K2-98b: A 32 M⊕ NEPTUNE-SIZE PLANET IN A 10 DAY ORBIT TRANSITING AN F8 STAR

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-circle dot, R-star = 1.311(-0.048)(+0.083) R-circle dot, and age of 5.2(-1.0)(+1.2) Gyr We derive a planetary mass and radius of M-p = 32.2 +/- 8.1 M-circle plus and R-p = 4.3(-0.2)(+0.3) R-circle plus.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 similar to 3 Gyr, due to the evolution of the latter toward the red giant branch.