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Description
Using radial velocity data from the Habitable Zone Planet Finder, we have measured the mass of the Neptune-sized planet K2-25b, as well as the obliquity of its M4.5 dwarf host star in the 600-800Myr Hyades cluster. 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.
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