We identify a sub-Neptune exoplanet ( R _ { p } = 2.5 \pm 0.2 \mbox { R } _ { \earth } ) transiting a solar twin in the Ruprecht 147 star cluster ( 3 Gyr , 300 pc , [ Fe/H ] = +0.1 dex ) .
The \sim 81 day light-curve for EPIC 219800881 ( V = 12.71 ) from K2 Campaign 7 shows six transits with a period of 13.84 days , a depth of \sim 0.06 % , and a Based on our analysis of high-resolution MIKE spectra , broadband optical and NIR photometry , the cluster parallax and interstellar reddening , and isochrone models from PARSEC , Dartmouth , and MIST , we estimate the following properties for the host star : M _ { \star } = 1.01 \pm 0.03 \mbox { M } _ { \odot } , R _ { \star } = 0.95 \pm 0.03 \mbox { R } _ { \odot } , and T _ { \mbox { \scriptsize eff } } = 5695 \pm 50 K. This star appears to be single based on our modeling of the photometry , the low radial velocity ( RV ) variability measured over nearly ten years , and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry .
Applying a probabilistic mass–radius relation , we estimate that the mass of this planet is M _ { p } = 7 + 5 - 3 \mbox { M } _ { \earth } , which would cause an RV semi-amplitude of K = 2 \pm 1 \mbox { m s } ^ { -1 } that may be measurable with existing precise RV facilities .
After statistically validating this planet with BLENDER , we now designate it K2-231 b , making it the second substellar object to be discovered in Ruprecht 147 and the first planet ; it joins the small but growing ranks of 22 other planets and 3 candidates found in open clusters .