due to the perturbation by an outer gaseous giant planet.

13 pages, 10 figures; published on Astronomy and Astrophysics, 707, A197 (2026)