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 $2.55\pm0.15\,R_\oplus$, 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 $ρ_p \geq 5.5$ g cm$^{-3}$.

Figures (14)

• Figure 1: TESS photometry of GJ 523. In the first two TESS Sectors, GJ 523b's transit (vertical red lines) falls in a data gap. In the latter three TESS Sectors, we plot the combined GP and transit fit to the data in purple. Rotational modulation can be seen in the photometry, with a period of 5.621 days.
• Figure 2: Lomb-Scargle periodograms (LSP) with the 1% and 5% false alarm levels plotted in purple and blue, respectively. The peak period of the BIS data is plotted as a green vertical line, and GJ 523b's period is plotted as a vertical red line. The top panel shows the LSP of the BIS, with a clear peak near 6.5 days and no peak at GJ 523b's period. The second and third panels show the LSPs of the NEID-DRP and SCALPELS RVs, respectively, and GJ 523b's period is well above the 1% false alarm level in both. In the fourth panel, GJ 523b's RV signal is removed from the SCALPELS RVs and the same period present in the BIS increases above the 1% false alarm level. The last panel is an LSP of the SCALPELS RVs with the planet and BIS-RV correlation removed. Here, the BIS period reduces below the 5% false alarm level.
• Figure 3: The first four PC scores of the singular value decomposition of the ACFs. Each panel shows the temporal evolution of a PC score over the observing window, illustrating the dominant modes of variability present in the ACFs. PC1 and PC2 exhibit observations with distinctly large-magnitude values, indicating days when the ACF structure departs strongly from the mean pattern. PC3 shows a mild linear trend over time, suggesting a gradual evolution in the ACF shape.
• Figure 4: Time-series of the NEID-DRP RVs (centered by their weighted mean), the shape-driven velocities, and the shift-driven velocities in $\text{m s}^{-1}$. For each observation, the NEID-DRP RV is equal to the sum of the shape-driven and shift-driven velocities.
• Figure 5: Top: Gemini optical speckle imaging 5$\sigma$ magnitude contrast curves in both filters as a function of the angular separation out to 1.2 arcsec. The inset shows the reconstructed 832 nm image of GJ 523 with a 1 arcsec scale bar. GJ 523 was found to have no close companions from the diffraction limit (0.02”) out to 1.2 arcsec to within the contrast levels achieved. Bottom: NIR AO imaging and sensitivity curves from the Palomar observations, in a narrowband $K_{cont}$ filter centered on 2.29 $\mu$m. The inset shows the central portion of the image.