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 ( [ \mathrm { Fe / H } ] = 0.26 \pm 0.05 ) G-dwarf ( T _ { eff } = 5430 \pm 70 K and \log g = 4.48 \pm 0.09 ) , based on observations with the High Dispersion Spectrograph ( HDS ) on the Subaru 8.2m 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 multi-color transit photometry with MuSCAT on the Okayama 1.88m telescope gives the orbital period of P = 8.266902 \pm 0.000070 days and consistent transit depths of R _ { p } / R _ { \star } \sim 0.035 or ( R _ { p } / R _ { \star } ) ^ { 2 } \sim 0.0012 .
The transit depth corresponds to a planetary radius of R _ { p } = 3.59 _ { -0.39 } ^ { +0.44 } R _ { \oplus } , 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 \sigma upper limit of 90 M _ { \oplus } ( 0.00027 M _ { \odot } ) 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 , spin-orbit ( mis ) alignment , as well as investigate the possibility of additional planets in the system .