The K2-ESPRINT project. VI. K2-105 b, a hot Neptune around a metal-rich G-dwarf

We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26 ± 0.05) G-dwarf (T_eff = 5430 ± 70 K and log g = 4.48 ± 0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2 m telescope. High spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be <10^-6 using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multicolor transit photometry with MuSCAT on the Okayama 1.88 m telescope gives an orbital period of P = 8.266902 ± 0.000070 d and consistent transit depths of R_p/R_{black star} ∼ 0.035 or (R_p/R_{black star})² ∼ 0.0012. The transit depth corresponds to a planetary radius of R_p = 3.59_-0.39^+0.44 R_⊕, indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3 σ upper limit of 90 M_⊕ (0.00027 M_⊙) on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, and spin-orbit (mis)alignment, as well as investigate the possibility of additional planets in the system.