The TESS-Keck Survey II: An Ultra-Short Period Rocky Planet and its Siblings Transiting the Galactic Thick-Disk Star TOI-561

Abstract

We report the discovery of TOI-561, a multi-planet system in the galactic thick disk that contains a rocky, ultra-short period planet (USP). This bright ($V=10.2$) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P=0.44 days, $R_b=1.45\pm 0.11R_{\oplus }$), c (TOI-561.01, P=10.8 days, $R_c=2.90\pm 0.13R_{\oplus }$), and d (TOI-561.03, P=16.3 days, $R_d=2.32\pm 0.16R_{\oplus }$). The star is chemically ([Fe/H]$=-0.41\pm 0.05$, [$\alpha$/H]$=+0.23\pm 0.05$) and kinematically consistent with the galactic thick disk population, making TOI-561 one of the oldest ($10\pm 3$Gyr) 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 $3.2\pm 0.8M_{\oplus }$ and $5.5_{-1.6}^{+2.0}$gcm${}^{-3}$, 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 $7.0\pm 2.3M_{\oplus }$ and $1.6\pm 0.6$gcm${}^{-3}$, 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-561 b 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.

Accepted at The Astronomical Journal; 25 pages, 10 figures

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