The Habitable Zone Planet Finder Reveals a High Mass and Low Obliquity for the Young Neptune K2-25b
Mahadevan, Suvrath; Terrien, Ryan; Wisniewski, John; Wright, Jason T.; Ford, Eric B.; Monson, Andrew; Kanodia, Shubham; Ninan, Joe P.; Hebb, Leslie; Bender, Chad F.; Diddams, Scott A.; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Levi, Eric; Maney, Marissa; Metcalf, Andrew J.; Robertson, Paul; Roy, Arpita; Schwab, Christian; Rajagopal, Jayadev; Dawson, Rebekah I.; Stefansson, Gudmundur; Cañas, Caleb; Cochran, William; Ramsey, Lawrence; Wolfgang, Angie; Allen, Lori; Winn, Joshua; Haase, Flynn
United States, Russia, Australia
Abstract
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 planet's radius and mass are 3.44 ± 0.12 R⊕ 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.