TOI-1246: Mass Measurements for Four Transiting sub-Neptunes

Multi-planet systems are valuable arenas for investigating exoplanet architectures and comparing planetary siblings. TOI-1246 is one such system, with a moderately bright K dwarf (V=11.6, K=9.9) and four transiting sub-Neptunes identified by TESS (4.31 d, 5.90 d, 18.66 d, 37.92 d). We collected 125 radial velocity (RV) observations with Keck/HIRES and TNG/HARPS-N to follow up the masses and orbital parameters of these planets. We refit the 14 sectors of TESS photometry to refine planet radii (2.97 ± 0.06 R_⊕, 2.47 ± 0.08 R_⊕, 3.46 ± 0.09 R_⊕, 3.72 ± 0.16 R_⊕). We confirm the four planets and find that the outermost transiting planet TOI-1246 e is substantially more massive than the three inner planets (8.3 ± 1.1 M_⊕, 8.8 ± 1.2 M_⊕, 5.3 ± 1.8 M_⊕, 15.2 ± 2.3 M_⊕). The two outer planets, TOI-1246 d and TOI-1246 e, lie just exterior to the 2:1 resonance (P_e/P_d = 2.03) and exhibit transit timing variations (TTVs). TOI-1246 is one of the brightest stars with four known transiting planets, making it particularly amenable for continued observations. It is also one of only six systems with measured masses and radii for all four transiting planets. The planet densities range from 0.70 ± 0.24 to 3.21 ± 0.44 g/cm³, implying that the planets have a range of bulk and atmospheric compositions. We also report a fifth planet candidate found in the RV data with a minimum mass of 24.9 ± 3.8 M_⊕, and discuss the implications for system architecture if it is confirmed to be planetary in nature. This planet candidate is exterior to TOI-1246 e with a candidate period of 93.8 d, and we are collecting additional RV observations with NEID to investigate the nature of this signal.