Program: GN-2023B-Q-237

Precise radial velocities from MAROON-X have suggested that TOI 561 b—the only planet with a precisely measured density orbiting a thick-disk star—is iron poor, although a larger sample of thick-disk planets is necessary for this result to be statistically significant. Using galactic kinematics and stellar abundance measurements from Gaia DR3, we have identified two other thick-disk terrestrial planets ideally suited to probing the compositional differences between planets orbiting thick- and thin-disk stars. We request 14.45 hours of observing time on Gemini North to collect 20 observations of each star with MAROON-X. With these observations, we will obtain precise mass estimates of the two planets, allowing us to measure the iron mass fraction and distinguish between Earth-like and iron-poor composition models. For the Earth, the mass ratio of the iron core to the silicate mantle matches precisely the ratio of these elements in the solar photosphere, born in the galactic thick disk of alpha-enhanced elements, it would be logical for these stars to be silicate rich and iron poor. This result would spur models of the interior structure, mantle convection, and atmospheric outgassing of these very different terrestrial worlds.