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


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 600-800 Myr 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 planets radius and mass are $3.44 \pm 0.12 R\oplus$ and $24.5{-5.2}^{+5.7}{M}_{\oplus}$ . 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 λ = 3° ± 16°, compatible with spin-orbit alignment, in contrast to other hot Neptunes that have been studied around older stars.