The TESS-Keck survey. II. An ultra-short-period rocky planet and its siblings transiting the galactic thick-disk star TOI-561

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

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


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.

Original languageEnglish (US)
Article numberabd409
JournalAstronomical Journal
Issue number2
StatePublished - Feb 1 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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