Photometry and RVs of K2-25b with HPF

This is one of the youngest planetary systems for which both of these quantities have been measured and one of the very few M dwarfs with a measured obliquity. Based on a joint analysis of the radial velocity data, time-series photometry from the K2 mission, and new transit light curves obtained with diffuser-assisted photometry, the planet's radius and mass are 3.44{+/-}0.12R_{Earth}_ and 24.5_-5.2_^+5.7^M_{Earth}_.

These properties are compatible with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass). We measure an orbital eccentricity of e=0.43{+/-}0.05. The sky-projected stellar obliquity is {lambda}=3{deg}{+/-}16{deg}, compatible with spin- orbit alignment, in contrast to other "hot Neptunes" that have been studied around older stars.