Radial velocity confirmation of K2-100b: a young, highly irradiated, and low density transiting hot Neptune

We present a detailed analysis of HARPS-N radial velocity observations of K2-100, a young and active star in the Praesepe cluster, which hosts a transiting planet with a period of 1.7 days. We model the activity-induced radial velocity variations of the host star with a multi-dimensional Gaussian Process framework and detect a planetary signal of $10.6 \pm 3.0 {\rm m,s^{-1}}$, which matches the transit ephemeris, and translates to a planet mass of $21.8\pm 6.2M_{\oplus }$. We perform a suite of validation tests to confirm that our detected signal is genuine. This is the first mass measurement for a transiting planet in a young open cluster. The relatively low density of the planet, $2.04_{-0.61}^{+0.66}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, implies that K2-100b retains a significant volatile envelope. We estimate that the planet is losing its atmosphere at a rate of ${10}^{11}-{10}^{12}\mathrm{g}{\mathrm{s}}^{-1}$ due to the high level of radiation it receives from its host star.