The GAPS programme at TNG. XL. A puffy and warm Neptune-sized planet and an outer Neptune-mass candidate orbiting the solar-type star TOI-1422

Context. Neptunes represent one of the main types of exoplanets and have chemical-physical characteristics halfway between rocky and gas giant planets. Therefore, their characterization is important for understanding and constraining both the formation mechanisms and the evolution patterns of planets.
Aims: We investigate the exoplanet candidate TOI-1422 b, which was discovered by the TESS space telescope around the high proper-motion G2 V star TOI-1422 (V = 10.6 mag), 155 pc away, with the primary goal of confirming its planetary nature and characterising its properties.
Methods: We monitored TOI-1422 with the HARPS-N spectrograph for 1.5 yr to precisely quantify its radial velocity (RV) variation. We analyse these RV measurements jointly with TESS photometry and check for blended companions through high-spatial resolution images using the AstraLux instrument.
Results: We estimate that the parent star has a radius of R_⋆ = 1.019_−0.013^+0.014 R_⊙, and a mass of M_⋆ = 1.019_−0.013^+0.014 M_⊙. Our analysis confirms the planetary nature of TOI-1422 b and also suggests the presence of a Neptune-mass planet on a more distant orbit, the candidate TOI-1422 c, which is not detected in TESS light curves. The inner planet, TOI-1422 b, orbits on a period of P_b = 12.9972 ± 0.0006 days and has an equilibrium temperature of T_eq,b = 867 ± 17 K. With a radius of R_b = 3.96_−0.11^+0.13 R_⊕, a mass of M_b = 9.0_−2.0^+2.3 M_⊕ and, consequently, a density of ρ_b = 0.795_−0.235^+0.290g cm⁻³, it can be considered a warm Neptune-sized planet. Compared to other exoplanets of a similar mass range, TOI-1422 b is among the most inflated, and we expect this planet to have an extensive gaseous envelope that surrounds a core with a mass fraction around 10% - 25% of the total mass of the planet. The outer non-transiting planet candidate, TOI-1422 c, has an orbital period of P_c = 29.29_−0.20^+0.21 days, a minimum mass, M_csin i, of 11.1_−2.3^+2.6 M_⊕, an equilibrium temperature of T_eq,c = 661 ± 13 K and, therefore, if confirmed, could be considered as another warm Neptune.