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, Daniel Huber, John M. Brewer, Karen A. Collins, David R. Ciardi, Elisabeth C. Matthews, Carl Ziegler, Steve B. Howell, Natalie M. Batalha, Ian J. M. Crossfield, Courtney D. Dressing, Benjamin J. Fulton, Andrew W. Howard, Howard Isaacson, Stephen R. Kane, Erik A. Petigura, Paul Robertson, Arpita Roy, Ryan A. Rubenzahl, Joseph D. Twicken, Zachary R. Claytor, Keivan G. Stassun, Mason G. MacDougall, Ashley Chontos, Steven Giacalone, Paul A. Dalba, Teo Močnik, Michelle L. Hill, Corey Beard, Joseph M. Akana Murphy, Lee J. Rosenthal, Aida Behmard, Judah Van Zandt, Jack Lubin, Molly Kosiarek, Michael B. Lund, Jessie L. Christiansen, Rachel A. Matson, Charles Beichman, Joshua E. Schlieder, Erica J. Gonzales, César Briceño, Nicholas M. Law, Andrew W. Mann, Kevin I. Collins, Phil Evans, Akihiko Fukui, Eric L. N. Jensen, F. Murgas, Norio Narita, Ε. Πάλλη, H. Parviainen, Richard P. Schwarz, Thiam-Guan Tan, Jack S Acton, Edward M. Bryant, Alexander Chaushev, Samuel Gill, Philipp Eigmüller, Jon M. Jenkins, G. Ricker, Sara Seager, Joshua N. Winn
Abstract
We report the discovery of TOI-561, a multiplanet system in the galactic thick disk that contains a rocky, ultrashort- 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, [a/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 ± 0.8M⊕ and 5.5+2.0-1.6g 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⊕ 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, groundbased 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.