TESS-Keck Survey IX: Masses of Three Sub-Neptunes Orbiting HD 191939 and the Discovery of a Warm Jovian Plus a Distant Sub-Stellar Companion

Abstract

Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI-1339), a bright (V=9) and Sun-like (G9V) star, which TESS found to host three transiting planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with similar sizes from 3 to 3.4 $R_{\oplus }$. To further characterize the system, we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES and APF/Levy. We find that $M_b=10.4\pm 0.9M_{\oplus }$ and $M_c=7.2\pm 1.4M_{\oplus }$, which are low compared to most known planets of comparable radii. The RVs yield only an upper-limit on $M_d$ (<5.8 $M_{\oplus }$ at 2$\ddot{I}ƒ$). The RVs further reveal a fourth planet (e) with a minimum mass of $0.34\pm 0.01M_{Jup}$ and an orbital period of 101.4 $\pm$ 0.4 days. Despite its non-transiting geometry, secular interactions between planet e and the inner transiting planets indicate that planet e is coplanar with the transiting planets ($\hat{I}”$i < 10${}^{\circ }$). We identify a second high mass planet (f) with 95% confidence intervals on mass between $2-11M_{Jup}$ and period between 1700-7200 days, based on a joint analysis of RVs and astrometry from $Gaia$ and $Hipparcos$. As a bright star hosting multiple planets with well-measured masses, HD 191939 presents many options for comparative planetary astronomy including characterization with JWST.

20 pages, 10 figures. Accepted for publication in The Astronomical Journal