HD 191939: Three Sub-Neptunes Transiting a Sun-like Star Only 54 pc Away

Abstract

We present the discovery of three sub-Neptune-sized planets transiting the nearby and bright Sun-like star HD 191939 (TIC 269701147, TOI 1339), a $K_s=7.18$ magnitude G8 V dwarf at a distance of only 54 parsecs. We validate the planetary nature of the transit signals by combining five months of data from the Transiting Exoplanet Survey Satellite with follow-up ground-based photometry, archival optical images, radial velocities, and high angular resolution observations. The three sub-Neptunes have similar radii ($R_b=3.42_{-0.11}^{+0.11}{R}_{\oplus }$, $R_c=3.23_{-0.11}^{+0.11}{R}_{\oplus }$, and $R_d=3.16_{-0.11}^{+0.11}{R}_{\oplus }$) and their orbits are consistent with a stable, circular, and co-planar architecture near mean motion resonances of 1:3 and 3:4 ($P_b=8.88$ days, $P_c=28.58$ days, and $P_d=38.35$ days). The HD~191939 system is an excellent candidate for precise mass determinations of the planets with high-resolution spectroscopy due to the host star's brightness and low chromospheric activity. Moreover, the system's compact and near-resonant nature can provide an independent way to measure planetary masses via transit timing variations while also enabling dynamical and evolutionary studies. Finally, as a promising target for multi-wavelength transmission spectroscopy of all three planets' atmospheres, HD 191939 can offer valuable insight into multiple sub-Neptunes born from a proto-planetary disk that may have resembled that of the early Sun.

arXiv comment24 pages, 14 figures, 6 tables

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