2016ApJ...818...36P

We report the discovery and confirmation of K2-24 b and c, two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42±0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68±0.56 R_⊕ and 7.82±0.72 R_⊕ and have orbital periods of 20.8851±0.0003 days and 42.3633±0.0006 days, near the 2:1 mean-motion resonance. We obtained 32 radial velocities with Keck/HIRES and detected the reflex motion due to K2-24 b and c. These planets have masses of 21.0±5.4 M_⊕ and 27.0±6.9 M_⊕, respectively. With low densities of 0.63±0.25 g/cm3 and 0.31±0.12 g/cm3, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 ± 4.3 M_⊕ and 16.1 ± 4.2 M_⊕, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offers rich opportunities for TTV follow-up. Finally, the low surface gravities of the K2-24 planets make them favorable targets for transmission spectroscopy by Hubble Space Telescope, Spitzer, and James Webb Space Telescope.