The Astronomical Journal (Jan 2023)
TOI-561 b: A Low-density Ultra-short-period “Rocky” Planet around a Metal-poor Star
- Casey L. Brinkman,
- Lauren M. Weiss,
- Fei Dai,
- Daniel Huber,
- Edwin S. Kite,
- Diana Valencia,
- Jacob L. Bean,
- Corey Beard,
- Aida Behmard,
- Sarah Blunt,
- Madison Brady,
- Benjamin Fulton,
- Steven Giacalone,
- Andrew W. Howard,
- Howard Isaacson,
- David Kasper,
- Jack Lubin,
- Mason MacDougall,
- Joseph M. Akana Murphy,
- Mykhaylo Plotnykov,
- Alex S. Polanski,
- Malena Rice,
- Andreas Seifahrt,
- Guðmundur Stefánsson,
- Julian Stürmer
Affiliations
- Casey L. Brinkman
- ORCiD
- Institute for Astronomy, University of Hawai’i , 2680 Woodlawn Drive, Honolulu, HI 96822 USA ; [email protected]
- Lauren M. Weiss
- ORCiD
- Department of Physics and Astronomy, University of Notre Dame , Notre Dame, IN 46556, USA
- Fei Dai
- ORCiD
- Division of Geological and Planetary Sciences , 1200 E. California Boulevard, Pasadena, CA 91125, USA
- Daniel Huber
- ORCiD
- Institute for Astronomy, University of Hawai’i , 2680 Woodlawn Drive, Honolulu, HI 96822 USA ; [email protected]
- Edwin S. Kite
- ORCiD
- Department of the Geophysical Sciences, University of Chicago , Chicago, IL 60637, USA
- Diana Valencia
- ORCiD
- Centre for Planetary Sciences, University of Toronto , 1265 Military Trail, Toronto, ON M1C 1A4, Canada
- Jacob L. Bean
- ORCiD
- Department of Astronomy & Astrophysics, University of Chicago , 5640 S. Ellis Avenue, Chicago, IL 60637, USA
- Corey Beard
- ORCiD
- Department of Physics & Astronomy, University of California Irvine , Irvine, CA 92697, USA
- Aida Behmard
- ORCiD
- Department of Astronomy, California Institute of Technology , Pasadena, CA 91125, USA
- Sarah Blunt
- ORCiD
- Department of Astronomy, California Institute of Technology , Pasadena, CA 91125, USA
- Madison Brady
- ORCiD
- Department of Astronomy & Astrophysics, University of Chicago , 5640 S. Ellis Avenue, Chicago, IL 60637, USA
- Benjamin Fulton
- ORCiD
- NASA Exoplanet Science Institute/Caltech-IPAC , MC 314-6, 1200 E. California Boulevard, Pasadena, CA 91125, USA
- Steven Giacalone
- ORCiD
- Department of Astronomy, 501 Campbell Hall, University of California , Berkeley, CA 94720, USA
- Andrew W. Howard
- ORCiD
- Department of Astronomy, California Institute of Technology , Pasadena, CA 91125, USA
- Howard Isaacson
- ORCiD
- Department of Astronomy, 501 Campbell Hall, University of California , Berkeley, CA 94720, USA; Centre for Astrophysics, University of Southern Queensland , Toowoomba, QLD, Australia
- David Kasper
- ORCiD
- Department of Astronomy & Astrophysics, University of Chicago , 5640 S. Ellis Avenue, Chicago, IL 60637, USA
- Jack Lubin
- ORCiD
- Department of Physics & Astronomy, University of California Irvine , Irvine, CA 92697, USA
- Mason MacDougall
- ORCiD
- Astronomy Department, 475 Portola Plaza, University of California , Los Angeles, CA 90095, USA
- Joseph M. Akana Murphy
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- Department of Astronomy and Astrophysics, University of California , Santa Cruz, CA 95064, USA
- Mykhaylo Plotnykov
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- Centre for Planetary Sciences, University of Toronto , 1265 Military Trail, Toronto, ON M1C 1A4, Canada
- Alex S. Polanski
- ORCiD
- Department of Physics and Astronomy, University of Kansas , Lawrence, KS 66045, USA
- Malena Rice
- ORCiD
- Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology , Cambridge, MA 02139, USA; Department of Astronomy, Yale University , New Haven, CT 06511, USA
- Andreas Seifahrt
- ORCiD
- Department of Astronomy & Astrophysics, University of Chicago , 5640 S. Ellis Avenue, Chicago, IL 60637, USA
- Guðmundur Stefánsson
- ORCiD
- Department of Astrophysical Sciences, Princeton University , 4 Ivy Lane, Princeton, NJ 08540, USA
- Julian Stürmer
- ORCiD
- Landessternwarte, Zentrum für Astronomie der Universität Heidelberg , Königstuhl 12, D-69117 Heidelberg, Germany
- DOI
- https://doi.org/10.3847/1538-3881/acad83
- Journal volume & issue
-
Vol. 165,
no. 3
p. 88
Abstract
TOI-561 is a galactic thick-disk star hosting an ultra-short-period (0.45-day-orbit) planet with a radius of 1.37 R _⊕ , making it one of the most metal-poor ([Fe/H] = −0.41) and oldest (≈10 Gyr) sites where an Earth-sized planet has been found. We present new simultaneous radial velocity (RV) measurements from Gemini-N/MAROON-X and Keck/HIRES, which we combined with literature RVs to derive a mass of M _b = 2.24 ± 0.20 M _⊕ . We also used two new sectors of TESS photometry to improve the radius determination, finding R _b = 1.37 ± 0.04 R _⊕ and confirming that TOI-561 b is one of the lowest-density super-Earths measured to date ( ρ _b = 4.8 ± 0.5 g cm ^−3 ). This density is consistent with an iron-poor rocky composition reflective of the host star’s iron and rock-building element abundances; however, it is also consistent with a low-density planet with a volatile envelope. The equilibrium temperature of the planet (∼2300 K) suggests that this envelope would likely be composed of high mean molecular weight species, such as water vapor, carbon dioxide, or silicate vapor, and is likely not primordial. We also demonstrate that the composition determination is sensitive to the choice of stellar parameters and that further measurements are needed to determine whether TOI-561 b is a bare rocky planet, a rocky planet with an optically thin atmosphere, or a rare example of a nonprimordial envelope on a planet with a radius smaller than 1.5 R _⊕ .
Keywords
- Exoplanets
- Extrasolar rocky planets
- Super Earths
- Exoplanet surface composition
- Exoplanet atmospheric composition
