Giant Outer Transiting Exoplanet Mass (GOT `EM) Survey. V. Two Giant Planets in Kepler-511 but Only One Ran Away

Abstract: Systems hosting multiple giant planets are important laboratories for understanding planetary formation and migration processes. We present a nearly decade-long Doppler spectroscopy campaign from the HIRES instrument on the Keck-I telescope to characterize the two transiting giant planets orbiting Kepler-511 on orbits of 27 days and 297 days. The radial velocity measurements yield precise masses for both planets: $0.100^{+0.036}_{-0.039}$ ($2.6 \sigma$) and $0.44^{+0.11}_{-0.12}$ (4$\sigma$) Jupiter masses respectively. We use these masses to infer their bulk metallicities (i.e., metal mass fraction $0.87 \pm 0.03$ and $0.22 \pm 0.04$ respectively). Strikingly, both planets contain approximately $25-30$ Earth masses of heavy elements but have very different amounts of hydrogen and helium. Envelope mass loss cannot account for this difference due to the relatively large orbital distance and mass of the inner planet. We conclude that the outer planet underwent runaway gas accretion while the inner planet did not. This bifurcation in accretion histories is likely a result of the accretion of gas with very different metallicities by the two planets or the late formation of the inner planet from a merger of sub-Neptunes. Kepler-511 uniquely demonstrates how giant planet formation can produce dramatically different outcomes even for planets in the same system.