Updated Masses for Kepler-90’s Gas Giants Via Transit-Timing Variation and Radial Velocity Observations

The eight-planet Kepler-90 system exhibits the greatest multiplicity of planets found to date, on par with only the solar system. All eight planets are transiting and were originally detected in photometry from the NASA Kepler primary mission. The two outermost planets, g and h (periods 211 and 332 days) exhibit significant transit-timing variations. Because of the paucity of transits (only 6 and 3 observed transits for planets g and h, respectively), masses for these outer planets are difficult to determine precisely given the number of free parameters needed to model their transit times. To overcome this difficulty, we have measured 34 radial velocities of the Kepler-90 host star over an eleven year baseline using the HIRES spectrograph on the W. M. Keck Observatory. We jointly model the transit-timing variations of the outer two planets as well as the radial velocity time series of their host star, arriving at the most precise masses yet for planets g and h. These results allow for clearer insights into the architecturally rich Kepler-90 system.