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.
Bonomo et al. (Thu,) studied this question.