The GAPS programme at TNG XYZ. A sub-Neptune suitable for atmospheric characterization in a multiplanet and mutually inclined system orbiting the bright K dwarf TOI-5789 (HIP 99452)

TL;DR

This study confirms TOI-5789 c, a sub-Neptune orbiting the bright K dwarf TOI-5789, and presents a detailed mass–radius characterization using 141 HARPS-N RVs and TESS photometry. The four-planet architecture includes a transiting c and three non-transiting siblings (b, d, e), with mutual inclinations suggesting a dynamically evolved, metastable inner system and no evidence for a perturbing outer giant. TOI-5789 c has a mass of $M_p\approx5.0\,M_\oplus$, radius $R_p\approx2.86\,R_\oplus$, and a low bulk density ($\rho_p\approx1.16\rm\ g\,cm^{-3}$), indicating a significant atmosphere. Transmission spectroscopy simulations show TOI-5789 c is an excellent candidate for atmospheric characterization with JWST and Ariel, enabling constraints on H$_2$O, CO, CO$_2$, CH$_4$, and SO$_2$ abundances and advancing our understanding of sub-Neptune compositions and formation in multiplanet systems.

Abstract

Sub-Neptunes with planetary radii of $R_{p} \simeq 2-4 R_{\oplus}$ 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 $R_{c}=2.86^{+0.18}_{-0.15} R_\oplus$, $M_{c}=5.00 \pm 0.50 M_\oplus$, and $ρ_{c}=1.16 \pm 0.23$ 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 detect three additional non-transiting planets, namely TOI-5789 b, d, and e, with orbital periods and minimum masses of $P_{b}=2.76$ d, $M_{b}\sin{i}=2.12 \pm 0.28 M_\oplus$, $P_{d}=29.6$ d, $M_{d}\sin{i}=4.29 \pm 0.68 M_\oplus$, and $P_{e}=63.0$ d, $M_{e}\sin{i}=11.61 \pm 0.97 M_\oplus$. 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 that these relatively high mutual inclinations are due to the perturbation by an outer gaseous giant planet.

Figures (21)

• Figure 1: HARPS-N RVs of TOI-5789 (blue circles) along with the four-planet Keplerian model (red solid line; see Sect. \ref{['RV_modeling']}). The error bars take the RV jitter into account.
• Figure 2: Stellar spectral energy distribution (SED). The broad-band measurements from the Tycho, APASS Johnson and Sloan, 2MASS and WISE magnitudes are shown in red, and the corresponding theoretical values with blue circles. The black solid line displays the non-averaged best-fit model.
• Figure 3: Corner plot of the posterior distributions of the GP hyper-parameters. The posteriors are obtained with a three-planet model and GP regression to account for the 29.8 d period. The posteriors of the parameters of the three planets are not shown for simplicity, but are fully consistent with those given in Table \ref{['tab:planet_parameters']}.
• Figure 4: Posterior distribution of the number of Keplerians $N_{\rm p}$ detected with kima in our RVs. The distributions are shown as the number of posterior samples for each value of $N_{\rm p}$ normalized to the effective sample size (ESS). The orange bar highlights the highest value for $N_{\rm p}$=4 corresponding to the most likely 4-planet model.
• Figure 5: Phase-folded transits of TOI-5789 c with TESS long-cadence (left panel) and fast-cadence (right panel) data.