TWO TRANSITING LOW DENSITY SUB-SATURNS FROM K2

Abstract

ABSTRACT 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 cm−3 and 0.31 ± 0.12 g cm−3, 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.