The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561

Weiss, Lauren M.; Dai, Fei; Huber, Daniel; Brewer, John M.; Collins, Karen A.; Ciardi, David R.; Matthews, Elisabeth C.; Ziegler, Carl; Howell, Steve B.; Batalha, Natalie M.; Crossfield, Ian J. M.; Dressing, Courtney; Fulton, Benjamin; Howard, Andrew W.; Isaacson, Howard; Kane, Stephen R.; Petigura, Erik A.; Robertson, Paul; Roy, Arpita; Rubenzahl, Ryan A.; Twicken, Joseph D.; Claytor, Zachary R.; Stassun, Keivan G.; MacDougall, Mason G.; Chontos, Ashley; Giacalone, Steven; Dalba, Paul A.; Mocnik, Teo; Hill, Michelle L.; Beard, Corey; Akana Murphy, Joseph M.; Rosenthal, Lee J.; Behmard, Aida; Van Zandt, Judah; Lubin, Jack; Kosiarek, Molly R.; Lund, Michael B.; Christiansen, Jessie L.; Matson, Rachel A.; Beichman, Charles A.; Schlieder, Joshua E.; Gonzales, Erica J.; Briceño, César; Law, Nicholas; Mann, Andrew W.; Collins, Kevin I.; Evans, Phil; Fukui, Akihiko; Jensen, Eric L. N.; Murgas, Felipe; Narita, Norio; Palle, Enric; Parviainen, Hannu; Schwarz, Richard P.; Tan, Thiam-Guan; Acton, Jack S.; Bryant, Edward M.; Chaushev, Alexander; Gill, Sam; Eigmüller, Philipp; Jenkins, Jon; Ricker, George; Seager, Sara; Winn, Joshua N.
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The Astronomical Journal

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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-561 b (TOI-561.02, P = 0.44 days, Rp = 1.45 ± 0.11 R⊕), c (TOI-561.01, P = 10.8 days, Rp = 2.90 ± 0.13 R⊕), and d (TOI-561.03, P = 16.3 days, Rp = 2.32 ± 0.16 R⊕). The star is chemically ([Fe/H] = -0.41 ± 0.05, [α/Fe] = +0.23 ± 0.05) and kinematically consistent with the galactic thick-disk population, making TOI-561 one of the oldest (10 ± 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 ± 0.8 M⊕ 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 ± 2.3 M⊕ and 1.6 ± 0.6 g 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-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.
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