Discovery and Validation of a High-Density sub-Neptune from the K2 Mission

Abstract

We report the discovery of BD+20594b, a high density sub-Neptune exoplanet, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission, ground-based radial velocity follow-up from HARPS and high resolution lucky and adaptive optics imaging obtained using AstraLux and MagAO, respectively. The host star is a bright ($V=11.04$, $K_s=9.37$), slightly metal poor ([Fe/H]$=-0.15\pm 0.05$ dex) solar analogue located at $152.1_{-7.4}^{+9.7}$ pc from Earth, for which we find a radius of $R_{\ast }=0.928_{-0.040}^{+0.055}{R}_{\odot }$ and a mass of $M_{\ast }=0.961_{-0.029}^{+0.032}{M}_{\odot }$. A joint analysis of the K2 photometry and HARPS radial velocities reveal that the planet is in a $\approx 42$ day orbit around its host star, has a radius of $2.23_{-0.11}^{+0.14}{R}_{\oplus }$, and a mass of $16.3_{-6.1}^{+6.0}{M}_{\oplus }$. Although the data at hand puts the planet in the region of the mass-radius diagram where we could expect planets with a pure rock (i.e. magnesium silicate) composition using two-layer models (i.e., between rock/iron and rock/ice compositions), we discuss more realistic three-layer composition models which can explain the high density of the discovered exoplanet. The fact that the planet lies in the boundary between "possibly rocky" and "non-rocky" exoplanets, makes it an interesting planet for future RV follow-up.

12 pages, 11 figures. Accepted for publication in ApJ