Large eccentricity, low mutual inclination: The three-dimensional architecture of a hierarchical system of giant planets

We establish the three-dimensional architecture of the Kepler-419 (previously KOI-1474) system to be eccentric yet with a low mutual inclination. Kepler-419b is a warm Jupiter at semi-major axis α = 0.370-0.006 ^+0.007 AU with a large eccentricity (e = 0.85_-0.07 ^+0.08) measured via the "photoeccentric effect." It exhibits transit timing variations (TTVs) induced by the non-transiting Kepler-419c, which we uniquely constrain to be a moderately eccentric (e = 0.184±0.002), hierarchically separated (α = 1.68±0.03 AU) giant planet (7.3±0.4M_Jup).We combine 16 quarters of Kepler photometry, radial-velocity (RV) measurements from the HIgh Resolution Echelle Spectrometer on Keck, and improved stellar parameters that we derive from spectroscopy and asteroseismology. From the RVs, we measure the mass of the inner planet to be 2.5±0.3M_Jup and confirm its photometrically measured eccentricity, refining the value to e = 0.83±0.01. The RV acceleration is consistent with the properties of the outer planet derived from TTVs. We find that despite their sizable eccentricities, the planets are coplanar to within 9_-6⁺⁸ degrees, and therefore the inner planet's large eccentricity and close-in orbit are unlikely to be the result of Kozai migration. Moreover, even over many secular cycles, the inner planet's periapse is most likely never small enough for tidal circularization. Finally, we present and measure a transit time and impact parameter from four simultaneous groundbased light curves from 1 m class telescopes, demonstrating the feasibility of ground-based follow-up of Kepler giant planets exhibiting large TTVs.