TOI-561 b: A Low-density Ultra-short-period “Rocky” Planet around a Metal-poor Star

TOI-561 is a galactic thick disk star hosting an ultra-short period (0.45 day orbit) planet with radius of 1.37 R$_{\oplus}$, making it one the most metal-poor ([Fe/H] = -0.41) and oldest ($\sim$10 Gyr) sites where Earth-sized has been found. We present new simultaneous radial velocity measurements (RVs) from Gemini-N/MAROON-X Keck/HIRES, which we combined literature RVs to derive mass M$_{b}$=2.24 $\pm$ 0.20 M$_{\oplus}$. also used two Sectors TESS photometry improve determination, finding R$_{b}$=$1.37 \pm 0.04 R_\oplus$, confirming that b lowest-density super-Earths measured date ($\rho_b$= 4.8 0.5 g/cm$^{3}$). This density consistent iron-poor rocky composition reflective host star's iron rock-building element abundances; however, low-density volatile envelope. The equilibrium temperature ($\sim$2300 K) suggests this envelope would likely be composed high mean molecular weight species, such as water vapor, carbon dioxide, or silicate not primordial. demonstrate determination sensitive choice stellar parameters, further are needed determine if bare planet, optically thin atmosphere, rare example non-primordial on smaller than 1.5 R$_{\oplus}$.