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)

Sub-Neptunes with planetary radii of ̊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 ̊m 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_ ̊m c =2.86^ +0.18 _ -0.15 ̊m R_⊕, M_ ̊m c =5.00 ± 0.50 ̊m M_⊕, and ρ_ ̊m c =1.16 ± 0.23 g,cm -3 , respectively, 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_ ̊m b =2.76,d, M_ ̊m b i =2.12 ± 0.28 ̊m M_⊕, P_ ̊m d =29.6,d, M_ ̊m d i =4.29 ± 0.68 ̊m M_⊕, and P_ ̊m e =63.0,d, M_ ̊m e i =11.61 ± 0.97 ̊m M_⊕. TOI-5789 is a mutually inclined system as the difference between the orbital inclinations of planets b and c must be higher than sim4,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.