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

TL;DR

This study confirms GJ 523b as a massive, dense, young mega-Earth with a high orbital obliquity, challenging existing planet formation theories and introducing a new classification for ultra-dense, sub-Neptune exoplanets.

Contribution

We provide the first detailed characterization of GJ 523b, establishing it as a young, ultra-dense mega-Earth with a high obliquity, and propose a new planet classification.

Findings

GJ 523b has a mass of 23.5±3.3 M⊕ and radius of 2.55±0.15 R⊕.

The system's age is approximately 170 Myr, indicating a young planetary system.

GJ 523b exhibits a high orbital obliquity of about 71 degrees.

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 $2.55\pm0.15\,R_\oplus$, a mass of $23.5\pm3.3\,M_\oplus$, and a zero-albedo equilibrium temperature of 538 K. GJ 523b's high bulk density of $7.8\pm1.8$ 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 $169^{+100}_{-48}$ 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 $17.6\pm5.0$ 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 $R_p \geq2.1\,R_\oplus$ and $\rho_p \geq 5.5$ g cm$^{-3}$.