摘要：We validate the discovery of a 2-Earth-radii sub-Neptune-sized planet around the nearby high-proper-motion M2.5 dwarf G 9-40 (EPIC 212048748), using high-precision, near-infrared (NIR) radial velocity (RV) observations with the Habitable-zone Planet Finder (HPF), precision diffuser-assisted ground-based photometry with a custom narrowband photometric filter, and adaptive optics imaging.At a distance of d = 27.9 $\,\mathrm{pc}$, G 9-40b is the second-closest transiting planet discovered by K2 to date.The planet's large transit depth (~3500 ppm), combined with the proximity and brightness of the host star at NIR wavelengths (J = 10, K = 9.2), makes G 9-40b one of the most favorable sub-Neptune-sized planets orbiting an M dwarf for transmission spectroscopy with James Webb Space Telescope, ARIEL, and the upcoming Extremely Large Telescopes.The star is relatively inactive with a rotation period of ~29 days determined from the K2 photometry.To estimate spectroscopic stellar parameters, we describe our implementation of an empirical spectral-matching algorithm using the high-resolution NIR HPF spectra.Using this algorithm, we obtain an effective temperature of ${T}_{\mathrm{eff}}=3404\pm 73\,{\rm{K}}$ and metallicity of $[\mathrm{Fe}/{\rm{H}}]=-0.08\pm 0.13$.Our RVs, when coupled with the orbital parameters derived from the transit photometry, exclude planet masses above 11.7M⊕ with 99.7% confidence assuming a circular orbit.From its radius, we predict a mass of $M={5.0}_{-1.9}^{+3.8}{M}_{\oplus }$ and an RV semiamplitude of $K={4.1}_{-1.6}^{+3.1}\,{\rm{m}}\ {{\rm{s}}}^{-1}$, making its mass measurable with current RV facilities.We urge further RV follow-up observations to precisely measure its mass, to enable precise transmission spectroscopic measurements in the future.
关键词：Exoplanet astronomy;Exoplanet systems;Radial velocity;Exoplanet detection methods;Transit photometry;Low mass stars;Mini Neptunes;Exoplanets
