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Using radial-velocity data from the Habitable-zone Planet Finder, we havemeasured the mass of the Neptune-sized planet K2-25b, as well as the obliquityof its M4.5-dwarf host star in the 600-800MYr Hyades cluster. This is one ofthe youngest planetary systems for which both of these quantities have beenmeasured, and one of the very few M dwarfs with a measured obliquity. Based ona joint analysis of the radial velocity data, time-series photometry from theK2 mission, and new transit light curves obtained with diffuser-assistedphotometry, the planet's radius and mass are $3.44\pm 0.12 \mathrm{R_\oplus}$and $24.5_{-5.2}^{+5.7} \mathrm{M_\oplus}$. These properties are compatiblewith a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass).We measure an orbital eccentricity of $e=0.43 \pm 0.05$. The sky-projectedstellar obliquity is $\lambda=3 \pm 16^{\circ}$, compatible with spin-orbitalignment, in contrast to other "hot Neptunes" that have been studied aroundolder stars.

The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b