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

February 2021 • 2021AJ....161...56W

Authors • 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.

Abstract • 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, R_p = 1.45 ± 0.11 R_⊕), c (TOI-561.01, P = 10.8 days, R_p = 2.90 ± 0.13 R_⊕), and d (TOI-561.03, P = 16.3 days, R_p = 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.

Links

• PREPRINT http://arxiv.org/abs/2009.03071
• ELECTR https://doi.org/10.3847%2F1538-3881%2Fabd409
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2021AJ....161...56W
• PDF https://iopscience.iop.org/article/10.3847/1538-3881/abd409/pdf
• PDF https://stacks.iop.org/1538-3881/161/56/pdf
• DATA http://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/161/56
• DATA https://archive.gemini.edu/searchform/publication=2021AJ....161...56W
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2021AJ....161...56W
• DATA https://irsa.ipac.caltech.edu/bibdata/2021/W/2021AJ....161...56W.html
• DATA http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?type=REFERENCE&bibcode=2020arXiv200903071W