EPIC 211391664b: A 32 Earth-mass Neptune-size Planet in a 10 Day Orbit Transiting an F8 Star

Abstract

We report the discovery of K2-98b (EPIC 211391664b), a transiting Neptune-size planet monitored by the K2 mission during its Campaign 5. We combine the K2 time-series data with ground-based photometric and spectroscopic follow-up observations to confirm the planetary nature of the object and derive its mass, radius, and orbital parameters. K2-98b is a warm Neptune-like planet in a 10 day orbit around a V = 12.2 mag F-type star with M_star = 1.074 ± 0.042 M_sun, R_star = 1.311 (-0.048, +0.083) R_sun, and age of 5.2 (-1.0, +1.2) Gyr. We derive a planetary mass and radius of M_planet = 32.2 ± 8.1 M_earth and R_planet = 4.3 (-0.2, +0.3) R_earth. K2-98b joins the relatively small group of Neptune-size planets whose mass and radius have been derived with a precision better than 25%. We estimate that the planet will be engulfed by its host star in ∼3 Gyr, due to the evolution of the latter toward the red giant branch.