摘要：While planets between the size of Uranus and Saturn are absent within the solar system, the star K2-24 hosts twosuch planets, K2-24b and c, with radii equal to 5.4ÅR and 7.5Å R , respectively. The two planets have orbital periods of 20.9days and 42.4days, residing only 1% outside the nominal 2:1 mean-motion resonance. In thiswork, we present results from a coordinated observing campaign to measure planet masses and eccentricities thatcombines radial velocity measurements from Keck/HIRES and transit-timing measurements from K2 and Spitzer.K2-24b and c have low, but nonzero, eccentricities of ~ ~ e e 0.08 1 2. The low observed eccentricities provideclues to the formation and dynamical evolution of K2-24b and K2-24c, suggesting that they could be the result ofstochastic gravitational interactions with a turbulent protoplanetary disk, among other mechanisms. K2-24b and c are - +19.0 2.1 2.2 Å M and - + 15.4 1.8 1.9 ÅM , respectively; K2-24c is 20% less massive than K2-24b, despite being 40% larger.Their large sizes and low masses imply large envelope fractions, which we estimate at - + 26 3 3 % and - + 52 3 5 %. Inparticular, K2-24c’s large envelope presents an intriguing challenge to the standard model of core-nucleatedaccretion that predicts the onset of runaway accretion when f env ≈50%.
关键词：planets and satellites: dynamical evolution and stability;planets and satellites: formation;planets and satellites: individual (K2-24b,K2-24c);techniques: radial velocities
