GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit

Abstract

We use WIYN/NEID radial velocity measurements to confirm the planetary nature and measure the mass of the TESS transiting exoplanet candidate around the mid-K dwarf GJ 523 ($V=9.23$, $K=6.525$). We find that GJ 523b is on a 17.75 day orbit and has a radius of $2.55\pm 0.15R_{\oplus }$, a mass of $23.5\pm 3.3M_{\oplus }$, and a zero-albedo equilibrium temperature of 538 K. GJ 523b's high bulk density of $7.8\pm 1.8$ g cm${}^{-3}$ and position on a mass-radius diagram implies a surprising low atmospheric mass fraction despite its relatively large mass. Additionally, we determine that the system has an age of $169_{-48}^{+100}$ Myr through a gyrochronological analysis of GJ 523 and its comoving companions. We also use the SED-derived stellar radius, the photometric rotation period, and the spectroscopic $v\sin i_{\star }$ to derive a stellar inclination of $17.6\pm 5.0$ degrees, implying that GJ 523b has a minimum orbital obliquity of $71.4_{-5.0}^{+4.7}$ degrees. GJ 523b's high mass, apparent lack of a gas envelope, young age, and high orbital obliquity present a challenge to typical planet formation pathways, and at the moment there is not enough data on the system to definitively determine how GJ 523b formed. Finally, we present a new observational classification for ultra-dense, sub-Neptune-sized exoplanets similar to GJ 523b: the mega-Earths, planets with $R_p\ge 2.1R_{\oplus }$ and $\ddot{I}{}_p\ge 5.5$ g cm${}^{-3}$.

26 pages, 14 figures, submitted to AJ