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

TL;DR

This study uses radial velocity data to measure the mass and obliquity of the young Neptune K2-25b, revealing a high mass, low obliquity, and a potential rocky core with a thin atmosphere, providing insights into young planetary systems.

Contribution

First measurement of both mass and obliquity for a young Neptune-sized exoplanet around an M dwarf star.

Findings

K2-25b has a mass of approximately 24.5 Earth masses.

The planet's obliquity is consistent with spin-orbit alignment.

The planet likely has a rocky core with a thin hydrogen-helium atmosphere.

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 600-800MYr 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 $3.44\pm 0.12 \mathrm{R_\oplus}$ and $24.5_{-5.2}^{+5.7} \mathrm{M_\oplus}$. These properties are compatible with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass). We measure an orbital eccentricity of $e=0.43 \pm 0.05$. The sky-projected stellar obliquity is $\lambda=3 \pm 16^{\circ}$, compatible with spin-orbit alignment, in contrast to other "hot Neptunes" that have been studied around older stars.