Three’s Company: An Additional Non-transiting Super-Earth in the Bright HD 3167 System, and Masses for All Three Planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial-velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02 ±0.38 M+for HD 3167 b, a hot super-Earth with a likely rocky composition (ρb = 5.60 +2.15-1.43= g cm-3), and 9.80 +1.30-1.24 M+for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (pc = 1.97 0.94-0.59 g cm-3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509 ±0.045 d (between planets b and c) and a minimum mass of 6.90 ±0.71 M⊕. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.°3 because we do not observe transits of planet d. From 1.°3 to 40°, there are viewing geometries invoking special nodal configurations, which result in planet d not transiting some fraction of the time. From 40° to 60°, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100 Myr. Above 60°, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.