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Description
We report the discovery of TOI-561, a multiplanet system in the galactic thick disk that contains a rocky, ultra-short-period planet. This bright (V=10.2) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561b (TOI-561.02, P=0.44days, Rp=1.45{+/-}0.11R{Earth}), c (TOI-561.01, P=10.8days, Rp=2.90{+/-}0.13R{Earth}), and d (TOI-561.03, P=16.3days, Rp=2.32{+/-}0.16R{Earth}). The star is chemically ([Fe/H]=-0.41{+/-}0.05, [{alpha}/Fe]=+0.23{+/-}0.05) and kinematically consistent with the galactic thick-disk population, making TOI-561 one of the oldest (10{+/-}3Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W.M.Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of 3.2{+/-}0.8M{Earth} and 5.5_-1.6_^+2.0^g/cm^3^, consistent with a rocky composition. Its lower- than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is 7.0{+/-}2.3M{Earth} and 1.6{+/-}0.6g/cm^3^, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up, and radial velocities. TOI-561b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.
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