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

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 G8V dwarf at a distance of only 54 parsecs. This multi-planetary system is one of the best candidates for atmospheric characterization to date, with all its known planets suitable for multi-wavelength transmission spectroscopy. 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 = 3.37^(+0.13)_(−0.13) R_⊕, R_c = 3.22^(+0.15)_(−0.14) R_⊕, and R_d = 3.16^(+0.11)^(−0.11) R_⊕) 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 prime target for transmission spectroscopy of all three planets' atmospheres, HD 191939 can offer a unique insight into multiple sub-Neptunes born from a proto-planetary disk that may have resembled that of the early Sun.