TOI-561 is a galactic thick disk star hosting an ultra-short period planet of 1.45 R⊕: one of the oldest known Earth-sized exoplanets (∼10 Gyr), and among the first discovered beyond the thin disk of our galaxy. As a thick disk star, TOI-561 is one of the most iron depleted (-0.41 dex) and alpha-enriched (0.23 dex) exoplanet hosts. To determine the composition of this planet, we collected high-precision Radial Velocity measurements and used equation-of-state modeling for iron and silicate rock. We have determined that the low density of TOI-561 b requires this planet to have a volatile envelope, despite its size and proximity to its host star.

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