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*=1.074 ± 0.042 Sun masses, R*=1.311 (+0.083)(-0.048) Sun radii, and age of 5.2 +/- 1.1 Gyr. We derive a planetary mass and radius of Mp=32.2 ± 8.1 Earth masses and Rp=4.3 ± 0.3 Earth radii. 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.