A High Quality Atmospheric Target in a Sparse Region of Mass-Radius Space around HD 63935

We present the discovery of the sub-Neptune-size planet orbiting HD 63935, a bright (V=8.6), sun-like (Teff=5560K) star. The planet, HD 63935 b (TOI509.01), was identified in TESS Sector 7 (at twice the correct orbital period). Our analysis of the photometric and radial velocity data yields a robust detection of the planet with a period of 9.0600+0.00064/-0.00070 days, radius of 3.058±0.065 Earth radii, and mass of 10.7±1.6 Earth masses. We identify two additional highly significant candidate signals at 21 and 115 days in the radial velocity data. We suggest these signals are most likely to be planetary in nature, although we examine other potential explanations. The candidate at 21 days has a crossing time consistent with the gap in the TESS sector 7 light curve. If it is transiting, we anticipate two transits in the upcoming TESS Sector 34. For the confirmed planet, we calculate a density of 2.1±0.3 g/cm³, consistent with interiors ranging from rocky to ice-dominated, with a hydrogen envelope making up a few percent of the planet mass. The planet resides on the sub-Neptune "occurrence cliff", a drop in short-period planet occurrence around 3 Earth radii, separating the sub-Neptune population from the giant planets. In addition to our characterization, we also describe our survey's efforts to choose the highest-quality atmospheric targets for followup with JWST. This planet is the best known atmospheric target on the "occurrence cliff", making it an intriguing prospect for additional followup observations. It also resides in a so-far unexplored region of parameter space comprising small (2.6R_earth < R_p < 4R_earth), moderately high irradiation (100 F_earth < F_p < 1000 F_earth) planets around G-stars.

No such planets with known masses have JWST SNR proxy values as high as HD 63935 b (~118) or published transmission spectra, emphasizing the new regions of parameter space being made accessible to exploration by the TESS mission in general and this system in particular.